Wyoming E coli O157:H7 outbreak showed hazards in small water systems

Apr 3, 2002 (CIDRAP News) – A 1998 outbreak of Escherichia coli O157:H7 in a small Wyoming town sickened 157 people and pointed up the vulnerability of small, unchlorinated water systems to contamination, according to a report in the April issue of Emerging Infectious Diseases.

The investigators concluded that surface water containing deer and elk feces seeped into the aquifer that provided the town's drinking water. The outbreak "confirms the potential of these small, unprotected and unchlorinated water systems to be an important source of infection with E coli O157:H7 and other pathogens," says the report by Sonja J. Olsen and colleagues from the Centers for Disease Control and Prevention (CDC) and other agencies.

The outbreak surfaced when physicians noted an increase in bloody diarrhea among residents of Alpine, Wyo., in late June of 1998. This led to a search for cases in Wyoming and neighboring states, including cohort studies of Alpine residents and of people who had attended a family reunion in Alpine from June 26 to 28. Subjects were asked about symptoms and about exposure to certain foods and to water from Alpine's system. A case of E coli O157:H7 infection was defined as either a stool culture yielding the organism or diarrhea with onset after June 25 (residents) or June 26 (reunion attendees). The investigation also included an inspection of the town water system.

The investigators found 157 sick people from 15 states, including three children and one adult who had hemolytic uremic syndrome. All four subsequently recovered. Few of the sick people had any exposure to foods known to be associated with E coli O157:H7 transmission, and no recalled food products had been shipped to the area.

In the family reunion cohort, 12 of 41 people interviewed met the case definition. People who drank water from the town system were nine times as likely to get sick as those who didn't (95% confidence interval [CI], 1.3 to 63.3), and this was the only factor significantly associated with the illness. Half of those who drank the water became sick. In the Alpine cohort, 54 (19%) of 284 people interviewed met the case definition. A multivariate analysis showed that drinking town water on the weekend of June 26 to 28 was the only significant risk factor for the illness (odds ratio, 10.1; 95% CI, 1.8 to 56.4). The attack rate for residents who drank water on the weekend in question was 27%—significantly lower than the visitors' 50% rate (P<.01). This suggested the possibility that residents had been exposed previously to E coli O157:H7 via the drinking water or other sources.

The town's water-collection system consisted of a series of perforated pipes and small boxes that collected water 7 to 10 feet below ground level and carried it to an underground storage tank. Pipes from the tank delivered unchlorinated water to the town. After the outbreak, inspectors found that a large pool of water had gathered on the surface over the collection pipes and that deer and elk feces were present from animals that drank from the pool.

Water taken from the storage tank on July 14 had a total coliform count of 108 colony-forming units per 100 mL and contained Enterococcus faecium. However, E coli O157:H7 was not found in five fecal samples or in the water. The authors comment that E coli O157:H7 can survive for long periods in water by changing into a "viable but nonculturable state," in which it cannot be detected by traditional plating methods.

Five of the 18 waterborne outbreaks of E coli O157:H7 infection reported to the CDC between 1982 and 1998 stemmed from contaminated drinking water, and all five involved small water systems or wells supplying rural townships or camps, the article states. "Stronger enforcement of existing regulations and perhaps broadening of current regulations . . . are needed to protect rural drinking water systems in the United States," the authors conclude.

Olsen SJ, Miller F, Breuer T, et al. A waterborne outbreak of Escherichia coli O157:H7 infections and hemolytic uremic syndrome: implications for rural water systems. Emerg Infect Dis 2002;8(4)
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