Biological Criteria

Chemical measurements are like taking snapshots of the ecosystem, whereas biological measurements are like making a videotape.
(Prof. David M. Rosenberg, Ph.D., University of Manitoba and the Freshwater Institute, DFO, Winnipeg. In: Bull. Entomol. Soc. Can. 1998. 30(4): 144-152).

This is one of a series of literature review documents on pathogenic organisms in water. The series includes documents on protozoans, helminth worms, viruses, fungi, algae, cyanophytes and bacteria. They are meant to list those organisms that may be of concern in water supplies, outline their life cycles, habitats, effects, sources and mechanisms of spread. We need to be aware of the full spectrum of pathogens that are potential contaminants in our water supplies so that we may devise withdrawal protocols or treatment methods to reduce or eliminate the risk to our health. The recommendations presented are those found in the literature, promoted by other jurisdictions or those of the author. These documents are compilations and presentations of information which may be of use in determining orders, codes of practice, policy, recommendations, guidelines or other actions but do not constitute policy, guidelines or recommendations.

Table of Contents


There is a glossary of water quality terminology on the Water Quality website.
Protozoan organisms that move by extending the cell membrane in some direction and flowing into the extension.
Organisms that resemble amoeba; movement by extending the cell membrane in some direction and flowing into the extension.
Organisms that move by waves of beating by many small hairs which cover their entire surface or only certain areas or zones on their surface.
Two organisms have a commensal relationship when one or both get some benefit from living together or sharing resources and neither is harmed.
A resting or dormant stage of a protozoan usually found in the environment where it awaits introduction into a new host.
Definitive host
The final and necessary host in which a pathogen or parasite completes its life cycle, undergoes sexual reproduction and sheds cysts to start the cycle again.
A chemical, radiation, physical or other technique that kills organisms.
An often fatal disease or infection of the spinal fluid and column, the brain and the nervous system of an organism.
An organism that originates and is, or was prior to transportation by man, restricted to a specific area as opposed to an organism with a widespread distribution.
Refers to organisms that have a discrete nucleus, mitochondria and various organelles such as chloroplasts, includes all multicellular organisms and many unicellular organisms but not bacteria.
Organisms that move by beating several long hairs that emanate from a specific location on their surface.
Refers to organisms that have male and female sex organs in the same individual.
Refers to a person whose immune system is fully functional and who is able to prevent or control infections by many pathogens without any medical help.
Refers to a person whose immune system is not fully functional and who is unable to prevent or control infections by many pathogens and thus is at risk of serious disease or death from many pathogens or other opportunistic organisms that do not cause a problem for immunocompetent people. AIDS, HIV and anti-rejection drugs used in transplant surgery all cause at least partially non-functional immune systems.
Refers to a person whose immune system is not fully functional and who is unable to prevent or control infections by many pathogens and thus is at risk of serious disease or death from many pathogens or other opportunistic organisms that do not cause a problem for immunocompetent people. AIDS, HIV and anti-rejection drugs used in transplant surgery all cause at least partially non-functional immune systems.
Refers to a person whose immune system is not fully functional and who is unable to prevent or control infections by many pathogens and thus is at risk of serious disease or death from many pathogens or other opportunistic organisms that do not cause a problem for immunocompetent people. AIDS, HIV and anti-rejection drugs used in transplant surgery all cause at least partially non-functional immune systems.
Refers to a person whose immune system is not fully functional and who is unable to prevent or control infections by many pathogens and thus is at risk of serious disease or death from many pathogens or other opportunistic organisms that do not cause a problem for immunocompetent people. AIDS, HIV and anti-rejection drugs used in transplant surgery all cause at least partially non-functional immune systems.
1/1,000,000 of a metre; 1/1,000 of a millimetre.
Newborn and often still immunoincompetent and thus susceptible to pathogens.
This refers to diseases or infections that are acquired while in a hospital or care facility setting.
An organism that is not normally a pathogen or parasite but takes advantage of people with non-functional immune systems to cause disease or damage.
Organisms that cause harm or death to their host organism when they live or breed on or within the host organism.
The length of time eggs or oocycts are voided in the feces. During this time, usually measured in days or weeks, a person is infective and actively spreading the disease.
These are organisms which cause disease or damage in another organism.
After death.
The time period between the first infection by a pathogen and the first appearance of eggs or oocycts in the feces. During this time, usually measured in days, a person can be an unknown carrier of a disease and travel all around the world.
Refers to organisms that have no discrete nucleus, mitochondria and various organelles such as chloroplasts, excludes all multicellular organisms and many unicellular organisms but includes bacteria, viruses and cyanophytes.
These are uni-cellular organisms such as amoebas, ciliates, microsporidians and flagellates as distinct from multi-cellular organisms.
A chemical substance produced by an organism for defense or offense against other organisms.
Free-living motile stages of protozoan pathogens.
Found in the blood serum, refers to circulating antibodies being present indicating some prior exposure to the pathogen; since the body has already prepared antibodies there must have been a previous exposure.
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There is an excellent AWWA reference manual, which covers many of the same topics as this document, which was published and became available to the author after this document was virtually complete. Some material from this AWWA manual has been incorporated into this report where appropriate. The AWWA manual is primarily concerned with drinking water pathogens while this document has a wider scope and deals with all water-borne pathogens which includes those that interfere with recreational, irrigation, industrial and livestock or wildlife uses of water. (AWWA. 1999. Waterborne Pathogens. American Water Works Association. Manual of Water Supply Practices. AWA M48. ISBN 1-58321-022-9).

British Columbia Water Quality Guidelines are written for physical, chemical and biological attributes of water. Most Guidelines deal with chemical attributes such as fluoride, silver and chlorophenols but there are some for physical attributes like pH, dissolved oxygen and suspended sediments. Biological attributes have not been as thoroughly covered. There is only one British Columbia biological Guideline that deals primarily with bacterial pathogens. It is dated 1988 and in need of major revision. Work on biological attributes of water quality is an active field of research and most documents more than about five years old are of limited value. The list of biological pathogens gives some indication of the scope of the field. This is an introductory document; subsequent reports will discuss the pathogenic protozoans, helminth worms, fungi, bacteria, toxic algae and viruses. Some of these reports could not have been written as little as ten years ago since the necessary research work had not yet been carried out. They may still be somewhat premature and may need to be revised before too long, especially the taxonomy. The understanding of viruses and protozoans are particularly subject to new research, the helminths and algae are better understood. However, there is a growing need for this information to be consolidated and reviewed; some guidelines are needed now. These documents will form the background survey of organisms and the problems they cause which are a necessary first step in devising water quality guidelines.

Until recently, water issues were overshadowed by other environmental concerns, such as climate change. But now water and its related problems have moved to the front of international environmental and health policy agendas. According to the World Health Organization, a third of the world population suffers from diseases derived from contaminated drinking water. Every year about 13 million people die from waterborne infections; of these, 2 million are children. The majority of these deaths occur in developing countries. However, water-borne pathogens, the agents that cause disease, are a growing hazard and a major economic burden in OECD countries too. For example, in the United States about 900,000 cases of illnesses and 900 deaths occur every year as a result of microbial contamination of drinking water. The annual cost to the US of waterborne diseases is about $19 billion.

The American Society for Microbiology, ASM, has prepared an expert report with data and forecasts, some of which are quite ominous. It says, for example, that more than 20% of the US ground water systems are contaminated. This has significant implications, since more than 100 million Americans rely on ground water as a source of drinking water. The ASM report also highlights how many of the outbreaks are associated with pathogen contamination of municipal water systems that operate according to governmental norms. This means that current methodologies are inadequate to monitor water quality or to detect failures of treatment systems. Current technologies lack the precision and specificity to measure low levels of pathogens and many micro-organisms, particularly viruses and parasites, can escape detection.

However, there are recent reports on the possible long-term effects of waterborne viral infections. Enteric viruses, such as Coxsackie B, appear to be associated with heart diseases, in particular myocarditis, an illness which affects the muscular wall. This could be extremely significant, given that most deaths in OECD countries are cardiovascular-related. The Environmental Protection Agency in the United States has recently published a list of likely new contaminants of drinking water. The list includes among other pathogens, Helicobacter pylori, a bacteria that has recently been associated with chronic gastric diseases. The situation in the United States is mirrored by other OECD countries. Disease outbreaks have recently been reported in Australia, Japan and Western Europe.

Today, the assessment of microbial quality of drinking water is based exclusively on culture techniques. Since these methods do not allow for the detection of specific water pathogens, 'indicator' bacteria showing the possible presence of pathogens are monitored. Most pathogens in drinking water are generally fecal in origin. That means they can be found in human and animal wastes. Thus the coliform bacteria, which are always present in the digestive systems of humans and animals, are commonly used as indicators. They are simply an indication that the water supply is contaminated and that disease-causing bacteria may be present. The method offers a good margin of safety against most bacterial pathogens, but is not effective against some other bacteria, viruses and protozoan parasites. For example, Escherichia coli O157 may be present even when fecal coliform measurements show negative. Furthermore, viruses and most protozoan parasites, such as Giardia and Cryptosporidium, are resistant to chlorination and filtration, which usually kill coliform bacteria. As a result, coliform bacteria cannot be accurate indicators in such cases, particularly in chlorinated waters. Furthermore, the indicator method usually requires cultivation on nutrient media which makes it impossible to obtain a reliable result within less than one day. By the time the results are available, pathogens might have spread wide in the water distribution system. There is clearly a need to find new approaches to monitor the microbiological quality of water.

Rapid advances in biotechnological research in the last few years have resulted in a wide range of new methods, principally based on the detection of nucleic acid material and its amplification, that is becoming available. They offer a more sensitive and specific way of detecting micro-organisms. They can also identify organisms that would not be detected with current culture techniques and can be used to track new pathogenic entities, including variants of otherwise harmless micro-organisms. There are countless reports on the application of these methods in bacteriology. For example, 16S-ribosomal RNA probes and antibodies tagged with fluorescent labels allow the direct microscopic detection of target organisms within hours. Similarly, designed gene probes help in the detection of specific nucleic acid sequences which signal the presence of a particular organism in the sample. Amplification methods, such as polymerase chain reaction (PCR), can then be used to increase sensitivity.

There are probably very few groups of micro-organisms that have not been located with these amplification techniques and indeed several test kits have already been commercialized. This offers some hope for isolating some pathogens. But there are some important technical bottlenecks to overcome before they can be used to assess water quality and the micro-biological safety of drinking water. At an OECD workshop several steps for improvement were agreed upon. Research would have to target micro-organisms at levels useful for risk management or investigation, micro-organisms would have to be recovered from large amounts of water and tests run within minutes or hours, or even in real time, researchers would have to discriminate between viable and non-viable micro-organisms and identify specific pathogens of public health concern, the running cost of monitoring would have to be made as affordable as possible.

To be useful, any methodologies for assaying microbial water quality must fulfil the needs of public health and environmental regulators. They also have to be cost effective enough not to impose an unacceptable burden on water suppliers and consumers. While it is not a viable option to have populations exposed to infected drinking water, it would not be realistic to raise operational standards of drinking water stations without considering the full economic and environmental costs. Today, in most OECD countries the adoption of safety standards, such as in pesticides control, has led to major capital investments. For example, England and Wales have invested over a billion pounds to comply with safety standards. But few reliable figures exist on the relative costs of conventional and new technologies, though some speculation may be made based on the history of biotechnology innovation.

Three issues of major economic significance have to be considered. First, the costs of new technologies are almost always highest at the beginning, and come down over time. In the case of new diagnostics, a cost factor is also the fact that first generation technologies can seldom replace standard technologies and are primarily utilized to complement them. The open question then is how long will it take for the costs to come down, and what will the time-lag be between first and better second generation technologies? In the case of outbreak investigation, where rapid and sensitive diagnosis is a decisive factor, there is every reason to assume that molecular methods will ultimately be more cost-effective.

Second, cost considerations must be the long-term economic price of not using a new technology. This cost will no doubt be different for rich countries than for poor ones and each country will have to make its own assessment on this. For grave diseases, such as cholera, the opportunity cost will be much more evident than for less severe ones. And even where the costs of non-action might seem low, pointing against the outlay, extending the cost-benefit analysis over a longer period of time could dramatically change the outlook. The example that water-borne infections can lead to heart and gastric diseases suggests the long-term economic costs of not using the best detection methods, whatever their price, could be very high indeed.

The third and final issue relates to the therapeutic gap between detection and prevention. That gap varies according to the disease and there are major economic implications when the gap is wide. In many cases, detection of diseases is moving ahead faster than our ability to prevent or cure them. However, perhaps more can be done, and faster, to prevent and cure water-borne infection than other diseases.

The demand for new molecular technologies raises diverse issues of its own. Different countries have varied problems. Some have plenty of water and others a scarcity, requiring recycling. In many OECD countries, the proportion of the population that can be reasonably connected to a community or municipal water plant is approaching its practical economic limits. Several countries have done well in providing water to small communities, and the application of appropriate technology can bring further progress at reasonable cost. On the other hand, to meet quality objectives for drinking water in densely populated areas, it has become necessary to treat sewage water. Soon cities will probably have to treat urban stormwater and wet-water overflows of sewage as well.

Most developing countries and many small community water systems in OECD countries do not have the necessary sophisticated laboratory infrastructure to comply with safe drinking water requirements. The differences in conditions underscore current variations in acceptable levels of pathogens and the fact that the concept of tolerable risk varies from country to country. Even in the United States there is no uniform view and standards vary from state to state. To date there are international agreements on legal limits for pathogens, as reflected by EU directives and the World Health Organization's international quality and safety standards, but only nationally-agreed measurement protocols. Yet common surveillance tools for water-borne disease are needed to reduce risk. That means standardizing methodologies and validating them, preferably on an international basis. A mechanism for sharing validated methods is required. And last, but by no means least, wider exchange of comparable information is crucial so that research can meet public health needs while taking management and economic realities fully into account.

The number of infections associated with the drinking water in the West Kootenays is high and increasing. About 0.1% of the population will be reported to health officials as having giardiasis and this is only the tip of the iceberg because a large majority of cases remain unreported. The overall infection rate is conservatively estimated to be 1-2% per year and unfortunately the rate is increasing in an alarming fashion, with reported cases tripling over the last ten years.

The chances of getting sick from your water are increased by road building and logging in the watershed. The risks are created by increased human access, increased sediment in the water and alterations in natural filtration systems. Increased access increases the risk of a disease being introduced into a watershed, loss of natural filtration increases the chance that a disease causing organism will end up in the water and increased sedimentation makes subsequent treatment less effective and more risky.

One way out of the indicator, organism, molecule sampling problem for drinking water quality control with its inherent costs, errors (type1 and type2), and risks is to abandon the pathogen enumeration approach and switch to a comprehensive treatment approach which will treat all source water to remove or inactivate all the possible pathogens. Since this ideal water supply now has no pathogens this removes the statistical risk involved in the other ways of estimating whether the quality is or is not adequate. It also removes the problem of sudden abnormal fluxes of pathogens which are statistical anomalies. Such an approach is certainly technically feasible and if long-term health costs and productivity losses are taken into account is also economical. Political will and an initial infrastructure cost are the main impediments.

If the current North American approach of treating all water to potable standards, instead of just the very small percentage that is actually used in drinking, cooking and bathing, were replaced with a two-tiered water system, potable and non-potable, the costs of treatment would drop drastically. There would be a one-time infrastructure cost to provide dual water lines to residences. Dual wastewater lines from residences should also be installed, one for black-water and one for grey-water, since this would greatly reduce the size and operating costs of wastewater treatment plants.

