Lake-associated outbreak of Escherichia coli O157:H7 in Clark County, Washington, August 1999

Context: Escherichia coli O157:H7, one of hundreds of strains of the gram-negative bacterium E coli, has been implicated in numerous lake-borne outbreaks of infection during the past decade. In August 1999, several children who later became ill with E coli O157:H7 infection reported swimming in a lake in Clark County, Washington. The lake was closed and an investigation begun. Objectives: To identify the source of the outbreak and determine risk factors for infection with E coli O157:H7. Design, Setting, and Patients: Two case-control studies were performed among residents of and visitors to Clark County in August 1999 by using community and campground-registrant control subjects. Main Outcome Measure: Risk factors for infection with E coli 0 157:H7 among Clark County residents or visitors. Results: We identified 37 case patients (including 29 primary-case patients) with a median age of 5-years (age range, 1-14 years for primary-case patients). Eight children were hospitalized, 3 with hemolytic uremic syndrome; none died. With analysis restricted to primarycase patients, illness was strongly associated with swimming in the lake (18 of 18 case patients vs I of 18 neighborhood-matched and age-matched control subjects; matched odds ratio undefined; P<.001). All primarycase patients were children younger than 15 years who swam in the lake. Illness was associated with placing the head underwater, getting lake water in the mouth, or swallowing lake water (26 of 27 case patients vs 43 of 62 control subjects; matched odds ratio= 11.5; P=.005). Cultures of lake water yielded E coli O157:H7 that matched the outbreak strain according to results of pulsed-field gel electrophoresis. Conclusions: To date, this is one of the largest documented outbreaks of E coli O157:H7 infection associated with unchlorinated recreational water and represents the first outbreak in which the strain was isolated from lake water. Guidelines are needed to decrease the risk of enteric illness associated with swimming in recreational lakes.