Skip booklet index and go to page content

Threats to Sources of Drinking Water and Aquatic Ecosystem Health in Canada

1. Waterborne Pathogens

Freshwater bodies require protection from pathogens that pose threats to drinking water supplies, recreational waters, agricultural and aquacultural source waters, and to aquatic ecosystems in general.Tom Edge,1 James M. Byrne,2 Roger Johnson,3 Will Robertson4 and Roselynn Stevenson5

1Environment Canada, National Water Research Institute, Burlington, ON
2University of Lethbridge, Water Resources Institute, Lethbridge, AB
3Health Canada, Laboratory for Foodborne Pathogens, Guelph, ON
4Health Canada, Water Quality Division, Ottawa, ON
5University of Guelph, Department of Microbiology, Guelph, ON


Current Status

The emergence and spread of infectious disease in plant, animal and human populations is a problem in Canada and around the world. Water is a common element in the ecology of many pathogens affecting these populations. Waterborne pathogens can pose threats to drinking water supplies, recreational waters, source waters for agriculture and aquaculture, as well as to aquatic ecosystems and biodiversity.

The World Heath Organization has stated that infectious diseases are the world's single largest source of human mortality (WHO 1996). Many of these infectious diseases are waterborne and have tremendous adverse impacts in developing countries. While developed countries have been more successful in controlling waterborne pathogens, water quality problems are still prevalent in Canada and the United States. Based on U.S.A. estimates (ASM 1999a), it is possible that about 90,000 cases of illness and 90 deaths occur annually in Canada as a result of acute waterborne infections. Waterborne pathogens can pose significant human health threats as the drinking water outbreak in Walkerton, Ontario, demonstrated last year, where seven people died and 2500 became ill. A drinking water incident in Milwaukee, Wisconsin, in 1993 resulted in 54 deaths and over 400,000 cases of illness (Hoxie et al. 1997).

Waterborne disease outbreaks in Canada have been monitored by Health Canada since 1974. Between 1974 and 1996, the last year for which data are available, over 200 outbreaks of infectious disease associated with drinking water were reported (Todd and Chatman 1974--1996). There were greater than 8000 confirmed cases linked to these outbreaks. However, depending upon the severity of the symptoms, the actual number of cases can be 10 to 1000 times greater than the number of reported cases. Public attention has recently focused on a variety of drinking water threats from bacteria (E. coli O157:H7 in Walkerton, Ont., in 2000), to protozoa (Toxoplasma gondii in Victoria, B.C., in 1995) and viruses (Hepatitis A in Ile d'Orleans, Que., in 1995).

Bacteria (primarily Salmonella, Shigella and Campylobacter) were responsible for 78 drinking water outbreaks in Canada between 1974 and 1996. Protozoa and unidentified pathogens were responsible for 59 and 43 outbreaks, respectively. Enteric viruses, primarily Norwalk and Hepatitis A, were responsible for 23 outbreaks. The number of outbreaks caused by protozoa is particularly interesting. In 1990, the cumulative total was only 20, but outbreaks caused by protozoa have tripled since then. Most of these outbreaks were caused by Giardia, but in 1993 the first reported outbreak caused by Cryptosporidium occurred in Canada. Since then, outbreaks of giardiasis and cryptosporidiosis have occurred in all regions of the country. In 1995, the first outbreak of toxoplasmosis (Toxoplasma gondii) linked to a municipal drinking water supply occurred in British Columbia. The large number of outbreaks caused by unknown agents is also of interest. It is probable that many of these were caused by enteric waterborne viruses.

Waterborne pathogens also pose threats to ambient recreational waters resulting in illnesses and economic impacts on local communities. From 1992 to 1995 there were 2839 beach closings reported for 169 public beaches on lakes Ontario, Erie, Huron and Superior (Health Canada 1998). Pathogens such as Cryptosporidium and Giardia are known to occur across Canada in aquatic ecosystems that serve as sources of recreation and drinking water. As an example, a study of waterborne pathogens has been underway in southern Alberta since mid-1999 (J. Byrne, personal communication). Health Canada and University of Lethbridge research scientists have monitored Salmonella, E. coli 0157:H7 and coliform/fecal coliform (C/FC) populations in the Oldman River basin. This area includes an extensive network of irrigation canals associated with intensive livestock production. In 1999 and 2000, a number of these pathogens were identified, with pathogen populations peaking in mid to late summer of both years. In late July 2000, almost half of the monitoring sites tested positive for one or more pathogens and researchers found E. coli 0157:H7 in Park Lake, Park Lake Provincial Park, one of the most popular recreation locations in southern Alberta. FC populations in the Oldman basin typically exceeded the Guidelines for Canadian Recreational Water Quality (GCRWQ) in most samples, and in many cases, FC counts exceeded GCRWQ by a factor of five or more.