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Importance and Typical Diseases
Parasitism, symbiosis and commensalism are common in organisms. In the competition for a habitat or niche in which to live, many organisms have evolved to live in or on other organisms. Much of evolution has been driven and shaped by such relationships. Cellular organelles like chloroplasts and mitochondria, which define the whole eukaryotic lineage, are likely endosymbionts of one prokaryotic organism within another. All groups of organisms, except viruses, have parasites. Viruses, which are possibly escaped genes, need to get into a cell and take over its protein manufacturing machinery to reproduce themselves. Prokaryotic parasites include plasmids and bacteriophages; protozoans harbour viruses and bacteria; fungi are infected by viruses and plasmids; metazoan animals and plants are host to viruses, bacteria, protozoans, fungi and other metazoans. 'Big fleas have little fleas...'. Higher organisms, including people are not individuals, they are ecosystems.

Parasite infections affect millions of people worldwide afflicting considerable human suffering and economic hardship. Far from declining, many parasite infections are increasing throughout the world. The impact of Human Immunodeficiency Virus (HIV) and AIDS has facilitated an explosion of opportunistic pathogens, as well as the increased prevalence of other recognized parasites, in these immunocompromised patients. Climatic changes induced through global warming have aided the spread of many parasite diseases, whilst starvation and the breakdown in sanitation that accompanies war has seen the re-emergence of others. The appearance of drug resistance has also dramatically influenced the ability to treat and control many parasite diseases. In first world countries parasite infections are relatively uncommon. However, outbreaks of cryptosporidiosis and giardiasis associated with drinking water supplies and cyclosporidiasis associated with imported fresh produce are of major concern even in North America. Generally, in immunocompetent people protozoan parasitism is rare and not a serious problem; the immune system has evolved to deal with these natural organisms. In the very young and very old protozoan parasitism, while still uncommon, is of some concern. In people who are immunocompromised by HIV, AIDS or anti-transplant rejection drugs, infections are epidemic and often fatal or life threatening.

In many of the descriptions of the life cycles of theses aquatic parasites there is a note concerning the geographic range of the parasite. This is the original, and still primary or historical, distribution range of the parasite, its area of evolutionary origin and current area of greatest density of infection. However, as noted elsewhere, such geographical limitations are rapidly becoming meaningless in the modern world. Travel is easy and fast. Visitors and immigrants bring in diseases and parasites that are then introduced into our health system, our sewage system and our environment. New cultural behaviours, diets and practices may encourage the survival and spread of these pathogens in their new aquatic habitats. It is no longer prudent to ignore a pathogen as of no local or regional concern because it is primarily found in the tropics or in another hemisphere; it can get here in less than a day.

Parasitic diseases are concentrated in tropical areas for two reasons, one biological and the second sociological. Sociologically, most tropical parts of the earth are poorer economically with the attendant poorer health care, water supplies and sewage disposal infrastructures and crowded slum conditions. This leads to ideal breeding grounds for pathogens. Biologically, many parasites have alternate hosts, often insects and other arthropods, whose ecological distribution is delimited by climate and centered in the tropics. The juxtaposition of these two factors leads to high parasitism rates in tropical peoples. The organisms dealt with in these reports, although they comprise a fairly long list, are only a few of the very many parasites that can infect man. Those dealt with in this series of reports have one factor in common, they are all transmitted, at least in part, in water.

While many life cycle descriptions do not specifically mention water, all the organisms dealt with can be distributed and acquired through water, even if water is not the optimal mechanism of spread. If the pathogen is waterborne our water supply is at risk, even though the risk may be small, and we should be aware of the potential problems and take the necessary steps to prevent disease outbreaks by treating sewage, irrigation and drinking water as required. Pathogens which are spread only through eating infected meat are not dealt with unless there is also an infective stage found in water. Insect vectors, particularly biting mosquitoes and flies, are the main problem with recreational waters. Many insects, and thus the diseases they transfer, are restricted to the close proximity of aquatic habitats used for recreation such as swimming, fishing and boating. Generally, pathogens spread only via insect vectors, and not directly through contact with the water, are not discussed. There are many pathogenic fungi but they tend to be spread by airborne spores in dust, in foods and by contact with soil or other people. While spores can be found in some aerosols, drinking, recreation or irrigation water is not a significant mechanism of fungal spread.

At the BCWWA Seminar in Surrey, British Columbia on June 23, 2000, Mr. Barry Boettger of the British Columbia Ministry of Health gave a presentation that covered details on the Walkerton, Ontario situation and background on British Columbia outbreaks. He indicated that outbreaks such as Walkerton are to be expected from time to time. About 10% of the population are susceptible to Escherichia coli O157: H7 infection and if infected, about 10% are expected to die. Prior to the Walkerton outbreak, the incidence of Escherichia coli O157: H7 was considered very high in the area and was correlated with cattle density. Cattle are common carriers but are not adversely affected. In Walkerton 7 deaths were attributed to the Escherichia coli outbreak and another 11 deaths are suspect. At least 70 people were hospitalized and at least an additional 20 patients were treated for kidney complications. Both the coroner's office and the Ontario Provincial Police conducted investigations. Class action suites against the province of Ontario were launched. Although probably not to the same degree as in Walkerton, there are illnesses and probably deaths resulting from consumption of contaminated water spread by water works systems in British Columbia on an ongoing basis. There have been at least 27 water-borne disease outbreaks recognized in British Columbia since 1970. Unless an aggressive proactive approach to waterborne pathogens is taken in British Columbia another 'Walkerton-like' event is to be expected sooner or later.

At this same conference Mr. Bob Smith also of the Ministry of Health gave an overview of drinking water systems in British Columbia which is served by 18 health regions. About 6% of the 3500 water systems in British Columbia were under boil water advisories. Some of these advisories had been in place for 20 years! There are no accurate data on the state of existing water systems and no accurate estimate of the number of systems using filtration. Since 1992, 23 waterworks improvements involving filtration have been instituted. Since 1980 some of the largest outbreaks were:

In May 2001 a Cryptosporidium outbreak occurred in the North Battleford, Saskatchewan water system due to contamination and a great many people became sick. Adequate filtration and UV sterilization would prevent these outbreaks; relying solely on source water contamination prevention and isolation procedures is not adequate.

Primarily, only diseases in which the infective stage can be transmitted directly through the water are dealt with in these documents; there may be other means of transmission as well. Diseases caused only by insect transmission, even though usually closely associated with water, are rarely discussed. Some of the insect vector diseases, primarily filarial worms, are given in the table below.

Insect Vectored Parasitic Diseases
Insect Vectored Parasitic Diseases
Insect Vectors Diseases Pathogenic Organisms
Flies leishmaniasis Leishmania donovani
Leishmania tropica
Leishmania mexicana
Flies loaisis, loiasis, loaiasis Loa loa
Flies filariasis Mansonella ozzardi
Flies onchocerciasis, river blindness Onchocerca volvulus
Flies sleeping sickness, Chagas' disease, trypanosomiasis Trypanosoma cruzi
Trypanosoma brucei
Midges filariasis Mansonella streptocerca
Mansonella perstans
Mansonella ozzardi
Mosquitos malaria Plasmodium falciparum
Plasmodium vivax
Plasmodium ovale
Plasmodium malariae
Mosquitoes elephantiasis, filariasis Wuchereria bancrofti
Brugia malayi
Brugia timori
Ticks babesiosis Babesia microti
Babesia divergens
List of Pathogens and Parasites

This list is by no means complete

Toxic Cyanophytes or Blue-Green Algae
Anabaena circinalis
Anabaena flos-aquae
Aphanizomenon flos-aquae
Aphanizomenon ovalisporum
Cylindrospermopsis raciborskii
Gloeotrichia echinata
Microcystis aeruginosa
Nodularia spumigena
the common cold
influenza, the flu
poliomyelitis, polio
yellow fever
dengue fever
Enterovirulent Escherichia coli Group (EEC Group)
Escherichia coli - enterotoxigenic (ETEC)
Escherichia coli - enteropathogenic (EPEC)
Escherichia coli -O157: H7 enterohemorrhagic (EHEC)
Escherichia coli -enteroinvasive (EIEC)
Aeromonas hydrophila
Bacillus cereus
Campylobacter jejuni
Clostridium botulinum
Clostridium perfringens
Helicobacter pylori
Listeria monocytogenes
Mycobacterium avium
Plesiomonas shigelloides
Staphylococcus aureus
Vibrio cholerae
Vibrio parahaemolyticus
Vibrio vulnificus
Yersinia enterocolitica
Yersinia pseudotuberculosis
Parasitic Protozoans
Brachiola connori*
Brachiola vesicularum*
Cryptosporidium parvum
Cyclospora cayetanensis
Encephalitozoon cuniculi*
Encephalitozoon hellem*
Encephalitozoon intestinalis*
Entamoeba histolytica
Enterocytozoon bieneusi*
Giardia lamblia
Isospora belli
Microsporidium africanum*
Microsporidium ceylonensis*
Naegleria fowleri
Nosema algerae*
Nosema ocularum*
Trachipleistophora anthropophthera*
Trachipleistophora hominis*
Vittaforma corneae*
* microsporidia
Helminth Worms
Ancylostoma braziliense
Ancylostoma ceylanicum
Ancylostoma duodenale
Ascaris lumbricoides
Austrobilharzia variglandis*
Capillaria aerophila
Capillaria hepatica
Capillaria philippinensis
Clonorchis sinensis
Diphyllobothrium latum
Diphyllobothrium pacificum
Diphyllobothrium cordatum
Diphyllobothrium ursi
Diphyllobothrium dendriticum
Diphyllobothrium lanceolatum
Diphyllobothrium dalliae
Diphyllobothrium yonagoensis
Dipylidium caninum
Dracunculus medinensis
Echinococcus granulosus
Echinococcus multilocularis
Echinococcus vogeli
Echinococcus oligarthrus
Enterobius vermicularis
Fasciola gigantica
Fasciola hepatica
Fasciolopsis buski
Heterobilharzia americanum*
Heterophyes heterophyes
Hymenolepis diminuta
Hymenolepis nana
Metagonimus yokogawai
Necator americanus
Opisthorchis viverrini
Opisthorchis felineus
Paragonimus westermani
Schistosoma haematobium
Schistosoma intercalatum
Schistosoma japonicum
Schistosoma mansoni
Schistosoma mekongi
Schistosoma spindale*
Schistosomatium douthitti*
Taenia saginata
Taenia solium
Toxocara canis
Toxocara cati
Trichobilharzia ocellata*
Trichobilharzia physella*
Trichobilharzia stagnicolae*
Trichuris trichiura
Uncinaria stenocephala
* swimmer's itch
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Parasitic infections are a major cause of illness in North America. While such parasitic diseases as malaria, amoebiasis and schistosomiasis particularly affect people living in tropical climates, the importance of other parasites in North America is generally not appreciated. Not only do parasitic diseases affect thousands, if not millions, in North America; they also disproportionally affect people with compromised immune systems, such as people with AIDS. Swimmer's itch will be common along all the major migratory bird flyways in the world since waterfowl are one of the primary hosts, the other is snails that live in weed beds that the birds feed on.

Protozoan parasites affect people the world over: Cryptosporidium parvum, Giardia lamblia, Cyclospora, Trichomonas vaginalis, Microsporidium and Toxoplasma gondii are important public health hazards. These parasites are organisms that live inside or on the body of an animal host, receiving nourishment and reproducing while causing diarrhea, gastrointestinal upset, vaginal irritation, swelling of lymph glands, fever, adverse outcomes of pregnancy or damage to the brain or nervous system. These single-cell organisms or protozoa differ from bacteria and viruses and can undergo complex morphological changes while in their host and often produce considerable physical damage to the host.

Up to 40 percent of US adults are infected with Toxoplasma gondii and up to three million women have acquired sexually transmitted Trichomonas vaginalis. Many parasitic diseases such as giardiasis and cryptosporidiosis are not always reported to health authorities, so the extent and impact of parasitic diseases in North America is underestimated. Accurately defining North American populations affected by these diseases would greatly help in the development of strategies to prevent, control and treat parasitic infections not only in North America, but in other countries as well.

The Center for Disease Control and Prevention, National Centers for Infectious Diseases, keeps track of the parasites that lead to diarrhea and other diseases in the United States. The most recent parasitic infection to attract public attention was the Milwaukee, Wisconsin epidemic of diarrhea and related gastrointestinal disorders that involved more than 400,000 people, stemming from water contaminated with the Cryptosporidium parvum parasite. More frequent, but less in the public eye, are Giardia lamblia infections. This parasite is frequently associated with poor hygiene practices or contaminated water. The CDC estimates between 100,000 and 1,000,000 US cases of Giardia occur annually. Both Giardia and Cryptosporidium infections are common problems in daycare centers. Hikers and campers drinking from contaminated streams also frequently become infected with Giardia.

Not all parasitic infections routinely cause debilitating illnesses. For example, healthy people who become infected with Toxoplasma gondii usually do not develop toxoplasmosis, which damages the brain and nervous system and sometimes kills. The disease frequently develops in those with weakened immune systems and affects 3 to 10 percent of AIDS patients. The disease also can occur among newborns if their mothers became infected during pregnancy. Toxoplasma gondii naturally infects cats and can spread to humans who handle garden soil or litter boxes where cats have defecated, drink contaminated water or eat raw or undercooked meat.

People with AIDS or other immunocompromising conditions are particularly susceptible to Cryptosporidium infections. Cryptosporidiosis affects 3 to 15 percent of AIDS patients, who develop incurable diarrhea that can lead to death. In addition, the parasite Microsporidium causes severe diarrhea in 20 to 25 percent of people with AIDS suffering from chronic diarrhea. Recently, scientists have identified another parasite, Cyclospora that can bring on diarrhea in AIDS patients and healthy individuals.

Emergence of New Diseases
Emerging infectious diseases can be defined as infections that have newly appeared in a population or have existed but are rapidly increasing in incidence or geographic range. Among recent examples are HIV/AIDS, hantavirus pulmonary syndrome, Lyme disease and hemolytic uremic syndrome, a foodborne infection caused by certain strains of Escherichia coli, such as O157: H7. Specific factors precipitating disease emergence can be identified in virtually all cases. These include ecological, environmental or demographic factors that place people at increased contact with a previously unfamiliar microbe or its natural host or promote dissemination; the ease, speed and prevalence of air travel is a prime factor. These factors are increasing in prevalence; this increase, together with the ongoing rapid evolution of viral and microbial variants and selection for drug resistance, suggests that infections will continue to emerge and probably increase and emphasizes the urgent need for effective surveillance and control.

Infectious diseases emerging throughout history have included some of the most feared plagues of the past; the Black Death decimated Europe; small pox and cholera decimated the indigenous peoples of North America. New infections continue to emerge today, while many of the old plagues are still with us. These are global problems as demonstrated by annual influenza epidemics; under suitable circumstances, a new infection first appearing anywhere in the world could traverse entire continents within days or weeks.

Although these occurrences may appear inexplicable, rarely if ever do emerging infections appear without reason. Specific factors responsible for disease emergence can be identified in virtually all cases studied. The known causes for a number of infections that have emerged recently are given in the table below. Infectious disease emergence can be viewed as a two-step process. Introduction of the agent into a new host population, whether the pathogen originated in the environment, possibly in another species, or as a variant of an existing human infection, followed by establishment and further dissemination within the new host population. Whatever its origin, the infection emerges when it reaches a new population. Factors that promote one or both of these steps will tend to precipitate disease emergence. Most emerging infections, and even antibiotic-resistant strains of common bacterial pathogens, usually originate in one geographic location and then disseminate to new places.

