In 1997 the first outbreak of Escherichia coli O157[ratio ]H7 infections involving 14 cases occurred in Finland. A case was defined as a resident of Alavus with an episode of diarrhoea between 5 and 17 July 1997, and from whom E. coli O157[ratio ]H7 was isolated from stool. The investigation included case searching and a population-based case control study. Five primary and eight symptomatic secondary cases of E. coli O157[ratio ]H7 illness were detected. In the 10 days before the outbreak, all 5 primary patients (aged 3–8 years), but only 6 of 32 population controls from the same age range (Fisher's test, P<0·001) and 4 of 10 sibling controls (P<0·05) had visited (but had not necessarily bathed in) a shallow beach popular among young children. Four out of 5 primary cases had remained within 5 m of the beach while swimming and had swallowed lake water compared to 1 of 5 population controls. These analytical epidemiologic findings incriminated fresh lake water as the vehicle of E. coli O157[ratio ]H7 transmission.
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