Pathogen contamination of aquatic ecosystems is known to occur from a range of sources including municipal wastewater effluents, agricultural wastes, and wildlife. This contamination can pose threats to water sources required for agriculture and aquaculture.

For example, pathogen contamination of irrigation waters or shellfish areas can pose risks to human food supplies. Such pathogen threats can lead to significant health care and economic impacts as well as significant trade implications and intrusive disease controls. Aquaculture exists in intimate contact with aquatic ecosystems. The presence of pathogens in source waters can severely limit success of food fish production for domestic consumption and export (Hedrick 1998).

Another critical aspect of waterborne disease is the threat that pathogens can pose to aquatic ecosystems and biodiversity. Much like the concern about emerging human pathogens, there is growing concern about non-human pathogens and their impacts on wildlife and biodiversity in Canada and around the world (CCWHC 1999; Daszak et al. 2000). For example, outbreaks of botulism caused by Clostridium botulinum have caused substantial waterfowl mortalities at locations across Canada. In addition, newly emerging fungal and viral pathogens have contributed to significant declines in amphibian populations around the world from frogs in South America to tiger salamanders in Saskatchewan (Carey 2000).

The current status of many pathogen threats to drinking water and aquatic ecosystems remains uncertain although they are likely underestimated. These threats are accompanied by growing concerns about pathogens as causative factors in chronic diseases such as ulcers, cancer and heart disease where infectious agents were not previously suspected of being involved (ASM 1999b). Additional surveillance and scientific research is required to better understand the nature of these pathogen threats.


The emergence and spread of waterborne pathogens is likely to be a growing problem in Canada and around the world. Increasing human population densities, along with rapidly expanding and increasingly intensive livestock/poultry production, pose significant animal and human waste management challenges. The possible threats from "pathogen pollution" will need careful surveillance since it can be anticipated there will be increasing releases of plant, animal and human pathogens into aquatic ecosystems in the future from a wide variety of sources including: municipal wastewaters, septic tanks, land application of sewage sludges, stormwater overflows, slaughter houses and rendering plants, livestock and poultry wastes, landfill sites, and biology-based research and manufacturing facilities. Surveillance efforts will also need to consider the fate of virulence and antibiotic resistance genes from microorganisms released into aquatic ecosystems. The spread of antibiotic resistance through gene transfer events is leading to the emergence of pathogens that are increasingly more difficult and expensive to treat.

Expanded human and agriculture densities can also compress wildlife populations, and increase the risk of amplification and dissemination of pathogens from wildlife into drinking water supplies and water-dependent food supplies. It is also possible there could be increasingly novel pathogen sources from biomedical wastes, or even use of biological weapons, that could lead to contamination of drinking water sources and aquatic ecosystems.

Together, these sources of pathogens will place greater demands on an inadequate and aging wastewater treatment infrastructure in Canada. A strategy will be required to manage the risks to humans, agriculture, aquaculture and aquatic ecosystems from the increasing quantities of potentially infectious wastes that will require storage, handling, transportation, treatment and disposal.

Globalization of travel and trade flows will also place increasing stress on our capacity to assess and control the threat of potentially infectious materials or organisms that may be imported into Canada and enter aquatic ecosystems. Pathogens can be transported across borders in ways such as contaminants in food, ship ballast waters and travelers. The introduction of alien species into aquatic ecosystems could also lead to spread of associated non-native pathogens. Additional pathogen threats to water quality could be posed by water diversions, changing agricultural practices and water use patterns, and global climate change. Climate factors and weather conditions can affect the prevalence and transmission of infectious diseases through effects on vectors like mosquitoes, as well as pathogen reservoirs and hosts.

Knowledge and Program Needs

While there has been considerable research to investigate the threats to water quality posed by environmental releases of toxic chemicals, comparatively little effort has been directed at studying the potential consequences of environmental releases of harmful microbial pathogens. The unique nature of microorganisms to replicate rapidly, evolve, transfer genes, and disperse in unusual ways, requires new knowledge and new approaches to understand, assess and control the potential environmental and human health threats of waterborne pathogens. The American Society for Microbiology (ASM) recently drew attention to waterborne pathogens by indicating that control of water pollution in the United States over the past two decades has focused on chemical risks, overshadowing the significant risks associated with microbial pollutants (ASM 1999a). The ASM considers microbial pollution of water in the United States to be a growing crisis in environmental and public health that is not being properly addressed through scientific research and risk assessment. It was indicated that waterborne pathogens pose increasing threats due to changing patterns of water use, increasing water pollution, aging wastewater treatment systems, out-moded risk assessment protocols, and an inadequate knowledge of the sources, occurrence, concentrations, survival and transport of specific microorganisms in the environment.