The numerous examples of infections originating as zoonosis suggest that the zoonotic pool introductions of infections from other species is an important and potentially rich source of emerging diseases; periodic discoveries of new zoonoses suggest that the zoonotic pool is still extensive and more diseases will be forthcoming. Once introduced, an infection might then be disseminated through other factors, although a rapid development and high mortality, combined with low transmissibility, are often limiting. However, even if a zoonotic agent is not able to spread readily from person to person and establish itself, other factors such as nosocomial infection, might transmit the infection. Additionally, if the reservoir host or vector becomes more widely disseminated, the microbe can appear in new places. Bubonic plague transmitted by rodent fleas and ratborne hantavirus infections are examples of diseases travelling around the world with their host vector, the rat.

The dramatic increase in drug-resistant microbes, combined with the lag in development of new antibiotics, the rise of megacities with severe health care deficiencies, environmental degradation and the growing ease and frequency of cross-border movements of people and produce have greatly facilitated the spread of infectious diseases. New and re-emerging infectious diseases will pose a rising global health threat. Infectious diseases are a leading cause of death, accounting for a quarter to a third of the estimated 54 million deaths world-wide in 1998. The spread of infectious diseases results as much from changes in human behaviour-including lifestyles and land use patterns, increased trade and travel, and inappropriate use of antibiotic drugs-as from mutations in pathogens.

Twenty well-known diseases-including tuberculosis (TB), malaria, and cholera-have re-emerged or spread geographically since 1973, often in more virulent and drug-resistant forms. At least 30 previously unknown disease agents have been identified since 1973, including HIV, Ebola, hepatitis C, and Nipah virus, for which no cures are available. Of the seven biggest killers world-wide, TB, malaria, hepatitis, and, in particular, HIV/AIDS continue to surge, with HIV/AIDS and TB likely to account for the overwhelming majority of deaths from infectious diseases in developing countries by 2020. Acute lower respiratory infections-including pneumonia and influenza-as well as diarrheal diseases and measles, appear to have peaked at high incidence levels.

Although the infectious disease threat in North America remains relatively modest as compared to that of non-infectious diseases, the trend is up. Annual infectious disease-related death rates in the United States have nearly doubled to some 170,000 annually after reaching an historic low in 1980. Many infectious diseases-most recently, the West Nile virus-originate North America and are introduced by international travellers, immigrants, returning military personnel, or imported animals and foodstuffs. The next major infectious disease threat may be, like HIV, a previously unrecognised pathogen. Barring that, the most dangerous known infectious diseases likely to pose a threat over the next two decades will be HIV/AIDS, hepatitis C, TB, and new, more lethal variants of influenza. Hospital-acquired infections and food or water-borne illnesses also will pose a threat.

Although multi-drug therapies have cut North American HIV/AIDS deaths by two-thirds to 17,000 annually since 1995, emerging microbial resistance to such drugs and continued new infections will sustain the threat. Some 4 million North Americans are chronic carriers of the hepatitis C virus, a significant cause of liver cancer and cirrhosis. The US death toll from the virus may surpass that of HIV/AIDS in the next five years. TB, exacerbated by multi-drug resistant strains and HIV/AIDS co-infection, has made a comeback. Although a massive and costly control effort is achieving considerable success, the threat will be sustained by the spread of HIV and the growing number of new, particularly illegal, immigrants infected with TB. Influenza now kills some 30,000 North Americans annually, and epidemiologists generally agree that it is not a question of whether, but when, the next killer pandemic will occur. Highly virulent and increasingly antimicrobial resistant pathogens, such as Staphylococcus aureus, are major sources of hospital-acquired infections that kill some 14,000 patients annually. The doubling of North American food imports over the last five years is one of the factors contributing to tens of millions of food-borne illnesses and 9,000 deaths that occur annually, and the trend is up.

Developing and former communist countries will continue to experience the greatest impact from infectious diseases-because of malnutrition, poor sanitation, poor water quality, and inadequate health care-but developed countries also will be affected. Sub-Saharan Africa-accounting for nearly half of infectious disease deaths globally-will remain the most vulnerable region. The death rates for many diseases, including HIV/AIDS and malaria, exceed those in all other regions. Sub-Saharan Africa's health care capacity-the poorest in the world-will continue to lag. Asia and the Pacific, where multi-drug resistant TB, malaria, and cholera are rampant, is likely to witness a dramatic increase in infectious disease deaths, largely driven by the spread of HIV/AIDS in South and Southeast Asia and its likely spread to East Asia. By 2010, the region could surpass Africa in the number of HIV infections. The former Soviet Union and, to a lesser extent, Eastern Europe also are likely to see a substantial increase in infectious disease incidence and deaths. In the former Soviet Union especially, the steep deterioration in health care and other services owing to economic decline has led to a sharp rise in diphtheria, dysentery, cholera, and hepatitis B and C. TB has reached epidemic proportions throughout the former Soviet Union, while the HIV-infected population in Russia alone could exceed 1 million by the end of 2000 and double yet again by 2002.

Latin American countries generally are making progress in infectious disease control, including the eradication of polio, but uneven economic development has contributed to widespread resurgence of cholera, malaria, TB, and dengue. These diseases will continue to take a heavy toll in tropical and poorer countries. The Middle East and North Africa region has substantial TB and hepatitis B and C prevalence, but conservative social mores, climatic factors, and the high level of health spending in the oil-producing states tend to limit some globally prevalent diseases, such as HIV/AIDS and malaria. The region has the lowest HIV infection rate among all regions, although this is probably due in part to above-average underreporting because of the stigma associated with the disease in Muslim societies. Western Europe faces threats from several infectious diseases, such as HIV/AIDS, TB, and hepatitis B and C, as well as from several economically costly zoonotic diseases (that is, those transmitted from animals to humans). The region's large volume of travel, trade, and immigration increases the risks of importing diseases from other regions, but its highly developed health care system will limit their impact.

The relationship between disease and political instability is indirect but real. A wide-ranging study on the causes of state instability suggests that infant mortality-a good indicator of the overall quality of life-correlates strongly with political instability, particularly in countries that already have achieved a measure of democracy. The severe social and economic impact of infectious diseases is likely to intensify the struggle for political power to control scarce state resources.

Infectious diseases remain a leading cause of death. Of the estimated 54 million deaths world-wide in 1998, about one-fourth to one-third were due to infectious diseases, most of them in developing countries and among children globally. Infectious diseases accounted for 41 percent of the global disease burden, as compared to 43 percent for non-infectious diseases and 16 percent for injuries. Although there has been continuing progress in controlling some vaccine-preventable childhood diseases such as polio, neonatal tetanus and measles, at least 29 previously unknown diseases that have appeared globally since 1973, many of them incurable, including HIV/AIDS, Ebola hemorrhagic fever, and hepatitis C. Most recently, Nipah encephalitis was identified. Twenty well-known diseases such as malaria, TB, cholera, and dengue have rebounded after a period of decline or spread to new regions, often in deadlier forms.

The seven infectious diseases that caused the highest number of deaths in 1998, according to WHO, will remain threats well into the next century. HIV/AIDS, TB, malaria, and hepatitis B and C, are either spreading or becoming more drug-resistant, while lower respiratory infections, diarrheal diseases and measles, appear to have at least temporarily peaked.

HIV/AIDS. Following its identification in 1983, the spread of HIV intensified quickly. Despite progress in some regions, HIV/AIDS shows no signs of abating globally. Approximately 2.3 million people died from AIDS world-wide in 1998, up dramatically from 0.7 million in 1993, and there were 5.8 million new infections. According to WHO, some 33.4 million people were living with HIV by 1998, up from 10 million in 1990, and the number could approach 40 million by the end of 2000. Although infection and death rates have slowed considerably in developed countries owing to the growing use of preventive measures and costly new multi-drug treatment therapies, the pandemic continues to spread in much of the developing world, where 95 percent of global infections and deaths have occurred. Sub-Saharan Africa currently has the biggest regional burden, but the disease is spreading quickly in India, Russia, China, and much of the rest of Asia. HIV/AIDS probably will cause more deaths than any other single infectious disease world-wide by 2020 and may account for up to one-half or more of infectious disease deaths in the developing world alone.

TB. WHO declared TB a global emergency in 1993 and the threat continues to grow, especially from multi-drug resistant TB. The disease is especially prevalent in Russia, India, Southeast Asia, Sub-Saharan Africa, and parts of Latin America. More than 1.5 million people died of TB in 1998, excluding those infected with HIV/AIDS, and there were up to 7.4 million new cases. Although the vast majority of TB infections and deaths occur in developing regions, the disease also is encroaching into developed regions due to increased immigration and travel and less emphasis on prevention. Drug resistance is a growing problem; the WHO has reported that up to 50 percent of people with multi-drug resistant TB may die of their infection despite treatment, which can be 10 to 50 times more expensive than that used for drug-sensitive TB. HIV/AIDS also has contributed to the resurgence of TB. One-quarter of the increase in TB incidence involves co-infection with HIV. TB probably will rank second only to HIV/AIDS as a cause of infectious disease deaths by 2020.

Malaria. Mainly a tropical disease that seemed to be coming under control in the 1960s and 1970s, is making a deadly comeback-especially in Sub-Saharan Africa where infection rates increased by 40 percent from 1970 to 1997. Drug resistance, historically a problem only with the most severe form of the disease, is now increasingly reported in the milder variety, while the prospects for an effective vaccine are poor. In 1998, an estimated 300 million people were infected with malaria, and more than 1.1 million died from the disease that year. Most of the deaths occurred in Sub-Saharan Africa. Sub-Saharan Africa alone is likely to experience a 7- to 20-percent annual increase in malaria-related deaths and severe illnesses over the next several years.

Hepatitis B and C. Hepatitis B, which caused at least 0.6 million deaths in 1997, is highly endemic in the developing world, and some 350 million people world-wide are chronic carriers. The less prevalent but far more lethal hepatitis C identified in 1989 has grown dramatically and is a significant contributor to cirrhosis and liver cancer. WHO estimated that 3 percent of the global population was infected with the hepatitis C virus by 1997, which means that more than 170 million people were at risk of developing the diseases associated with this virus. Various studies project that up to 25 percent of people with chronic hepatitis B and C will die of cirrhosis of the liver and liver cancer over the next 20 to 30 years.

Lower respiratory infections, especially influenza and pneumonia, killed 3.5 million people in 1998, most of them children in developing countries, down from 4.1 million in 1993. Owing to immuno-suppression from malnutrition and growing microbial resistance to commonly used drugs such as penicillin, these children are especially vulnerable to such diseases and will continue to experience high death rates.

Diarrheal diseases-mainly spread by contaminated water or food-accounted for 2.2 million deaths in 1998, as compared to 3 million in 1993, of which about 60 percent occurred among children under five years of age in developing countries. The most common cause of death related to diarrheal diseases is infection with Escherichia coli. Other diarrheal diseases include cholera, dysentery, and rotaviral diarrhea, prevalent throughout the developing world and, more recently, in many former communist states. Such waterborne and food-borne diseases will remain highly prevalent in these regions in the absence of improvements in water quality and sanitation.

Measles Despite substantial progress against measles in recent years, the disease still infects some 42 million children annually and killed about 0.9 million in 1998, down from 1.2 million in 1993. It is a leading cause of death among refugees and internally displaced persons during complex humanitarian emergencies. Measles will continue to pose a major threat in developing countries particularly Sub-Saharan Africa, until the still relatively low vaccination rates are substantially increased. It also will continue to cause periodic epidemics in areas such as South America with higher, but still inadequate, vaccination rates.

With few exceptions, the resurgence of the infectious disease threat is due as much to dramatic changes in human behaviour and broader social, economic, and technological developments as to mutations in pathogens. Changes in human behaviour include population dislocations, living styles, and sexual practices; technology-driven medical procedures entailing some risks of infection; and land use patterns. They also include rising international travel and commerce that hasten the spread of infectious diseases; inappropriate use of antibiotics that leads to the development of microbial resistance; and the breakdown of public health systems in some countries owing to war or economic decline. In addition, climate changes enable diseases and vectors to expand their range. Several of these factors interact, exacerbating the spread of infectious diseases.

Population growth and urbanisation, particularly in the developing world, will continue to facilitate the transfer of pathogens among people and regions. Frequent and often sudden population movements within and across borders caused by ethnic conflict, civil war, and famine will continue to spread diseases rapidly in affected areas, particularly among refugees. As of 1999, there were some 24 major humanitarian emergencies world-wide involving at least 35 million refugees and internally displaced people. Refugee camps, found mainly in Sub-Saharan Africa and the Middle East, facilitate the spread of TB, HIV, cholera, dysentery, and malaria. Well over 120 million people lived outside the country of their birth in 1998, and millions more will emigrate annually, increasing the spread of diseases globally. Behavioural patterns, such as unprotected sex with multiple partners and intravenous drug use, will remain key factors in the spread of HIV/AIDS.

Although technological breakthroughs will greatly facilitate the detection, diagnosis, and control of certain infectious and non-infectious illnesses, they also will introduce new dangers, especially in the developed world where they are used extensively. Invasive medical procedures will result in a variety of hospital-acquired infections, such as Staphylococcus aureus. The globalisation of the food supply means that non-hygienic food production, preparation, and handling practices in originating countries can introduce pathogens endangering foreign as well as local populations. Disease outbreaks due to Cyclospora, Escherichia coli and Salmonella in several countries, along with the emergence, primarily in Britain, of Bovine Spongiform Encephalopathy, or "mad cow" disease, and the related new variant Creutzfeldt-Jakob disease affecting humans, result from such food practices.

Changes in land and water use patterns will remain major factors in the spread of infectious diseases. The emergence of Lyme disease in the United States and Europe has been linked to reforestation and increases in the deer tick population, which acts as a vector, while conversion of grasslands to farming in Asia encourages the growth of rodent populations carrying hemorrhagic fever and other viral diseases. Human encroachment on tropical forests will bring populations into closer proximity with insects and animals carrying diseases such as leishmaniasis, malaria, and yellow fever, as well as heretofore unknown and potentially dangerous diseases, as was the case with HIV/AIDS. Close contact between humans and animals in the context of farming will increase the incidence of zoonotic diseases-those transmitted from animals to humans. Water management efforts, such as dam-building, will encourage the spread of water-breeding vectors such as mosquitoes and snails that have contributed to outbreaks of Rift Valley fever and schistosomiasis in Africa.

The increase in international air travel, trade, and tourism will dramatically increase the prospects that infectious disease pathogens such as influenza-and vectors such as mosquitoes and rodents-will spread quickly around the globe, often in less time than the incubation period of most diseases. Earlier in the decade, for example, a multi-drug resistant strain of Streptococcus pneumoniae originating in Spain spread throughout the world in a matter of weeks, according to the director of WHO's infectious disease division. The cross-border movement of some 2 million people each day, including 1 million between developed and developing countries each week, and surging global trade ensure that travel and commerce will remain key factors in the spread of infectious diseases.

Infectious disease microbes are constantly evolving, often into new strains that are increasingly resistant to available antibiotics. As a result, an expanding number of strains of diseases-such as TB, malaria, and pneumonia-will remain difficult or virtually impossible to treat. At the same time, large-scale use of antibiotics in both humans and livestock will continue to encourage development of microbial resistance. The first-line drug treatment for malaria is no longer effective in over 80 of the 92 countries where the disease is a major health problem. Penicillin has substantially lost its effectiveness against several diseases, such as pneumonia, meningitis, and gonorrhea, in many countries. Eighty percent of Staphylococcus aureus isolates in the United States, for example, are penicillin-resistant and 32 percent are methicillin-resistant. A CDC study found a 60-fold increase in high-level resistance to penicillin among one group of Streptococcus pneumoniae cases in the United States and significant resistance to multi-drug therapy as well. Influenza viruses, in particular, are particularly efficient in their ability to survive and genetically change, sometimes into deadly strains. HIV also displays a high rate of genetic mutation that will present significant problems in the development of an effective vaccine or new, affordable therapies.

