There are 215,000 water systems in the United States classified as public systems that are regulated under the Safe Water Drinking Act and the Surface Water Treatment Rule.2 These regulations require water utilities to disinfect surface water and groundwater by using parameters such as turbidity, coliform counts, and the presence of human enteric viruses and Giardia lamblia cysts as target organisms to assess efficacy of treatment.2 The surveillance system for waterborne-disease outbreaks is similar to that for foodborne illnesses. Criteria for a waterborne outbreak is the presence of a similar illness in two or more people after the ingestion of drinking water or after exposure to water used for recreational purposes. Epidemiologic evidence must implicate water as the probable source of infection. 2
The CDC identified 30 outbreaks of waterborne disease in the United States associated with drinking water causing illness in 405,366 people from 1993 to 1994. 2 A single outbreak of cryptosporidiosis, the largest waterborne-disease outbreak ever reported in the United States, accounted for 403,000 cases from which 4400 people were hospitalized. The outbreak was traced to a community water system. Surface water from Lake Michigan had been filtered and chlorinated, but a deterioration in the water quality and the decreased effectiveness of the coagulation-filtration process led to an inadequate removal of Cryptosporidium parvum oocysts. The treated water had met all state and federal quality standards that were then in effect. The outbreak was recognized by widespread absenteeism among employees of hospitals and among students and school teachers, a shortage of antidiarrheal drugs citywide, and a marked increase in the number of emergency department visits.2 A single waterborne outbreak associated with Salmonella, infecting 625 people, occurred when municipal water-storage towers were inadequately protected from wild-bird droppings.2
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