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.