Alone or in combination, war and natural disasters, economic collapse, and human complacency are causing a breakdown in health care delivery and facilitating the emergence or re-emergence of infectious diseases. While Sub-Saharan Africa is the area currently most affected by these factors, economic problems in Russia and other former communist states are creating the context for a large increase in infectious diseases. The deterioration of basic health care services largely accounts for the re-emergence of diphtheria and other vaccine-preventable diseases, as well as TB, as funds for vaccination, sanitation, and water purification have dried up. In developed countries, past inroads against infectious diseases led to a relaxation of preventive measures such as surveillance and vaccination. Inadequate infection control practices in hospitals will remain a major source of disease transmission in developing and developed countries alike.

Climatic shifts are likely to enable some diseases and associated vectors-particularly mosquito-borne diseases such as malaria, yellow fever, and dengue-to spread to new areas. Warmer temperatures and increased rainfall already have expanded the geographic range of malaria to some highland areas in Sub-Saharan Africa and Latin America and could add several million more cases in developing country regions over the next two decades. The occurrence of waterborne diseases associated with temperature-sensitive environments, such as cholera, also is likely to increase.

Recent examples of emerging infections and probable factors in their emergence
Emerging infections and probable factors in their emergence
Infection or Agent Factor(s) contributing to emergence
Argentine, Bolivian hemorrhagic fever Changes in agriculture favoring rodent hosts
Bovine spongiform encephalopathy Changes in rendering processes and recycling of brain tissue as animal feed stock
Dengue, dengue hemorrhagic fever Transportation, travel and migration; urbanization; human demographics and behaviour
Ebola, Marburg Unknown in the wild; in Europe and the United States, importation of monkeys
Hantaviruses Ecological or environmental changes increasing contact with rodent hosts
Hepatitis B, C Transfusions, organ transplants, contaminated hypodermic needles, sexual transmission, spread from infected mother to child
HIV Migration to cities and travel; after introduction, sexual transmission, spread from infected mother to child, contaminated hypodermic needles including intravenous drug use, transfusions, organ transplants
HTLV Contaminated hypodermic needles
Influenza, pandemic Probably pig-duck agriculture facilitating re-assortment of avian and mammalian influenza viruses
Lassa fever Urbanization favoring rodent hosts, increasing exposure, usually in homes
Rift Valley fever Dam building, agriculture, irrigation, economic development and land use, possibly change in virulence or pathogenicity of viruses
Yellow fever Conditions favoring mosquito vector in new areas, economic development and land use, climate change
Brazilian purpuric fever, Haemophilus influenzae, biotype aegyptius Probably a new strain
Cholera In a recent epidemic in South America, probably introduced from Asia by ship, with spread facilitated by reduced water chlorination; a new strain, type O139, from Asia recently disseminated by travel, similarly to past introductions of classic cholera
Helicobacter pylori Probably long widespread, now recognized, associated with gastric ulcers, possibly other gastrointestinal disease
Hemolytic uremic syndrome Mass food processing technology allowing, Escherichia coli O157:H7 contamination of meat
Legionella, Legionnaires' disease Cooling and plumbing systems, organism grows in biofilms that form on water storage tanks and in stagnant plumbing
Lyme borreliosis, Borrelia burgdorferi Reforestation around homes and other conditions favoring tick vector and deer, a secondary reservoir host, economic development and land use
Streptococcus, group, invasive, necrotizing Uncertain
Toxic shock syndrome, Staphylococcus aureus Ultra-absorbency tampons and sanitary napkins
Cryptosporidium, and other waterborne pathogens Contaminated surface water, inadequate water sterilization
Giardiasis Human demographics and behaviour
Malaria In new areas, travel or migration, economic development and land use, climate change
Schistosomiasis Dam building, economic development and land use
Dengue/dengue hemorrhagic fever, sexually transmitted diseases, giardiasis Human demographics and behaviour
Toxic shock syndrome, nosocomial infections, hemorrhagic colitis/hemolytic uremic syndrome Technological and industrial changes
Lyme disease, malaria, plague, rabies, yellow fever, Rift Valley fever, schistosomiasis Economic development and land use
Malaria, cholera, pneumococcal pneumonia International travel and commerce
Influenza, HIV/AIDS, malaria, Staphylococcus aureus infections Microbial adaptation and change
Rabies, tuberculosis, trench fever, diphtheria, whooping cough (pertussis), cholera Breakdown of public health measures
Malaria, dengue, cholera, yellow fever Climate change
Continual re-appearances of influenza are due to two distinct mechanisms: Annual or biennial epidemics involving new variants due to antigenic drift, point mutations, primarily in the gene for the surface protein and hemagglutinin and pandemic strains, arising from antigenic shift, genetic reassortment, generally between avian and mammalian influenza strains.

Most emerging infections appear to be caused by pathogens already present in the environment, brought out of obscurity or given a selective advantage by changing conditions and afforded an opportunity to infect new host populations, on rare occasions, a new variant may also evolve and cause a new disease. The process by which infectious agents may transfer from animals to humans or disseminate from isolated groups into new populations can be called microbial traffic. A number of activities increase microbial traffic and as a result promote emergence and epidemics. In some cases, including many of the most novel infections, the agents are zoonotic, crossing from their natural animal hosts into the human population; because of the many similarities, included here are vector-borne diseases. In other cases, pathogens already present in geographically isolated populations are given an opportunity to disseminate further. Surprisingly often, disease emergence is caused by human actions, however inadvertently; natural causes, such as changes in climate, can also at times be responsible. Although this discussion is confined largely to human disease, similar considerations apply to emerging pathogens in agricultural and other species.

There are some clusters of mostly human activities that lead to emergence of new diseases or spread of existing diseases as epidemics from formerly isolated locations. Ecological changes, including those due to economic development and land use. These include changes in agriculture, dam construction, changes in aquatic systems, deforestation, reforestation, floods, droughts, famine and climate changes. Human demographics and behaviour which includes societal changes, population growth and migration, movement from rural to urban environments, wars, civil strife and infrastructure breakdown, slums, high density living conditions, institutions, intravenous drug use, sexual activities and family/clan/small group breakdown. International travel and commerce leads to the worldwide movement of people and goods by air so rapidly that diseases may be moved around undetected while still in their incubation stages, before there are any overt signs. Technology and industry changes have lead to globalization of food supplies, processing and packaging; tissue and organ transplantation, the use of immunosuppressive drugs and the widespread abuse of antibiotics have also contributed to new disease outbreaks. A breakdown in public health measures are due to complacency, reduced budgets, a reduction in prevention activities, inadequate sanitation, cross connections between water, wastewater and storm water drains and inadequate vector control. Microbial adaptation and evolution in response to changes in the environment, usually caused by man, leads to evolution of new strains.

Ecological interactions can be complex, with several factors often working together or in sequence. For example, population movement from rural areas to cities can spread a once-localized infection. The strain on infrastructure in the overcrowded and rapidly growing cities may disrupt or slow public health measures, perhaps allowing establishment of the newly introduced infection. Finally, the city may also provide a gateway for further dissemination of the infection to other parts of the world. Most successful emerging infections, including HIV, cholera and dengue, have followed this route.

Consider HIV as an example. Although the precise ancestry of HIV-1 is still uncertain, it appears to have had a zoonotic origin. Ecological factors that would have allowed human exposure to a natural host carrying the virus that was the precursor to HIV-1 were, therefore, instrumental in the introduction of the virus into humans. This probably occurred in a rural area. A plausible scenario is suggested by the identification of an HIV-2-infected man in a rural area of Liberia. His virus strain resembled a virus isolated from the sooty mangabey monkey, an animal widely hunted for food in rural areas and the putative source of HIV-2, more closely than it did strains circulating in the city. Such findings suggest that zoonotic introductions of this sort may occur on occasion in isolated populations but may well go unnoticed, and eventually die out naturally, so long as the recipients remain isolated. But with increasing movement from rural areas to cities, such isolation is increasingly rare. After its likely first move from a rural area into a city, HIV-1 spread regionally along highways, then by long distance routes, including air travel, to more distant places. This last step was critical for HIV and facilitated today's global pandemic. Social changes that allowed the virus to reach a larger population and to be transmitted despite its relatively low natural transmissibility were instrumental in the success of the virus in its newfound human host. For HIV, the long duration of infectivity allowed this normally poorly transmissible virus many opportunities to be transmitted and to take advantage of such factors as human behavior, sexual transmission, intravenous drug use, changing technology and early spread through blood transfusions and blood products.

Ecological changes, including those due to agricultural or economic development, are among the most frequently identified factors in emergence. They are especially frequent as factors in outbreaks of previously unrecognized diseases with high case-fatality rates, which often turn out to be zoonotic introductions. Ecological factors usually precipitate emergence by placing people in contact with a previously unknown, but usually already present, often zoonotic or arthropod-borne infection reservoir or host for an infection, either by increasing proximity, or, often by changing conditions so as to favor an increased population of the microbe or its natural host. The emergence of Lyme disease in the United States and Europe was probably due largely to reforestation that increased the population of deer and the deer tick, the vector of Lyme disease. The movement of people into these areas placed a larger population in close proximity to the vector.

Agricultural development, one of the most common ways in which people alter and insert themselves into the environment, is often a factor. Hantaan virus, the cause of Korean hemorrhagic fever, causes over 100,000 cases a year in China and has been known in Asia for centuries. The virus is a natural infection of the field mouse, Apodemus agrarius. The rodent flourishes in rice fields; people usually contract the disease during the rice harvest from contact with infected rodents. Junin virus, the cause of Argentine hemorrhagic fever, is an unrelated virus with a history remarkably similar to that of Hantaan virus. Conversion of grassland to maize cultivation favored a rodent that was the natural host for this virus, and human cases increased in proportion with expansion of maize agriculture. Other examples, in addition to those already known, are likely to appear as new areas are placed under cultivation.

Pandemic influenza appears to have an agricultural origin, integrated pig/duck farming in China. Strains causing the frequent annual or biennial epidemics generally result from mutation and antigenic drift; pandemic influenza viruses do not generally arise by this process. Instead, gene segments from two influenza strains re-assort to produce a new virus that can infect humans. Evidence indicates that waterfowl, such as ducks, are major reservoirs of influenza and that pigs can serve as mixing vessels for new mammalian influenza strains. Pandemic influenza viruses have generally come from China. Integrated pig/duck agriculture, an extremely efficient food production system traditionally practiced in certain parts of China for several centuries, puts these two species in contact and provides a natural laboratory for making new influenza recombinants. With high-intensity agriculture and movement of livestock across borders, suitable conditions may now also be found in Europe.

Water is also frequently associated with disease emergence. Infections transmitted by mosquitoes or other arthropods, which include some of the most serious and widespread diseases, are often stimulated by expansion of standing water, simply because the mosquito vectors breed in water. There are many cases of diseases transmitted by water-breeding vectors, most involving dams, water for irrigation or stored drinking water in cities. The incidence of Japanese encephalitis, another mosquito-borne disease that accounts for almost 30,000 human cases and approximately 7,000 deaths annually in Asia, is closely associated with flooding of fields for rice growing. Outbreaks of Rift Valley fever in some parts of Africa have been associated with dam building as well as with periods of heavy rainfall. In the outbreaks of Rift Valley fever in Mauritania in 1987, the human cases occurred in villages near dams on the Senegal River. The same effect has been documented with other infections that have aquatic hosts, such as schistosomiasis.

Because humans are important agents of ecological and environmental change, many of these factors are anthropogenic. This is not always the case, and natural environmental changes, such as climate or weather anomalies, can have the same effect. The outbreak of Hantavirus pulmonary syndrome in the southwestern United States in 1993 is an example. It is likely that the virus has long been present in mouse populations but an unusually mild and wet winter and spring in that area led to an increased rodent population in the spring and summer and thus to greater opportunities for people to come in contact with infected rodents, and hence, with the virus. It has been suggested that the weather anomaly was due to large-scale climatic effects. The same causes may have been responsible for outbreaks of hantaviral disease in Europe at approximately the same time. With cholera, it has been suggested that certain organisms in marine environments are natural reservoirs for Cholera vibrios, and that large-scale effects on ocean currents may cause local increases in the reservoir organism with consequent flare-ups of cholera.

Human population movements or upheavals, caused by migration or war, are often important factors in disease emergence. In many parts of the world, economic conditions are encouraging the mass movement of workers from rural areas to cities. The United Nations has estimated that, largely as a result of continuing migration, by the year 2025, 65% of the world population, also expected to be larger in absolute numbers, including 61% of the population in developing regions, will live in cities. As discussed above for HIV, rural urbanization allows infections arising in isolated rural areas, which may once have remained obscure and localized, to reach larger populations. Once in a city, the newly introduced infection would have the opportunity to spread locally among the population and could also spread further along highways and interurban transport routes and by airplane. HIV has been, and in Asia is becoming, the best known beneficiary of this social change, but many other diseases, such as dengue, stand to benefit. The frequency of the most severe form, dengue hemorrhagic fever, which is thought to occur when two types of dengue virus sequentially infect a person, is increasing as different dengue viruses have extended their range and now overlap. Dengue hemorrhagic fever is now common in some cities in Asia, where the high prevalence of infection is attributed to the proliferation of open containers needed for water storage as the growing population exceeds the existing drinking water infrastructure capacity. These open containers provide breeding grounds for the mosquito vector. In urban environments, rain-filled tires or plastic bottles are often the breeding grounds of choice for certain mosquito vectors. The resulting mosquito population boom is complemented by the high human population density in such situations, increasing the chances of stable transmission cycles between infected and susceptible persons. Even in industrialized countries such as the United States, infections such as tuberculosis can spread through high-population density settings like day care centers or prisons.

Human behavior can have important effects on disease dissemination. The best known examples are sexually transmitted diseases, and the ways in which such human sexual practices or intravenous drug use have contributed to the emergence of HIV are now well known. Other factors responsible for disease emergence are influenced by a variety of human actions. Human behavior in the broader sense is thus very important and motivating appropriate individual behavior and constructive action, both locally and in a larger scale, will be essential for controlling emerging infections. Ironically, as AIDS prevention efforts have demonstrated, influencing risky human behavior, especially where ignorance, sexual activity or substance abuse are involved, remains one of the weakest links in our efforts to control diseases.

The dissemination of HIV through travel has already been mentioned. In the past, an infection introduced into people in a geographically isolated area might, on occasion, be brought to a new place through travel, commerce or war. Trade between Asia and Europe, perhaps beginning with the silk route and continuing with the Crusades, brought the rat and one of its infections, the bubonic plague, to Europe. Beginning in the 16th and 17th centuries, ships bringing slaves from West Africa to the New World also brought yellow fever and its mosquito vector, Aedes aegypti, to the new territories. Similarly, smallpox escaped its Old World origins to decimate populations in the New World. In the 19th century, cholera had similar opportunities to spread from its probable origin in the Ganges plain to the Middle East and, from there, to Europe and much of the remaining world. Each of these infections had once been localized and took advantage of opportunities to be carried to previously unfamiliar parts of the world.

Similar scenarios are being repeated today, but opportunities in recent years have become far richer, far faster and more numerous, reflecting the increasing volume, scope, and speed of traffic in an increasingly mobile world. Rats have carried hantaviruses virtually worldwide. Aedes albopictus, the Asian tiger mosquito, was introduced into the United States; Brazil and parts of Africa in shipments of used tires from Asia. Since its introduction in 1982, this mosquito has established itself in at least 18 states of the United States and has acquired local viruses including eastern equine encephalomyelitis, a cause of serious disease. Another mosquito-borne disease, malaria, is one of the most frequently imported diseases in non-endemic-disease areas, and cases of airport malaria are occasionally identified.

