Swimming-associated haemorrhagic colitis due to Escherichia coli O157[ratio ]H7 infection: evidence of prolonged contamination of a fresh water lake

Abstract

We describe an Escherichia coli O157[ratio ]H7 outbreak associated with a fresh water lake at a county park. Campers were surveyed for diarrhoeal illness within 10 days of their visit, and a case-control study of day visitors was conducted. A confirmed case was a symptomatic person with a stool culture positive for E. coli O157[ratio ]H7 and a probable case was a person with bloody diarrhoea. Clinical isolates of E. coli O157 were subtyped by pulsed field gel electrophoresis (PFGE). In the camper survey, 12 (38%) of 32 swimmers had a diarrhoeal illness (relative risk [RR]=12·4; 95% confidence interval [RR]=1·7–89·7). For the case-control study, the 12 cases were more likely than controls to have purposefully ingested lake water (odds ratio [OR]=6·9, 95% CI=0·9–55·8). The PFGE patterns of six clinical isolates were indistinguishable. This report further demonstrates that contaminated fresh-water lakes can be the source of community outbreaks of E. coli O157[ratio ]H7.