A multistate outbreak of Escherichia coli O157:H7 infections associated with consumption of mesclun lettuce

Background: An outbreak of Escherichia coli O157:H7 infections in Connecticut and Illinois during May 28 to June 27, 1996, was investigated to determine the source of infections. Methods: Independent case-control studies were performed in both states. Pulsed-field gel electrophoresis (PFGE) was performed on E coli O157:H7 isolates. A case-patient was defined as a Connecticut or northern Illinois resident with diarrhea whose stool culture yielded E coli O157:H7 of the outbreak-associated PFGE subtype. Controls were town-, age-, and sex-matched to case-patients. We traced implicated lettuce to the farm level and performed environmental investigations to identify unsafe lettuce production practices. Results: In Connecticut and Illinois, infection was associated with consumption of mesclun lettuce (Connecticut matched odds ratio [MOR], undefined; 95% confidence interval [CI], 3.4 to ∞; and Illinois MOR, undefined; 95% CI, 1.4 to ∞). We traced implicated lettuce to a single grower-processor. Cattle, a known E coli O157:H7 reservoir, were found near the lettuce fields. Escherichia coli (an indicator of fecal contamination) was cultured from wash water and finished lettuce. A trace- forward investigation identified 3 additional states that received implicated lettuce; E coli O157:H7 isolates from patients in 1 of these states matched the outbreak-associated PFGE subtype. Conclusions: This multistate outbreak of E coli O157:H7 infections was associated with consumption of mesclun lettuce from a single producer. Molecular subtyping facilitated the epidemiological investigation. This investigation increased the knowledge about current production practices that may contribute to the contamination of lettuce by microbial pathogens. Lettuce production practices should be monitored for microbiological safety.