A classic bacterial disease, cholera, recently entered both South America, for the first time this century, and Africa. Molecular typing shows the South American isolates to be of the current pandemic strain, supporting the suggestion that the organism was introduced in contaminated bilge water from an Asian freighter. Other evidence indicates that cholera was only one of many organisms to travel in ballast water; dozens, perhaps hundreds, of species have been exchanged between distant places through this means of transport alone. New bacterial strains, such as the recently identified Vibrio cholerae O139, or an epidemic strain of Neisseria meningitidis, also examples of microbial adaptation and change, have disseminated rapidly along routes of trade and travel, as have antibiotic-resistant bacteria.

High-volume rapid movement characterizes not only travel, but also other industries in modern society. In operations, including food production, that process or use products of biological origin, modern production methods yield increased efficiency and reduced costs but can increase the chances of incidental contamination and amplify the effects of such contamination. The problem is further compounded by globalization, allowing the opportunity to introduce agents from far away. A pathogen present in some raw material may find its way into large batches of many final products, as happened with the contamination of hamburger meat by E. coli strains causing hemolytic uremic syndrome. In the United States the implicated E. coli strains are serotype O157: H7; additional serotypes have been identified in other countries. Bovine spongiform encephalopathy, BSE, which emerged in Britain within the last few years, was likely an interspecies transfer of scrapie from sheep to cattle that occurred when changes in rendering processes led to incomplete inactivation of scrapie agent in sheep byproducts, brain tissue, fed to cattle.

The concentrating effects that occur with blood and tissue products have inadvertently disseminated infections unrecognized at the time, such as HIV and hepatitis B and C. Medical settings are also at the front line of exposure to new diseases, and a number of infections, including many emerging infections, have spread nosocomially in health care settings. Among the numerous examples, in the outbreaks of Ebola fever in Africa many of the secondary cases were hospital acquired, most transmitted to other patients through contaminated hypodermic needles, and some to the health care staff by contact. Transmission of Lassa fever to health care workers has also been documented.

On the positive side, advances in diagnostic technology can also lead to new recognition of agents that are already widespread. When such agents are newly recognized, they may be labeled, in some cases incorrectly, as emerging infections. Human herpesvirus 6, HHV-6, was identified only a few years ago, but the virus appears to be extremely widespread and has recently been implicated as the cause of roseola, exanthem subitum, a very common childhood disease. Because roseola has been known since at least 1910, HHV-6 is likely to have been common for decades and probably much longer. Another recent example is the bacterium Helicobacter pylori, a probable cause of gastric ulcers and some cancers. We have lived with these diseases for a long time without knowing their cause. Recognition of the agent is often advantageous, offering new promise of controlling a previously intractable disease, such as treating gastric ulcers with specific antimicrobial therapy.

Microbes, like all other living things, are constantly evolving. Due to their short life cycles the change is very rapid in terms of human life cycles. The emergence of antibiotic-resistant bacteria as a result of the ubiquity of antimicrobials in the environment is an evolutionary lesson on microbial adaptation, as well as a demonstration of the power of natural selection. Selection for antibiotic-resistant bacteria and drug-resistant parasites has become frequent, driven by the wide and sometimes inappropriate use of antimicrobial drugs in a variety of inappropriate applications. Pathogens can also acquire new antibiotic resistance genes from other, often nonpathogenic, species in the environment, selected or perhaps even driven by the selection pressure of antibiotics.

Many viruses show a high mutation rate and can rapidly evolve to yield new variants. A classic example is influenza. Regular annual epidemics are caused by antigenic drift in a previously circulating influenza strain. A change in an antigenic site of a surface protein, usually the hemagglutinin protein, allows the new variant to reinfect previously infected persons because the altered antigen is not immediately recognized by the immune system.

On rare occasions, perhaps more often with nonviral pathogens than with viruses, the evolution of a new variant may result in a new expression of disease. The epidemic of Brazilian purpuric fever in 1990, associated with a newly emerged clonal variant of Hemophilus influenzae, biogroup aegyptius, may fall into this category. It is possible, but not yet clear, that some recently described manifestations of disease by group A Streptococcus, such as rapidly invasive infection or narcotizing fasciitis, may also fall into this category.

Classical public health and sanitation measures have long served to minimize dissemination and human exposure to many pathogens spread by traditional routes such as water or preventable by immunization or vector control. The pathogens themselves often still remain, albeit in reduced numbers, in reservoir hosts, in the environment or in small pockets of infection and, therefore, are often able to take advantage of the opportunity to re-emerge if there are breakdowns in preventive measures. Our infrastructure and our responses to diseases are many years out of date and very difficult, much too slow and expensive to change; the organisms responsible for the diseases are not under any such handicap. We rely too much on treatment and not enough on prevention.

Re-emerging diseases are those, like cholera, that were once decreasing but are now rapidly increasing again. These are often conventionally understood and well recognized public health threats for which, in most cases, previously active public health measures had been allowed to lapse, a situation that unfortunately now applies all too often in both developing countries and the inner cities of the industrialized world. The appearance of re-emerging diseases may, therefore, often be a sign of the breakdown of public health measures and should be a warning against complacency in the war against infectious diseases. There is now a whole generation that has never known the ravages of many once common diseases and do not believe that the necessary prevention methods are worth the cost, effort and minimal risk. There are an increasing number of people in society that do not believe in pasteurization of milk, vaccination for childhood diseases or disinfection of water supplies. Not only do they put themselves and their children at risk but they also constitute a reservoir of disease organisms that put the rest of the population at risk too as has often been demonstrated in childcare facilities.

Cholera, for example, has recently been raging in South America and Africa, for the first time in this century. The rapid spread of cholera in South America may have been abetted by recent reductions in chlorine levels used to treat water supplies. The success of cholera and other enteric diseases is often due to the lack of a reliable water supply. These problems are more severe in developing countries, but are not confined to these areas. The US outbreak of waterborne Cryptosporidium infection in Milwaukee, Wisconsin, in the spring of 1993, with over 400,000 estimated cases, was in part due to a non-functioning water filtration plant, similar deficiencies in water purification have been found in other cities in the United States and recently in Canada.

There is a long history of infectious diseases and infections that, from the dawn of history to the present, have traveled with the caravans and followed the invading armies. The history of infectious diseases has been a history of microbes on the march, often in our wake, and of microbes that have taken advantage of the rich opportunities offered them to thrive, prosper, and spread. The historical processes that have given rise to the emergence of new infections throughout history continue today unabated; in fact, they are accelerating because the conditions of modern life ensure that the factors responsible for disease emergence are more prevalent than ever before. Speed of travel and global reach are further borne out by studies modeling the spread of influenza epidemics and HIV.

Humans are not powerless, however, against this relentless march of microbes. Knowledge of the factors underlying disease emergence can help focus resources on the key situations and areas worldwide and develop more effective prevention strategies. If we are to protect ourselves against emerging diseases, the essential first step is effective global disease surveillance to give early warning of emerging infections. This must be tied to incentives, such as national development, and eventually be backed by a system for an appropriate rapid response. Political will is lacking and political response times are far too slow; there is too much dependence on reactive treatment and too little on proactive prevention. World surveillance capabilities are critically deficient. Efforts, such as the CDC plan, now under way in the United States and internationally to remedy this situation are the essential first steps and deserve strong support. Research, both basic and applied, will also be vital. Early warning of emerging and reemerging infections depends on the ability to identify the unusual as early as possible.

Frequency of Infections by Primary Vector and Country

The water borne bacterial, viral, helminth and protozoan diseases are dealt with in their respective reports. This document lists only a few non-water, animal, insect, soil and food borne diseases.
Infections by Vector and Country
Geographic Region Occurrence [Deaths]
Plasmodium falciparum, Plasmodium vivax
Canada 483 (imported)
United States 910
Central America 500,000
Caribbean 100,000
South America 1,200,000 [240,000]
Africa 23,000,000 [2,600,000]
Australia 93(imported)
Chinaa 74,000
Indian sub-continent 2,100,000 [400,000]
Middle East 46,000 [2000]
Southeast Asia 400,000
World Total 27,521,486 [3,242,000]
Insect-trypanosomiasis, chagas
Trypanosoma cruzi, Trypanosoma species
Central America 8,000,000
South America 18,000,000 [45,000]
Africa 300,000
World Total 26,300,000 [45,000]
Insect-filariases, elephantiasis
Wuchereia bancrofti, Brugia malayi, Brugia timori
South America 2,000,000
Africa 18,000,000
Southeast Asia 12,500,000
World Total 32,500,000
Insect-leishmaniasis, kala-azar
Leishmania chagasi, Leishmania mexicana Leishmania braziliensis
South America 12,000
Africa 3,000,000
Middle East 1,300,000
Central America 150,000
Indian sub-continent 500,000
World Total 4,962,000
Insect-river blindness
Onchocerca volvulus
South America 500,000
Insect-dirofilariasis, heart worm
Dirofilaria immitis
United States 118 (since 1961)
Strongyloides stercoralis
United States up to 1,000,000
Central America ~ 3,300,000
South America ~ 7,400,000
Africa ~ 16,300,000
Indian sub-continent ~ 7,000,000
Europe up to 2,000,000
Southeast Asia ~ 5,300,000
World Total ~ 42,300,000
Trichinella spiralis
Canada 10
United States 40
Europe sporadic
Eurasia 1,500,000
World Total 1,500,050
Frequency of all Infections by Country
Those that are water borne and dealt with in these reports are marked with an *. Compare their frequencies of occurrence with those of other parasites, generally having animal, mostly insect, vectors, to get a measure of the relative importance to the health of the human population from water borne, as opposed to non-waterborne, parasites.
United States Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Balamuthia mandrillaris amoebic encephalitis * rare, few/yr., [usually]
Cryptosporidium parvum cryptosporidiosis * 33/yr
Dirofilaria immitis pulmonary dirofilariasis 118 (since1961)
Echinococcus hydatidosis 7100/yr
Entamoeba histolytica amoebiasis * 2983 (in 1994)
Enterobius vermicularis enterobiasis 50,000,000/yr
Giardia lamblia giardiasis * 141/yr
Plasmodium malaria 910/yr
Plasmodium vivax malaria 464/yr
Plasmodium falciparum malaria 282/yr
Strongyloides stercoralis strongyloidiasis up to 1,000,000/yr
Trichinella spiralis trichinellosis sporadic, about 40/yr
Vampirolepis nana vampirolepiasis 2,600,000/yr
Canada Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Diphyllobothrium diphyllobothriasis a few isolated cases/yr
Entamoeba histolytica amoebiasis * 1778/yr
Enterobius vermicularis enterobiasis 2,000,000/yr
Giardia lamblia giardiasis * 7042/yr
Plasmodium vivax
Plasmodium falciparum
malaria 483/yr
Trichinella spiralis trichinellosis sporadic, about 5 to 10/yr
Central America Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Entamoeba histolytica amoebiasis * 5,000,000/yr
Leishmania chagasi
Leishmania mexicana
cutaneous leishmaniasis 150,000/yr
Plasmodium malaria 500,000/yr
Strongyloides stercoralis strongyloidiasis about 3,300,000/yr
Taenia solium cystercercosis 6,500,000/yr
Trypanosoma cruzi chagas, trypanosomiasis 8,000,000/yr
Caribbean Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Ascaris lumbricoides ascariasis 1,000,000/yr
Plasmodium malaria 100,000/yr
Schistosoma mansonii schistosomiasis 10,000/yr
Trichuris trichiura trichuriasis, whipworm disease 1,000,000/yr
South America Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Leishmania chagasi visceral leishmaniasis 3000/yr
Leishmania braziliensis cutaneous leishmaniasis 19000/yr
Onchocerca volvulus river blindness 500,000/yr
Plasmodium malaria 1,200,000/yr
Schistosoma mansonii schistosomiasis 45,000,000/yr
Strongyloides stercoralis strongyloidiasis about 7,400,000/yr
Trypanosoma cruzi chagas, trypanosomiasis 16 to 18,000,000/yr [4500]
Wuchereia bancrofti filariasis 2,000,000/yr
Australia Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Echinococcus granulosus hydatidosis 15/yr
Plasmodium malaria 93/yr (imported)
New Zealand Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Echinococcus granulosus hydatidosis 5000/yr
China Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Ascaris lumbricoides ascariasis about 100,000,000/yr
Opisthorchis chinensis Chinese liver fluke 5,000,000/yr
Paragonimus paragonimiasis about 1,000,000/yr
Plasmodium malaria 74,000/yr
Schistosoma japonicum schistosomiasis greater than 870,000/yr
Africa Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Ancylostoma duodenale
Necator americanus
hookworm, ancylostomiasis greater than 1,800,000/yr
Ascaris lumbricoides ascariasis greater than 200,000,000/yr
Entamoeba histolytica amoebiasis * 5,000,000/yr, [5000]
Leishmania leishmaniasis 2 to 3,000,000/yr
Onchocerca volvulus
Loa loa
Wuchereia bancrofti
river blindness, filariasis 18,000,000/yr
Plasmodium malaria 23,000,000/yr [2,600,000]
Strongyloides stercoralis strongyloidiasis about 16,300,000/yr
Schistosoma mansonii
Schistosoma haematobium
schistosomiasis 100,000,000/yr [10000]
Trichuris trichiura trichuriasis occurs
Trypanosoma trypanosomiasis more than 300,000/yr
Japan Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Anisakis anisakiasis about 10,000,000/yr
Clonorchis sinensis clonorchiasis about 20,000,000/yr
Diphyllobothrium diphyllobothriasis about 1,000,000/yr
Paragonimus paragonimiasis greater than 1,000,000/yr
Schistosoma japonicum schistosomiasis greater than 1,000,000/yr
Southeast Asia Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Brugia malayi filariasis 12,500,000/yr
Opisthorchis chinensis Chinese liver fluke 19,000,000/yr
Paragonimus westermani paragonimiasis 2,000,000/yr
Plasmodium malaria 500,000/yr
Strongyloides stercoralis strongyloidiasis greater than 5,300,000/yr
Eurasia Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Diphyllobothrium diphyllobothriasis 2,000,000/yr
Echinococcus granulosus hydatidosis 1,000,000/yr
Giardia lamblia giardiasis * 10,000,000/yr
Trichinella spiralis trichinellosis 1,500,000/yr
Vampirolepis nana vampirolepiasis 1,700,000/yr
Middle-East Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Ancylostoma duodenale hookworm, ancylostomiasis greater than 60,000,000/yr [2000]
Ascaris lumbricoides ascariasis 100,000,000/yr
Leishmania tropica
Leishmania major
cutaneous leishmaniasis 1,300,000/yr
Plasmodium malaria 46000/yr [2000]
Schistosoma mansonii
Schistosoma haematobium
schistosomiasis 50,000,000/yr [5000]
Europe Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Diphyllobothrium diphyllobothriasis 7,000,000/yr
Echinococcus granulosus hydatidosis 5000/yr
Strongyloides stercoralis strongyloidiasis up to 2,000,000/yr
Taenia solium cystercercosis sporadic outbreaks
Trichinella spiralis trichinellosis sporadic outbreaks
India, Pakistan and Sri Lanka Parasitic Disease Frequencies
Parasite Disease Frequency [Deaths]
Ancylostoma duodenale hookworm, ancylostomiasis greater than 300,000,000/yr
Ascaris lumbricoides ascariasis 400,000,000/yr
Dracunculus medinensis dracunculiasis 39,792 (in 1994) 60 (in 1995)
Leishmania leishmaniasis, kala-azar 500,000/yr
Plasmodium malaria 2,100,000/yr [400,000]
Strongyloides stercoralis strongyloidiasis about 7,000,000/yr
World Totals Parasitic Disease Frequencies
Parasite/Disease Frequency
Intestinal roundworms, ascariasis 1,400,000,000
Schistosomiasis 200,000,000
Lymphatic filariasis 120,000,000
Amoebiasis (protozoan) * 40,000,000
Food borne trematode infections 40,000,000
Chagas trypanosomiasis 16,000,000
Leishmaniasis 12,000,000
African trypanosomiasis 300,000
Dracunculiasis 100,000
Nature of Infections
There are many different kinds of diseases caused by these pathogens, many different organs and parts of the body are affected, many different routes of access to the body are used and the outcomes or severity of the diseases caused range from non-symptomatic to fatal. Virtually all organs are affected but the chief or most severe diseases affect the brain, heart, liver, lungs, bile ducts, lymph system and lower gut. Some round worms take the 'grand tour' and wander around much of the body, causing damage as they go, until they settle in their preferred location. Access may be via food or water that is ingested, air or aerosols that are inhaled, direct penetration though the skin, access through the eye, nose and ear or transmitted through sexual activity.

