health risks resulting from microorganisms in water. These approaches include (1) assessments of epidemiologic evidence for waterborne-based outbreaks; (2) human volunteer studies showing that a known or potential waterborne pathogen is infectious by the oral ingestion route and capable of causing infection and disease at particular doses (dose-response studies); (3) various types of retrospective and prospective epidemiologic studies for health effects assessments; (4) estimates of health risks by linking epidemiologic evidence for disease to measured concentrations of either pathogens or indicators in the water; (5) estimates of the ratios of pathogens to indicators in the exposure vehicle (e.g., feces, sewage, fecally contaminated water); and (6) quantitative microbial risk assessments that integrate human exposure and health effects data for quantitative risk estimations or characterizations.

Health Effects Concerns and Early Studies of Microbial Water Quality

Outbreak Investigations and Risk Estimates from Pathogen-to-Indicator Ratios in Water

As discussed in Chapter 1, concerns about the sanitary quality of drinking water and the risks of enteric infectious diseases in the United States go back to at least the late 1800s, when enteric disease outbreaks were first recognized and linked at least tentatively to these exposure routes. Similar concerns for U.S. recreational and shellfish waters started in the 1920s. The initial recognition of and concern about infectious disease risks from these sources of exposure focused on enteric bacterial diseases, and early health effects assessments of enteric bacterial pathogens and waterborne outbreaks date back to the early 1920s. Human health risks from enteric viruses and parasites in water were first recognized and addressed during and shortly after World War II. However, civilian risks from these waterborne pathogens were not widely documented and appreciated until studies of waterborne outbreaks and waterborne pathogen occurrence were first reported in the 1960s. The recognized viral and protozoan pathogens of initial concern were infectious hepatitis viruses, polio, and other enteroviruses, and Entamoeba histolytica and Giardia lamblia, respectively.

Perhaps the first attempts at linking health effects assessments of waterborne pathogens to microbial water quality were based on ratios of Salmonella typhi to fecal indicator (coliform) bacteria in feces and sewage and the allowable limits of coliforms in drinking water and, later, in recreational bathing and shellfish waters (Kehr and Butterfield, 1943; Prescott et al., 1945).

Early Health Effects Assessments of Enteric Pathogens from Human Dose-Response Studies

The first human health effects dose-response studies appear to be with the

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