Swimming-associated outbreak of Escherichia coli O157 : H7

In 1997 the first outbreak of Escherichia coli O157:H7 infections involving 14 cases occurred in Finland. A case was defined as a resident of Alavus w ith an episode of diarrhoea between 5 and 17 July 1997, and from whom E. co li O157:H7 was isolated from stool. The investigation included case searchi ng and a population-based case control study. Five primary and eight sympto matic secondary cases of E. coli O157:H7 illness were detected. In the 10 d ays 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 < 0.0 01) and 4 of 10 sibling controls (P < 0.05) had visited (but had not necess arily 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 I of 5 population controls. These analyti cal epidemiologic findings incriminated fresh lake water as the vehicle of E, coli O157:H7 transmission.