Disease severity ranges from rapidly fatal encephalitis to low grade chronic infections of the gut that may be non-symptomatic for many years. Most commonly there are respiratory problems or diarrhea which are non-life threatening. Tapeworms absorb nutrients from the gut and lead to chronic malnutrition and malaise in heavy infestations. Some worms block passages responsible for bile, lymph or blood flow and the resulting obstruction leads to swelling and tissue damage as well as loss of function. Many pathogens can set up autoinfection cycles within a host whereby they are able to keep re-infecting the host from within without having to shed eggs to the environment and re-infect again. The tables below indicate the main areas of the body which are affected by protozoan and helminth parasites. These are the areas affected during the migration stage and the final destination for growth and reproduction.

Parts of the Body Affected by Specific Organisms
Location in Body Protozoan Organisms Responsible
Central nervous system, Brain Trypanosoma, Naegleria fowleri, Toxoplasma gondii, Plasmodium, Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Balamuthia mandrillaris, Hartmannella veriformis
Eye Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi, Brachiola vesicularum, Brachiola connori, Vittaforma cornea, Nosema algerae, Nosema ocularum, Microsporidium ceylonensis, Microsporidium africanum, Microsporidium buyukmihcii
Genito-urinary system Giardia lamblia, Entamoeba histolytica, Cryptosporidium parvum, Isospora belli, Balantidium coli, Blastocystis hominis, Cyclospora cayetanensis, Dientamoeba fragilis, Enterocytozoon bieneusi, Encephalitozoon intestinalis, Trichomonas vaginalis, Entamoeba histolytica
Kidney Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi
Liver Leishmania, Entamoeba histolytica, Encephalitozoon intestinalis
Lymphatic system Wuchereria bancrofti
Mouth amoebae, flagellates (usually non-pathogenic)
Respiratory tract, Lungs Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Balamuthia mandrillaris, Hartmannella veriformis, Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi
Skin Leishmania, Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Balamuthia mandrillaris
Spleen Leishmania
Widespread Encephalitozoon hellem, Encephalitozoon cuniculi, Thelohania apodemi
Location in the Body Helminth Organisms
Central nervous system, Brain Taenia solium, Heterophyes heterophyes, Metagonimus yokogawai, Paragonimus westermani, Schistosoma haematobium
Circulatory system, Heart Ancylostoma duodenale, Ascaris lumbricoides, Heterophyes heterophyes, Metagonimus yokogawai, Schistosoma mansoni, Schistosoma haematobium, Schistosoma japonicum, Schistosoma mekongi, Schistosoma intercalatum, Echinococcus granulosus, Echinococcus multilocularis, Echinococcus vogeli, Echinococcus oligarthrus
Eye Toxocara cati, Toxocara canis
Genito-urinary system Schistosoma haematobium
Gut, Colon, Duodenum, Ileum, Small intestine, Lumen, Intestinal tract Diphyllobothrium latum, Dipylidium caninum, Hymenolepis diminuta, Hymenolepis nana, Taenia saginata, Taenia solium, Ancylostoma duodenale, Ascaris lumbricoides, Capillaria philippinensis, Enterobias vermicularis, Necator americanus, Trichuris trichiura, Clonorchis sinensis, Fasciola gigantica, Fasciola hepatica, Fasciolopsis buski, Heterophyes heterophyes, Metagonimus yokogawai, Opisthorchis viverrini, Opisthorchis felineus, Schistosoma mansoni, Schistosoma haematobium, Schistosoma japonicum, Schistosoma mekongi, Schistosoma intercalatum, Echinococcus granulosus, Echinococcus multilocularis, Echinococcus vogeli, Echinococcus oligarthrus
Liver Taenia solium, Capillaria hepatica, Toxocara cati, Toxocara canis, Clonorchis sinensis, Fasciola gigantica, Fasciola hepatica, Opisthorchis viverrini, Opisthorchis felineus, Echinococcus granulosus, Echinococcus multilocularis, Echinococcus vogeli, Echinococcus oligarthrus
Muscle Taenia saginata, Taenia solium, Ancylostoma duodenale
Pancreas Opisthorchis viverrini, Opisthorchis felineus
Respiratory tract, Lungs Ancylostoma duodenale, Ascaris lumbricoides, Capillaria aerophila, Necator americanus, Paragonimus westermani, Echinococcus vogeli
Skin Ancylostoma ceylanicum, Ancylostoma duodenale, Ancylostoma braziliense, Uncinaria stenocephala, Necator americanus, Schistosoma mansoni, Schistosoma haematobium, Schistosoma japonicum, Schistosoma mekongi, Schistosoma intercalatum, Austrobilharzia variglandi, Gigantobilharzia, Heterobilharzia americanum, Microbilharzia, Schistosoma spindale, Schistosomatium douthitti, Trichobilharzia ocellata, Trichobilharzia physella, Trichobilharzia stagnicolae, Dracunculus medinensis
Widespread Taenia solium, Paragonimus westermani, Echinococcus granulosus, Echinococcus vogeli
Sources of Infection

While water borne transmission is important for many of these pathogens it is not the only, or in some case the most important, means of spread. The anal/oral shortcut is faster and more efficient than discharge to the environment and infection or re-infection from contaminated water. This maintains a reservoir of pathogens from which the water is continuously re-contaminated and in some cases, where no alternate host is required in the life cycle, or currently available, may maintain a pathogen population in isolation from the water.


Water as a Reservoir of Nosocomial Pathogens

Understanding the means of acquisition, sources and reservoirs of nosocomial pathogens is crucial for developing methods to reduce the incidence of these infections. Important water reservoirs in the hospital include potable water, sinks, faucet aerators, showers, tub immersion, toilets, dialysis water, ice and ice machines, water baths, flower vases, eyewash stations and dental-unit water stations. While virtually all of these pathogen records, except for the protozoan Hartmannella at the eyewash station and Cryptosporidium in ice machines, document bacterial diseases it should be noted that contaminated water was the source and could well be the source for protozoan, or other, parasites as well. Often there was not even an analysis carried out for protozoans or any other parasites. These examples are from North American institutions which are relatively well run by world standards!

Potable Water
Potable water has been described as a reservoir in several outbreaks. Most commonly, equipment has been rinsed with potable water, resulting in contamination of the equipment and subsequent nosocomial infections. Outbreaks have involved tracheal suction tubing, otologic equipment, material used for discectomies and endoscopic equipment. Rinsing of burn patients with tap water as first aid when entering the hospital has led to serious wound infection and sepsis.

There are 19 reported cases of nosocomial pulmonary infections from several hot-water generators and water taps in hospital wards. Two outbreaks have been reported in patients receiving hemodialysis with reprocessed dialyzers. Legionella can be isolated from more than 50% of the potable water supplies and more than 10% of the distilled water supplies in hospitals.

A number of reports have demonstrated the presence of gram-negative bacteria in hospital sinks, but patients rarely seem to acquire nosocomial pathogens from this source. The ability of gram-negative bacilli to survive wet environments for long periods of time, up to 250 days, helps to explain their common occurrence in sink drains. Not surprisingly, many of these bacteria isolated from sinks possess resistance to antibiotics. Several studies that have isolated pathogens colonizing or infecting patients from hand washing sinks. Although organisms may pass to patients from employees through hand washing at a contaminated sink, this route does not seem to be an important factor in endemic nosocomial infections.

Faucet Aerators and Showers
The faucet aerator has been identified as a reservoir and possible source of infection within the hospital. Premature infants became infected from delivery room resuscitation equipment that was contaminated by a faucet aerator. Contaminated hand-held showerheads have been linked to infections. Legionella has been isolated from showerheads. In one case despite sterilization of the showerheads with ethylene oxide, rapid recontamination occurred, suggesting that the potable water was contaminated.

Ice and Ice Machines
Contaminated ice and ice machines may be a source for nosocomial infections. There was an epidemic among hospital staff because of the use of ice obtained from a fecally contaminated river. During the last 25 years, several reports have linked nosocomial epidemics or pseudo-epidemics to contaminated ice or ice machines. An outbreak of serious pulmonary and wound infections was traced to an ice machine with bad plumbing. Patients were presumed to have acquired the epidemic pathogens by sucking on ice or ingesting iced drinks. An outbreak of cryptosporidiosis, involving both HIV-positive and -negative subjects, was traced to an ice machine contaminated by an incontinent, psychotic patient. An outbreak in 14 patients was traced to the use of nonsterile ice. Investigation revealed that the ice machine in the intensive-care unit served as the reservoir for the epidemic pathogen.

The cold-water dispensers of 8 of 14 ice machines yielded Legionella pneumophila. Positivity of these sites was linked to the incoming cold-water supply by recovering Legionella pneumophila from the cold-water storage tank, which was supplied directly by the incoming municipal water line. Recently, nosocomial legionellosis in two heart-lung transplant patients was linked to a contaminated ice machine and nosocomial legionellosis in a renal transplant unit was linked to contamination in the ice machine and shower water. The Centers for Disease Control and Prevention (CDC), however, has published a set of recommendations designed to minimize ice- and ice-machine-associated infections. A regular program of disinfection of ice machines is included in these recommendations.

Eyewash Stations
Stationary and portable eyewash stations, which may be unused for months or years, represent a reservoir for potential pathogens. An analysis of 40 eyewash stations revealed the following contamination rates: heterotropic bacteria, the majority of isolates were Pseudomonas species, 95%; Legionella species, 7.5%; amoebae, such as Hartmannella, 47.5%; and fungi, 42.5%. This high contamination rate raises concerns that serious eye infections could result if such eyewash stations were used to flush injured eyes. Because the source water may stand in the incoming pipes at room temperature for a year or more, the American National Standards Institute has recommended that eyewash stations be flushed weekly. Ideally, eyes should be flushed with unused bottles of sterile water or other solutions. However, a disadvantage of the eyewash bottles is the small volume. When a chemical spill to the eyes occurs, copious amounts of water are required to flush the injured eye.

Dental-Unit Water Systems
Dental units are equipped with plastic tubing to bring water to the different dental hand pieces, the air-water syringe, the ultrasonic scaler, and the high-speed hand piece. Potable water normally supplies these dental units. With use, the water lines in the dental units may become heavily contaminated with microorganisms. One study reported that all dental units were contaminated with at least 5 logs of bacteria.

Dialysis Water
It was demonstrated in the 1970s that excessive levels of gram-negative bacteria in the dialysate of hemodialyzers were responsible for pyrogenic reactions or bacteremia. This hazard is caused either by the organism gaining entrance to the blood from the dialysate or by endotoxins from gram-negative bacteria associated with the water and dialysate passing intact through membranes and causing pyrogenic reactions. In one study the attack rates of pyrogenic reactions were related directly to the levels of gram-negative bacteria in the dialysate. It also has been demonstrated that certain types of bacteria, especially the waterborne bacteria, have the capability of surviving and reproducing in distilled, deionized, reverse-osmosis and softened water, all of which have been used to supply water for hemodialysis.

Water Baths
Several outbreaks of serious nosocomial infections have been traced to contaminated 37C water baths that were used to thaw fresh plasma or cyroprecipitate or used to warm bottles of peritoneal dialysate before use. In each outbreak, the water bath was heavily contaminated with the disease-causing organism and contamination of the infusate occurred when the fluid was being prepared for administration. The problem of contaminated water baths and dialysis bottles may be eliminated quite simply by using warm-air cabinets or a microwave to preheat the dialysis solution before use.

Ice Baths for Thermodilution Cardiac Outputs
Thermodilution is the most common method of measuring cardiac output. Until recently, the most frequently used technique involved the injection of cooled saline. Often, cooling was accomplished by placing individual syringes or bottles of saline in ice baths derived from mixing nonsterile ice with water, which led to several outbreaks ascribed to contamination of the ice-water baths.

Tub Immersion
Tub immersion is used in the hospital, as an aid in physical therapy, for cleaning burn wounds and bathing babies, and, until recently, was required for kidney lithotripsy. Skin infections related to water immersion have been recognized for many years. The majority of these infections have been associated with contaminated whirlpools or hot tubs. Tub immersion of hospitalized patients could lead to infection via cross-transmission, transmission from an environmental reservoir or autotransmission and infection of the wound by fecal flora. Frequently, immersion of hospitalized patients will contaminate the tub environment, including the tub water, drains, agitators, floors and walls. Contamination of a baby bath was linked to a disease outbreak among neonates. Prevention of nosocomial transmission of nosocomial infections requires adherence to strict disinfection protocols. In addition, the epidural catheter used for anesthesia in patients undergoing lithotripsy should be covered with a transparent occlusive dressing.

Hospital Toilets
The microbiology of hospital toilets has been investigated by obtaining cultures of the air, water and surfaces of hospital toilets and the bacteria in the air or splashes after toilet flushing. The frequency and level of contamination of the air, water, and surface of hospital toilets by fecal bacteria was surprisingly low at 27%, 39% and 6%, respectively. The likelihood of generating a bacterial aerosol was studied in artificial flushing experiments; at least 1010 bacteria/100 mL of toilet water were required to produce an aerosol. The bacterial counts in toilet water were reduced 100-fold by a single flush. It seems that the hospital toilet is an unlikely source of infection. This may not be true if the surfaces are coated heavily with feces, as could happen in hospitals for mentally impaired patients, pediatric wards, daycare centers for children or with neurologically impaired adults.

Concern has been expressed that cut flowers may represent a reservoir of pathogens even though no actual outbreaks of nosocomial infections have been definitively linked to cut flowers as a source. Cultures of tap water made 72 hours after the water was placed in vases yielded approximately 107 to 1010 bacteria/mL. Bacteria isolated from vases located on hospital floors have included Acinetobacter species, Klebsiella species, Enterobacter species, Pseudomonas aeruginosa, Burkholderia cepacia, Pseudomonas fluorescens, Pseudomonas putida, Aeromonas hydrophilia, Serratia marcescens and Flavobacterium. Studies have failed; however, to link pathogens isolated from flower vases or potted plants to pathogens isolated from nearby patients.

Other Water Sources
Improperly maintained devices for producing distilled water or containers used for storing distilled water have led to disease outbreaks. An outbreak in a neonatal nursery was linked to use of a contaminated wash basin. An outbreak was associated with a contaminated water reservoir of the intra-aortic balloon pump. Four cases of postoperative infection were linked to contaminated ventilation-system humidifier water and an outbreak of gastrointestinal illness to drinking from an institutional water cooler. There is a case of hospital-acquired infection in a burn patient after exposure to contaminated holy water.


Food processing operations
Few studies have been conducted on the effect of food processing operations on oocysts of Cryptosporidium, Cyclospora and Giardia cysts. Most studies in water have been conducted with Cryptosporidium, which is regarded as the most resistant of these organisms.

From limited studies it is probable that standard pasteurization procedures will inactivate these organisms. Heating Cryptosporidium cysts in water to 72.4C for one minute or holding cysts in water at 64.2C for two minutes rendered cysts non-infective to mice (International Journal of Food Microbiology 31, 1996, 1-26). The thermal death time point, the temperature at which organisms are destroyed, of Giardia cysts has been reported as 62C (Journal of Food Protection 56, 1993, 451-456).

