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Lower Grand Watershed Interactive Tool (WIT) - Pathogens

Pathogens are human-disease-causing bacteria or viruses

If pathogens levels are too high in a water body, it can affect the state-designated uses of full body contact or partial body contact. For more information, download the DEQ factsheet on bacteria.


  • The determination of water-borne pathogens as the causative factor in an outbreak uses the classical detective work of epidemiological studies.
  • In 1854, a cholera outbreak in London, England was shown to be linked to a pump that derived its water from a polluted section of the River Thames. People served by a pump that obtained its water upstream of London had a low incidence of cholera.
  • In 1855-1856, Dudd and Shaw showed that there was an association between typhoid fever in a street in Bristol, England and the use of a particular pump for water. People on an alternate pump did no become infected. This was 30 years before the causative agent for typhoid was identified. The appreciation of water as a carrier of disease-producing organisms only came about in the mid-1880's with the establishment of the germ theory by Pasteur.
  • At the beginning of this century, cities in the Ohio River watershed commonly had death rates due to typhoid on the order of 75 deaths per 100,000 people. Upon the installation of drinking water treatment plants, the death rate fell to about 15/100,000.

The occurrence of pathogens in drinking water supplies is currently a serious problem in developing countries but is also of concern in developed countries. The number of water-borne disease outbreaks in the U.S. decreased steadily from 1920 - 1960 in direct relation to the increase of chlorination systems, but has increased since then. Whether this is a real trend, or a change in the ability to identify the disease source, is arguable.

Pathogen Characteristics

Most water-borne pathogens may be classified as viruses, bacteria, or protozoa. They typically cause intestinal diseases, leaving the host in the fecal material, contaminating the water supply, and then entering the recipient by ingestion. Their survival period in water varies widely and is influenced by many factors such as salinity, temperature, etc. It may be generalized that cellular viruses last longer than bacteria, while protozoa can extend their survival time by forming protective cysts.


  • Although there are over 100 known water-borne human enteric viruses, infectious hepatitis A, poliovirus and viral gastroenteritis are of practical concern as water-borne viruses.
  • All of the enteric viruses, with the exception of the infectious hepatitis agent, have been found in sewage and polluted rivers.
  • Tests for the presence of viruses in water supplies are difficult and uncertain and so little is known of the survival time and concentration distribution of viruses in water sources.
  • In general, enteric viruses survive less than three months in the environment but have been reported surviving up to five months in sewage.
  • There is dispute over whether a minimum infectious dose is necessary as for bacteria, some researchers claiming that a single virus is sufficient for infection.
  • Due to their small size and surface properties, viruses tend to be adsorbed onto surfaces.


  • Bacteria comprise the largest group of water-borne pathogens.
  • A minimum infectious dose of several hundred to several thousand organisms is necessary to cause bacteriological infection.
  • Pathogenic bacteria are usually poor competitors at low substrate levels found in natural waters and so tend to be eliminated by competition and predation.
  • Low temperatures, sediment adsorption and anoxic conditions occasionally prolong their survival.

The most common bacteriological diseases are:

  • Shigella sp., the cause of dysentery, is almost strictly a human affliction (minor in other primates). Most shigellosis and salmonellosis epidemics in developed countries are food-borne but a few are caused by drinking water. Transmission by drinking water is still the major route of infection in underdeveloped countries. Methodology for detection is not reliable. Die-off is rapid in sewage, although low salt concentrations and temperatures will extend survival times (25 days at 13 °C, 4 days at 37 °C).
  • Salmonellosis incidence (a form of food poisoning) is low, and peaks seasonally in mid to late summer due to favorable conditions for food-borne salmonella. Salmonella species are carried by humans (1-4% of population), farm animals (13-17% incidence) and wild animals. Most Salmonella species cause gastrointestinal diseases, while one, which is strictly a human pathogen, causes typhoid.
  • Enteropathogenic E. coli produce gastroenteritis and urinary infections. Carrier rates vary but may be 16% in mothers of newborn infants, 7% in food handlers, and 3% in children. Farm animals are also carriers. Concentrations of E. coli in effluents to natural waters reduce to 5% of original levels in less than 5 days.
  • Tularemia is passed principally by ticks, rodents, and direct contact with sewage. Water contamination occurs from rodents. The disease spreads via the lymphatics and bloodstream. It grows intracellularly and causes lesions in the lungs, liver, spleen and brain.
  • Leptospirosis, which begins as a wound infection, is an occupational disease among workers in close contact with polluted water. Pigs, dogs, rodents and humans are carriers, and it is excreted in urine of infected animals.
  • Cholera is a serious, highly contagious disease causing dramatic and fatal loss of water and electrolytes. Healthy carriers may make up 1-9% or even 25% of the population


The variety of pathogenic organisms in water supplies is large but their concentrations are low. Testing for each one in order to monitor water quality is an expensive and unsure proposition. Tests for some pathogens are unreliable and may require an unacceptably long incubation period for the quick response that may be required for public safety. Thus, the concept of an indicator organism is used to indicate the possible presence of disease-causing constituents.

An indicator organism should behave as follows:

  • be applicable to all water - be present when pathogens are
  • have no aftergrowth in water - be absent when pathogens are
  • have constant characteristics - persist longer than pathogens
  • be harmless to humans - correlate quantitatively with pathogens
  • be present in greater numbers than pathogens
  • be easily, accurately and quickly detected

Why are Escherichia coli and coliforms used as indicator organisms?

The coliform group nearly fulfills the criteria listed above. Escherichia coli is considered a reliable indicator of bacterial pathogens. Protozoans and viruses, however, usually survive longer than E. coli and may also survive disinfection that is otherwise adequate for bacteria. Filtration is usually successful in extracting protozoans and viruses and should be used in conjunction with disinfection procedures.

Source: University of Waterloo. Environmental Microbiology 447. "Module 4. Pathogens in Water" Canada. 10/7/2002.

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Page last modified January 19, 2011