At present there are only limited surveillance data and knowledge of waterborne pathogens in Canada. Enhanced surveillance and research will be required to understand and control the sources of pathogens entering aquatic ecosystems as well as the epidemiological factors associated with infectious disease outbreaks. For example, new knowledge is needed about the ecology of pathogens in aquatic ecosystems, including their survival, transmission, reservoirs, host ranges and adaptive responses to environmental conditions. This knowledge requirement is particularly acute for non-human pathogens and the threats these pathogens may pose to aquatic ecosystems and biodiversity. Much of the concern and attention about infectious diseases has arisen from a human health and agricultural perspective. At present, there is a significant gap in scientific understanding of the infectious disease issue from a broader ecosystem perspective.

The identification and control of threats posed by waterborne pathogens will also require effective pathogen detection techniques. The value and limitations of traditional pathogen indicators like coliform counts must be evaluated since such indicators are unable to accurately assess the presence of certain protozoan and viral pathogens (OECD 1999). Related to this will be the need to develop, evaluate and validate newer molecular tools for pathogen detection such as PCR techniques and DNA microarrays. Rapid advances in fields such as genomics offer the potential to develop improved pathogen detection tools. The responsibility of governments to carefully monitor advances in pathogen detection technology, and apply these technologies in a timely manner to prevent infectious disease outbreaks, was an integral aspect of the Krever Commission's review of the "public health disaster" related to pathogen contamination of the Canadian blood supply.

  • Water management programs in Canada need to take a preventative approach to pathogen pollution and emphasize the importance of source water protection and defining sources of water contamination. Canada should have a source water protection program for all major rivers.
  • Canada needs to develop a National Monitoring Program for: i) collection and analysis of baseline data on waterborne pathogens; and ii) identification of hot spot areas that require targeted research and monitoring.
  • Wastewater management and water supply systems in Canada need to be re-examined to consider the need for enhanced regulations and guidance for preventing pathogen contamination of aquatic ecosystems.
  • Strategic support is needed to enhance funding for waterborne pathogen research in Canada to: i) assess the value of current pathogen indicators; ii) investigate and validate newer molecular detection tools; iii) better understand the ecology of pathogens in aquatic ecosystems; iv) better understand environmental risk factors for predicting disease outbreaks; and v) evaluate current emergency response capacity for pathogens.
  • Canada needs to establish a network for research on pathogens in aquatic ecosystems.
  • All levels of government need to ensure pathogen pollution prevention concerns are integrated into other water management programs.


  • American Society for Microbiology (ASM). 1999a. Microbial pollutants in our nation's water. American Society for Microbiology, Washington, D.C. 16 p.
  • American Society for Microbiology (ASM). 1999b. Congressional briefing--infectious disease threats. American Society for Microbiology, Washington, D.C. 12 p.
  • Canadian Cooperative Wildlife Health Centre (CCWHC). 1999. "Emerging" diseases. Wildlife Health Centre Newslett. 6(1): 3-4.
  • Carey, C. 2000. Infectious disease and worldwide declines of amphibian populations, with comments on emerging diseases in coral reef organisms and in humans. Environ. Health Perspect. 108: 143-150.
  • Daszak, P., A.A. Cunningham and A.D. Hyatt. 2000. Emerging infectious diseases in wildlife--threats to biodiversity and human health. Science 287: 443-449.
  • Health Canada. 1998. Health-related indicators for the Great Lakes basin population. Minister of Public Works and Government Services Canada.
  • Hedrick, R.P. 1998. Relationships of the host, pathogen, and environment: implications for diseases of cultured and wild fish populations. J. Aquat. Animal Health 10: 107-111.
  • Hoxie, N.J., J.P. Davis, J.M. Vergeront, R.D. Nashold and K.A. Blair. 1997. Cryptosporidiosis--associated mortality following a massive waterborne outbreak in Milwaukee, Wisconsin. Am. J. Public Health 87(12): 2032-2035.
  • Organisation for Economic Co-operation and Development (OECD). 1999. Molecular technologies for safe drinking water: results from the Interlaken workshop, Switzerland, 5-8 July 1998. Directorate for Science, Technology and Industry. DSTI/STP/BIO(98)11/FINAL. 26 p.
  • Todd, E.C.D. and P. Chapman. 1974--1996. Foodborne and waterborne disease in Canada. Annual summaries. Laval: Polyscience Publications Inc.
  • World Health Organization (WHO). 1996. World health report 1996: fighting disease, fostering development. Geneva: World Health Organization.