It must be stressed that the resistance of these cysts in a complex food medium such as milk may be different from that exhibited in water. Laberge and co-authors (International Journal of Food Microbiology 31, 1996, 1-26) in reviewing available data conclude that it is unclear whether HTST pasteurization, 72C/15 sec, would lower the level of infective oocysts below the infective dose for humans. The level of contamination used in the study reported above, 1 million oocysts/mL, is much higher than one would expect in raw milk.

There is one reported environmental/laboratory study of cryptosporidiosis attributed to an acid food, fresh pressed apple juice (Journal of the American Medical Association 272, 1994, 1592-1596). This is in fact the best-documented outbreak attributed to food. Unfortunately the pH of the juice involved is not recorded but it may be assumed to be around pH 4.0. Staff and students drank the juice the same afternoon it was prepared at a school agricultural fair in Maine, USA, in 1995. State Bureau of Health workers subsequently obtained a sample of the partially fermented apple juice ten days after the fair. The juice was then frozen and stored for six weeks before Cryptosporidium oocysts were counted. Up to 750 oocysts/L were recorded and the investigators believe the count was probably much higher. This indicates the ability of the oocysts to survive at the pH of the apple juice for several days at ambient temperature and weeks during frozen storage.

Two experimental studies confirm the relative tolerance of Cryptosporidium oocysts to low pH. Friedman and co-workers (Journal of Food Safety 17, 1997, 125-132) report a study in which oocysts were inoculated into beer, cola, orange juice and infant formula. The beer and cola were carbonated; the beer had a pH of 3.8 while the cola had a pH of 2.5. The pH of the orange juice was 3.9 and that of the infant formula 6.6. A pH 4.0 buffer as well as ethanol solutions were used as controls. Samples were incubated at 4C and/or 22C for 24 hours and viability of inoculated oocysts determined. The authors calculated that after this time there was a loss of >85% oocyst viability in beer or cola stored at 4C while the loss of viability in water, orange juice and infant formula was equal to or less than 35 percent. The study did not permit estimation of the time required to achieve complete inactivation of Cryptosporidium oocysts.

The second study (FEMS Microbiology Letters 142, 1996, 203-208) used a laboratory medium with pH adjusted to 2.0 with hydrochloric acid, 4.0 with acetic acid, 6.0, 8.0 and 9.5. Cryptosporidium parvum, the common human pathogen, was one of two Cryptosporidium species tested. The initial population of cysts used was about 1 million/ml. These workers reported apparent zero viability of cysts after 60 minutes at pH 2.0 but 85 percent viability after the same length of time at pH 4.0. No other incubation time was studied.

The study reported with apple juice indicates that oocysts of Cryptosporidium are able to survive at least some weeks of frozen storage in a suitable medium. An experimental study (Applied and Environmental Microbiology 58, 1992, 3494-3500) using purified water as the medium showed that a small proportion of oocysts survived 750 hours at -22C after slow freezing. Snap freezing using liquid nitrogen resulted in 100 per cent loss of viability. The initial population used in this study was again about one million cysts/mL. The case cited above where frozen tripe acted as a vehicle for infection is further confirmation that freezing cannot be relied upon to destroy all Cryptosporidium cysts.

No published studies have investigated the effect of drying of oocysts in or on foods. However experimental studies have shown that drying of cysts suspended in water on glass surfaces at ambient temperature resulted in 97 percent loss of viability after two hours and total loss after four hours.

It is clear from the above that we need to know a lot more about the behaviour of waterborne parasites especially Cryptosporidium and Giardia in food systems. While this knowledge is being accumulated, it is essential that food processors that use domestic drinking water, either for washing foods or as an ingredient in foods, for which there is no terminal heat process, develop contingency plans in case of similar incidents in the future.

Drinking Water

Drinking water is not used just for drinking; in fact this is probably its most insignificant use in terms of volume used. It is used for cooking food, preserving food, washing food, washing people, as ice cubes in drinks, watering the garden, washing the house and objects in it, filling wading pools and filling humidifier reservoirs. People in a shower and kids playing under a sprinkler are exposed to aerosols.


There are a number of jobs in which the workers are exposed to feces, which may be contaminated with pathogens. The use of protective clothing, masks, rubber gloves and the practice of washing thoroughly with soap and using a nailbrush to clean under the fingernails will help to reduce the risk. A partial list of jobs and activities with high pathogen contact risk includes, sewer workers, sewage treatment plant workers, medical laboratory staff, hospital staff, child and adult daycare staff, caring for infants, farm workers where contaminated irrigation water is used, animal care staff in laboratories, zoos, feedlots, pet stores and organizations like the SPCA, commercial laundry staff, composting, recycling and garbage collection workers, abattoir workers, farm workers where animals are raised and ambulance and other emergency response staff. Everyone is exposed daily when they go to the toilet.

Aquatic Recreation

Natural lakes and other bodies of water may be contaminated and continuously re-contaminated, particularly if heavily used by people. For most pathogens warmer waters are more of a risk and spas, hot tubs and hotsprings are pathogen reservoirs. Lack of flow and replacement of the water allows pathogens to build up. Surface waters are generally relatively clean and swimming from a dock or raft is a lower risk activity than wading in the shallows and stirring up the bottom sediments where pathogen concentrations are orders of magnitude higher. Ironically, this puts toddlers at a higher risk than teens. For some organisms the risk is greatly increased by submersion of the head while diving or swimming since the infection route is through the nasal passages. Aerosols are also a problem and splashing increases the risk. The wading pool in the back yard that is freely accessible to cats and dogs and has been incubating for some weeks in the sun without a water change is a prime source of pathogens.

For artificial pools, chlorination is not necessarily adequate as a treatment, especially if not properly maintained at high levels. Ozonation is also not adequate for some organisms. High intensity UV irradiation will control many spores; Giardia and Cryptosporidium, but may not kill encysted Ascaris ova. Sub-micron filtration is necessary to remove all the pathogens.

Sexual Activities

There are a number of sexual practices, which greatly increase the risk of anal/oral spread of many pathogens. Some ways to reduce, but not eliminate, such risk include use of protective devices such as condoms, washing with soap before and after such activities, avoiding such activities when one of the partners has an active infection and choosing a sexual partner with care.
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Water Treatment

Treatment Plants
Multiple barriers, combinations such as clarification, filtration and disinfection, are key to minimizing protozoan oocyst presence in drinking water. Facilities with disinfection but not filtration are clearly at risk. However, as of 1995, at least three outbreaks of cryptosporidiosis were attributed to oocysts in drinking water supplies subject to multiple barrier treatment, so current water treatment processes are not entirely adequate in removing or inactivating oocysts. Problems arise under two scenarios. Firstly, when some identifiable breakdown in the system occurs resulting in direct/indirect fecal contamination of the water supply. Maintaining vigilance can minimize their occurrence, but there is no way to entirely prevent such accidents. The second scenario is some combination of atypical, coincidental, occurrences, heavy runoff plus slightly less than optimal treatment leading to turbidity fluctuations being an example. Mixing of filter effluents may produce suitable final water according to regulations, yet on a filter-by-filter basis, some treatment streams may not perform as well as others.

Simple regulatory compliance is not good enough in an instance like this and improvement is possible. Blending of potentially oocyst-carrying, higher turbidity water with another lower turbidity water to meet turbidity guidelines may lead to an oocyst presence in the blended waters sufficient to initiate illness. With respect to potential oocyst contamination, operators need to view each treatment stream individually, rather than judging the quality of their product on the final, mixed water leaving the plant. The backwash process appears particularly critical with respect to oocyst presence in final waters. Some authorities recommend no recycling of backwash water and filtering to waste after backwashing until the filter restabilizes. Solids from backwashing may contain considerable numbers of oocysts, and should be handled appropriately as infectious wastes.

Various authorities note that communication between water producers and public health officials needs to improve, to lessen the possibility of cryptosporidiosis outbreaks and to more quickly gain control if an outbreak ensues. Health authorities should be informed immediately when abnormal turbidity or particle counts are experienced, or when oocyst numbers are atypical if monitoring is being conducted, and water producers should be informed of unusually high incidence of gastroenteritis noted in a community. Sewage treatment plants may be contributors of oocysts to raw water supplies depending upon where they are sited and they are certainly the primary repositories of oocysts produced by infected individuals within a community; monitoring data from sewage treatment facilities also fits into an improved communication network. Increasing oocyst numbers at the sewage treatment plant could serve as an early warning sign that a cryptosporidiosis problem is on the way. Water treatment facilities must be geared to handling peak incidences of oocysts rather than normal levels, to ensure that unacceptably high risks of infection from Cryptosporidium, and other cyst forming pathogens do not occur.

There are suggested yardsticks for gauging the significance of oocysts in finished waters, based upon monitoring studies and oocyst levels recorded or estimated during documented cryptosporidiosis outbreaks. When finished water concentrations are greater than 10-30 oocysts/100 L the possibility of an epidemic outbreak exists. In the lower part of this range, or below, outbreaks may occur but not be detectable. If a single sample is at or above the action level, further sampling should be quickly initiated and supplemental performance data should be examined and/or collected, turbidity, particle data, individual filter performance data, fecal coliform counts, to support any subsequent decision making. The American Water Works Association guideline is that finished drinking water consistently measure 0.5 NTU, and that utilities set a goal of 0.1 NTU.

Implications for ground water management are not entirely clear at the present, but ground water sources are certainly of secondary importance relative to surface waters, with respect to contamination by cyst forming protozoans such as Cryptosporidium. In the US, a Groundwater Disinfection Rule is under development. The workgroup associated with this rule is not considering any requirements with respect to Giardia or Cryptosporidium. The conventional wisdom is that contamination of ground water with protozoa indicates surface water influence, and ground water-associated outbreaks have been associated with distribution system deficiencies. Treatment facilities can gauge the importance of Cryptosporidium to their produced water quality by considering factors such as degree of watershed development, presence and location of upstream urban areas, animal husbandry operations, sewage treatment facilities and the in-plant treatment train, especially the presence or absence of filtration. When an investigative monitoring program is embarked upon, some assessment of the watershed should be included, to identify potential oocyst sources. Within the plant, use of particle counting, as a surrogate measure should be investigated.

Process alterations may lead to changing circumstances, with respect to the risk of oocyst presence, or oocyst survivability, as may have been the case in Milwaukee. The outbreak in Kitchener/Waterloo took place after the area had shifted from complete ground water dependence to a mixed source of ground water augmented by some surface water, a change that increased the possibility of oocyst presence in the plant intake water. This may have been a factor in the outbreak.

It is crucial that water managers are aware of the potential biological hazards associated with their product; the problem is likely to get worse, not better. Increased knowledge among water producers, and improved education of water users are two keys to preventing over-reaction and a good deal of wasted money. The currently perceived waterborne protozoan threats to public health are Cryptosporidium and Giardia. However, many other protozoans are already emerging as the waterborne pathogens of the future. Cyclospora is one example; the only known, pre-1995, cyclosporiasis outbreak in the US was traced to contaminated water supply at a Chicago hospital. A second eastern US/Canada outbreak was recently associated with raspberries and possibly the water used to irrigate the crop. A toxoplasmosis outbreak, Toxoplasma gondii, has been linked to a British Columbia water reservoir; and other microsporidia, Enterocytozoon, Septata, Encephalitozoon, Nosema and Pleistophora represent future waterborne threats.

No one should ever view the water supply as guaranteed pathogen-free. Historical data record a downward trend in waterborne disease in developed societies, particularly since the advent of filtration and then chlorination early in this century, but never a cessation of waterborne illness. Any microbiologist would concur that the microbiological hazards associated with drinking water are always of far greater immediate importance than small amounts of chemical pollutants or the halo-organic byproducts of chlorination practices. The rational goal is to strive to reduce total lifetime risk since to promise no risk is irrational. Assessment and subsequent management of newly identified risks will be constantly needed.

When feeding very young infants, we carefully sterilize bottles and other equipment, to avoid pathogen exposure, because infant immune systems are not completely developed. Society views this practice as common sense. The same special care must be exercised in the case of other children or adults with imperfect or failing immune systems; this too is only common sense. Provision of point-of-use filtration devices, use of boiled drinking water and other avoidance practices for high-risk individuals, seem more economically reasonable solutions to potential Cryptosporidium exposure than a massive overhaul of current water treatment facilities. Again, we need improved communication to educate these individuals and their medical advisors.

It is unfortunately true that little effort has generally been expended on upgrading of public utilities, only on maintenance of the status quo. In some cases not even preventive maintenance is kept up. It is also unfortunate that even existing laws and legislation regarding filtration and disinfection, where they exist, are not enforced. Such existing regulations are not adequate as written but would go a long way towards ensuring cleaner water and preventing outbreaks if they were routinely practiced and inspection ensured compliance. Lack of enforcement may lead to expensive lawsuits if large numbers of people become ill or die as a result of political or administrative inaction or lack of funding, when there is adequate scientific and engineering knowledge to provide better quality drinking water.

Particularly problematic systems should certainly be improved as is feasible over the short term, and replaced over the long term; new facilities should be designed to incorporate the most cost-effective, modern techniques to remove biological contaminants. Although this is the case, reaction to emerging pathogens sometimes seems a bigger problem than the emerging pathogen and its effects. Even within the scientific literature, unreasonable statements appear. That...'outbreaks of cryptosporidiosis became more common during the next 5 years'... for example, is quite unlikely. It is far more likely that existing and previously un-recognized outbreaks, and their causes, became more commonly recognized. Consumers, water providers, politicians and regulators need to be educated to appreciate this.

Bottled Water
Many people use bottled water for health and taste reasons. However, differing water sources and methods for treating bottled water, and a lack of uniform legislation and consistent inspection, creates inconsistent quality, which may not be any better, and may be worse, than tap water. In many cases it is simply cold, bottled, expensive, tap water. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Carbon Filtration
Carbon filtering reduces chlorine and organic contaminants like algal toxins, but few carbon filters remove viruses, microsporidia, Cryptosporidium or Giardia. Water quality depends on the filter being replaced routinely and the filter pore size being adequate; sub-micron filters are required but rarely provided. There is no residual action to maintain sterility in case of subsequent contamination.

Chlorine or chlorine dioxide are used to disinfect water since they are strong oxidants and destroy living organic matter. A minimal contact time is required and the dose must be adjusted to leave some free residual chlorine after all the organic matter has been reacted. For the process to work efficiently and safely and minimize the amount of chlorine used, filtration should occur first to remove as much organic material as possible. Organic reaction products, which are toxic, such as trihalomethanes, are produced during chlorination if organic materials are present. There is residual action to maintain sterility in case of subsequent contamination if excess chlorine is used.

Water is boiled and the resulting steam is then cooled, condensing into fresh water. The contaminants that can not evaporate are left behind and then drained away. This method will not remove trace amounts of some volatile organic chemicals with boiling points near that of water. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Filtration Beds
Filtration through beds of sand is the standard process. To be effective for small spores the pore size needs to be less than 1 micron. This requires very fine sand and results in slow filtration rates with rapid clogging. Pre-filtering with coarser sand will likely be required to make the process effective and economical. Backwash waters used to unclog filter beds must not be recycled as product water but must be sent to waste due to the potentially high spore density. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

The use of a chemical, often alum, ammonium or potassium aluminum sulphate, to cause particles to coagulate or clump together is often used in water treatment. The fewer, larger, heavier particles produced are more effectively removed by filtration, sedimentation or other means. This is an effective pre-treatment process used prior to filtration. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Ordinary slow freezing is not very effective for some protozoan spores but flash freezing at liquid nitrogen temperatures will usually kill most spores and organisms. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Heating water will kill spores and most protozoan pathogens but the temperature needs to be raised to about 75 degrees Celsius and held there for about 5 minutes. Boiling for several minutes is required for some other parasitic organisms, bacteria and viruses. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Ionizing Radiation
Ionizing radiation, protons, electrons and gamma rays, will destroy the nuclear structure of pathogenic organisms and kill them, or at least prevent them from reproducing. The water should be filtered first to remove heavy metals and as much suspended material as possible to reduce the necessary dose and unwanted secondary re-radiation. This technique is used successfully for long-term preservation of food. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

This process uses ozone, which is a strong and unstable oxidant, to destroy organisms and toxins. A minimal contact time is required and the dose must be adjusted to leave some free residual ozone after all the organic matter has been reacted. For the process to work efficiently and safely and minimize the amount of ozone used, filtration should occur first to remove as much organic material as possible. Ozone is very reactive and breaks down quickly; there is no residual disinfection action to maintain sterility in case of subsequent contamination.

Reverse Osmosis
Pressure forces water through membranes leaving behind most but not all-organic, inorganic and biological contaminants. However, reverse osmosis uses three to ten volumes of water to produce one volume of drinking water. The membranes need regular replacing and many soluble organic contaminants are not removed. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Allowing water to sit undisturbed in a reservoir for a minimal period of time will permit many particles to settle out. However, this can only be used as a pre-treatment to reduce the particle density since some organisms are motile and some spores are small enough to be kept in suspension by Brownian motion and weak water currents due to thermal density differences or seiche waves.

Ultraviolet light
UV light kills most biological contaminants such as viruses and bacteria. However, the light must be in contact long enough to be effective and the water must first be filtered to allow the light to reach the organisms. There is no residual disinfection action to maintain sterility in case of subsequent contamination.

Spore Size

The US Surface Water Treatment Rule states that all surface water that may potentially be used for drinking water must be filtered. Unfortunately, problems with Cryptosporidium, Giardia, and protozoans like Cyclospora, which is larger than Cryptosporidium and thus more easily filtered, are still occurring, even in ground water sources. Also, because Cryptosporidium is pliable, it can fold up and pass through one-micron pores, thus slipping through most public utilities filtration systems. The only water treatment devices that can effectively filter Cryptosporidium are those certified for sub-micron filtration or less than one micron.

Sporocysts and oocysts may be quite different in size. The full range of reported values is given in the text for either kind of cyst. The trophozoite and amoeboid stages may be quite different in size from the cysts. It is the cysts which are generally the resistant and infective stage which need to be filtered out of drinking water. Many are not round but ovoid or other more complex shapes. In some cases the organisms are amoeboid and flexible and can pass through smaller pores than their spores. It is the minimum size of any water transmittable stage that is important for designing drinking water filtration processes so that is what is reported in the table below.

Spore Sizes for several Pathogenic Protozoans
Protozoan Parasites Size of trophozoites/oocysts/sporocysts
smallest reported dimension in microns
Acanthamoeba 7
Balantidium coli 20
Balantidium hominis 5
Cryptosporidium parvum 2
Cyclospora cayetanensis 8
Dientamoeba fragilis 7
Encephalitozoon cuniculi 1
Encephalitozoon hellem 1
Encephalitozoon intestinalis 1.5
Entamoeba histolytica 10
Enterocytozoon bieneusia 1
Giardia lamblia 5
Isospora belli 7
Microsporidia 1
Nosema connori 2
Nosema corneum 2
Pleistophora 2.8
Toxoplasma gondii 4

Resistance to Disinfection

The resistance of many protozoan cysts to standard water disinfection procedures has been extensively reported. Cryptosporidium is resistant to the usual chlorine disinfection concentrations. Of the disinfectants commonly used to treat water, ozone and UV appear to be effective in destroying the oocysts of Cryptosporidium parvum, which is the most resistant of the protozoans. Notable outbreaks occurred in Milwaukee, WI in 1993, and Las Vegas, NV, in 1994. Milwaukee authorities installed ozone treatment plants to supplement filtration after the 1993 incident. Giardia can be killed only by long contact with chlorine. Recent outbreaks have occurred in Amsterdam, NY, Oregon, Georgia, and in Golden, CO.

Korich and co-workers (Applied and Environmental Microbiology 56 (5) 1990, 1423-1428) reported that 1 mg/L of ozone for 5 minutes achieved greater than 90 percent inactivation of Cryptosporidium parvum cysts. These authors concluded that with the possible exception of ozone, the use of disinfectants alone should not be expected to inactivate Cryptosporidium parvum in drinking water. However, 90% inactivation is inadequate for organisms where the initial concentration may be in the order or 104 to 107 or more. Consider that a 99% kill rate of 1,000,000, or 106, organisms still leaves one with 10,000, or 104, viable organisms. When the infectious dose may be in the order of 10 to 100 organisms this is quite inadequate performance. It is common to see performance figures quoted as log reductions when it is actually the number of remaining viable organisms that is important, not how many were killed. While a 99% reduction sounds good it is quite inadequate when the potentially very large numbers of many pathogenic micro-organisms are considered.

Killing blue-green algae within a treatment plant is not an acceptable procedure. The dead cells lyse releasing their deadly endotoxins which are water soluble complex organic molecules. These cells must therefore be filtered out of the input stream before any disinfection steps. Only activated charcoal will remove the dissolved organic toxins from the water supply.

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"Current testing methods cannot determine with certainty whether Cryptosporidium detected in drinking water is alive or whether it can infect humans. In addition, the current method often requires several days to get results, by which time the tested water has already been used by the public and is no longer in the community's water pipes. ...Analytical method limitations prevent using Cryptosporidium monitoring data to accurately assess risk or even to set an acceptable level of risk for Cryptosporidium in drinking water. Water utilities must be vigilant in applying source water protection and appropriate treatment to protect customers against this organism"

This is a quote from Pontius and Clancy, 2000, AWWA, 14-22. To get around this problem all water should be filtered and treated with ultraviolet; it is rare that even one of these treatments is applied to most drinking.

Sampling and analyzing for many pathogens is a difficult process. Their intrinsic distribution pattern is statistically non-random and non-normal. The numbers tend to fluctuate over orders of magnitude within small spatial and temporal intervals. Determining the actual effective numbers present with any accuracy or precision is rarely economical and sometimes not possible; it is likely not even relevant. Converting such numbers to a risk factor is often not possible or even meaningful. Epidemiological techniques, though they have inherent high variability and low consistency values, are still the best practical tools available. However, they still do not give good distinction between morbidity and mortality or good values for low-grade morbidity, which may leave a person susceptible to other infections.

Within the laboratory, detection, isolation, culture and quantification techniques are also difficult, or impossible, for many of these organisms; some are obligate intra-cellular parasites and can not be cultured outside their living host cells. False positive and false negative results are common. Since we are concerned here with the actual pathogens, and not with indicators as with fecal coliforms or E. coli as for bacterial guidelines, the relevant safe number or infectious dose is often zero. The way to achieve this is to set treatment standards designed to remove or kill all pathogens that are present rather than the impossible task of finding water supplies with no pathogens in them and maintaining them in that state, or trying to monitor small fluctuating numbers of organisms. This is a departure from the usual physical or chemical guideline, which is justifiably a non-zero number since organisms have evolved to cope with background levels of all naturally occurring substances in the environment. For biological contaminants which the immune system can not handle, the number is zero since the organisms are capable of reproducing to ultimately large numbers if even one is originally present.

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Many of these organisms are relatively rare in the environment or, even if common, very rarely cause disease in normal immunocompetent people. The risk of disease is extremely low for some of them and should be of little concern to normal healthy people; however, the consequences may be fatal for those few who do acquire some pathogens. It is the immunoincompetent, such as AIDS and transplant patients that are at greatest risk. One reason that many of these diseases have come into prominence recently is likely due to the recent AIDS epidemic, the prevalence of intravenous drug users and high risk sexual practices among such vulnerable people. Some organisms are, of course, extremely pathogenic and must be removed from water supplies unless the entire population can be vaccinated or in some way rendered immune.

While these reports are primarily concerned with water borne pathogens many of these organisms are also opportunistic and will spread directly from man to man, often via the fecal/oral route. Human behaviour is the weak link here; sexual practices and poor hygiene contribute to self-re-infection and person-to-person spread. Once in man they are then released again into the water to continue the cycle. It is important to break the cycle in as many places and as often as possible to reduce the pathogen load back to the water. Thus, many of the recommendations given are not directly related to water transmission but are related to human behaviour. The recommendations given will also control bacterial and viral disease incidence since they relate to drinking water and domestic sewage treatment processes and to human behaviour.

Existing guidelines for many physical and chemical contaminants in water, and also for biological contaminants like bacteria, through the use of a surrogate measure like fecal coliforms or E. coli, are based on numerical standards. Numerical standards are appropriate for physical and chemical contaminants but not for biological pathogens where both the geographical and statistical distributions are intrinsically not normal, either spatially or temporally. For pathogens which can reproduce, a fixed number at a given place and time does not lead to predictable infectious doses delivered to the host. For such contaminants, helminth worms, protozoans, bacteria and viruses, it is more productive, and offers better protection from infection, to prescribe treatment regimes designed to remove the organisms rather than define numbers that have no defensible risk factor attached to them. Measurement of numbers for such pathogens is often impossible, rarely reproducible, inaccurate, commonly rife with false positives and false negatives, changes constantly, correlates poorly if at all with disease or infection risk and does not necessarily determine whether or not the detected cyst is actually viable.

The risk of infection is a function of many factors including but not limited to the numbers of spores or other infectious agents, the number of people in the habitat where the organisms are found, the behaviour of the people, sewage treatment processes, drinking water treatment processes, watershed conditions, the number of already infected people, the prevalence of alternate hosts, their infection rate and climatic conditions. Only a few of these can be controlled to influence pathogen numbers and distribution. Drinking water and sewage treatment processes are the main influences under our direct and immediate control. Since such processes and infrastructures are already in place it is easiest, and likely the most economical, to modify them to the extent that pathogen control is achieved, rather than devise new control mechanisms. If we can reduce the number of organisms getting into people and kill most of those leaving people we can greatly reduce the disease incidence even if we can not influence behaviour.

There are a number of instances of co-infection or dependent species pairs involved in some diseases and in such cases control of the species of concern may actually necessitate control of the other species too. Dientamoeba fragilis is often linked to the pinworm, Enterobius vermicularis, and may gain access to the body in pinworm eggs or worms. Controlling pinworm spread may reduce the rate of Dientamoeba infections. The protozoan Hartmannella veriformis is a host protozoan within which the bacteria Legionella pneumophila multiplies to very large numbers. While Hartmannella may not cause a serious disease in man, Legionella does; one infected Hartmannella cell may introduce an infective dose of Legionella into the human body.

Water Borne

These techniques are all designed to reduce the number of pathogens below the infective dose, which in some cases is as low as one organism, ideally to zero. In order that the necessary sub-micron filters work and have a reasonable life span the bulk of the suspended materials in the water needs to be first removed by conventional sand filtration, possibly with prior flocculation/coagulation/sedimentation. This pre-filtration will also remove strains of blue-green algae which release water soluble endotoxins when they lyse. These must be removed early in the water treatment process rather than killed later preventing the dead cells from releasing their toxins into the water supply. Sterilization techniques such as chlorination, ultraviolet light, ionizing radiation and ozone will not work effectively or efficiently unless such pre-filtration occurs to remove organic compounds and particulate materials. Disinfection must follow filtration since amoeboid protozoans; bacteria and viruses may penetrate filtration processes designed to remove spores and trophozoites.

Some of these techniques are not practical, economical, socially acceptable or useful for other reasons on a large scale but do have some value for restricted uses of small quantities of water. On a practical and economical basis it is impossible to guarantee sterile water on a large scale with an extensive distribution system. What can be done is reduce the risk to an acceptable level and permit the immune system of healthy people to cope with the residual. There ill always be an additional need to provide more expensive, smaller quantities of absolutely sterile water to people at special risk, for example kidney dialysis and extensive burn washing, as is currently done in hospitals.

Immunocompromised patients with diseases such as AIDS also need sterile water. Immunocompromised persons traveling to Latin America, Africa or other developing regions should exercise precautions when eating and drinking, in order to avoid infection with multiple gastrointestinal pathogens including Isospora, Cyclospora, microsporidium and Cryptosporidium. These precautions include the use of sub-micron filters or boiling for purifying water, eating well-cooked foods and avoiding many recreational water activities.

Bottled water is not necessarily an alternative to boiled tap water. Bottled water suppliers rarely test for Cryptosporidium or other parasites and oocysts that can live for weeks in water, even refrigerated water. Bottled water from a ground water source, such as an artesian well, would have less likelihood of becoming contaminated with Cryptosporidium and other parasites than would surface water. Bottled waters with labels that specifically indicate treatment with ozone, ultraviolet light, sub-micron filtration or reverse osmosis are probably safest but not absolutely safe.

Other Vectors

Many, but not all, of these are designed to break the anal/oral transmission cycle.

Drinking Water Treatment

All drinking should be disinfected. However, chlorine is known to be ineffective against many spores, including Cryptosporidium, and UV has been demonstrated to kill these spores. UV should be required as the main disinfectant with chlorine used for residual action against bacteria in distribution systems.

Sewage Treatment

Administrative, Political and Regulatory

Given what science knows about these and other diseases and their spread some serious questions need to be addressed concerning political, administrative and regulatory inaction, complacency, non-enforcement, non-compliance and counter-productive laws and regulations. We know how to do a better job, not perfect but much better, but we are not doing it. We need a faster, more interactive and proactive way to enact water safety provisions as new information becomes available instead of a cumbersome, reactive process which only comes into play years after people have suffered and died. Our whole medical administrative system is geared towards treatment instead of prevention.
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Pathogens, particularly parasites, are generally quite species-specific and rarely infect another host. They may, however, be found in other species, which act as reservoirs from which they may be passed to man. In some cases other species are the necessary alternate host for the completion of the life cycle of the pathogen before it can re-infect man. Generally, it is not practical, possible or ecologically acceptable, to eliminate the alternate hosts from the environment and it is up to man to prevent the spread from the other organism to man and from man-to-man. This may be as simple as cooking meat well or removing your bathing suit immediately after swimming and drying with a towel. It may also be as complex as flocculation, sedimentation and sub-micron filtration of raw drinking water followed by ozonation, chlorination, ultraviolet irradiation or exposure to ionizing radiation.
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It is rarely possible, and quite unnecessary, to use sterile or pathogen-free water for irrigation. Ground water sources will reduce or eliminate pathogen levels in irrigation water. However, the soil is also a reservoir for pathogens, which will reach the crop by splashing, cultivation, wind-blown dust and direct contact. Crops meant to be eaten raw, such as salad greens, need to be rinsed at the grower, wholesale, retail and consumer level to remove such pathogens. Sub-surface, drip and flood irrigation offer safer ways to prevent the spread of some pathogens to the crops and to the farm workers. Spray irrigation produces aerosols, which may be a health hazard to farm workers and percolates into the heads of leafy crops. If the water supply is known to be contaminated then the workers may need to use protective clothing and masks at certain times.
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Water used in industrial processes and as cooling water is normally recycled which leads to a build-up of pathogens. Air conditioners, humidifiers, de-humidifiers, refrigerators and similar devices have water reservoirs, which can become contaminated. In many industrial processes aerosols are formed from recycled water which can be quite high in pathogens. Workers may need protective clothing and masks in some situations. The exhaust vents for such devices should be located so that the general public is not directly exposed. Generally, these are bacterial pathogens; most worm or protozoan pathogens will not multiply under these conditions.
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Internet Pages

Paper Documents

Papers dealing strictly with clinical, diagnostic and therapeutic aspects have been eliminated from this list.

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For further information
Phone: (250) 387-9513
Fax: (250) 356-8298
Email: Dr. Patrick Warrington

This page was last updated November 6, 2001