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 Connectic ut and Illinois during May 28 to June 27, 1996, was investigated to determi ne the source of infections. Methods: Independent case-control studies were performed in both states. Pu lsed-field gel electrophoresis (PFGE) was performed on E coli O157:H7 isola tes. A case-patient was defined as a Connecticut or northern Illinois resid ent with diarrhea whose stool culture yielded E coli O157:H7 of the outbrea k-associated PFGE subtype. Controls were town-, age-, and sex-matched to ca se-patients. We traced implicated lettuce to the farm level and performed e nvironmental investigations to identify unsafe lettuce production practices . Results: In Connecticut and Illinois, infection was associated with consump tion of mesclun lettuce (Connecticut matched odds ratio [MOR], undefined; 9 5% confidence interval [CI], 3.4 to infinity; and Illinois MOR, undefined; 95% CI, 1.4 to infinity). We traced implicated lettuce to a single grower-p rocessor. Cattle, a known E coli O157:H7 reservoir, were found near the let tuce fields. Escherichia coli (an indicator of fecal contamination) was cul tured from wash water and finished lettuce. A trace-forward investigation i dentified 3 additional states that received implicated lettuce; E coli O157 :H7 isolates from patients in 1 of these states matched the outbreak-associ ated PFGE subtype. Conclusions: This multistate outbreak off coli O157:H7 infections was assoc iated with consumption of mesclun lettuce from a single producer. Molecular subtyping facilitated the epidemiological investigation. This investigatio n increased the knowledge about current production practices that may contr ibute to the contamination of lettuce by microbial pathogens. Lettuce produ ction practices should be monitored for microbiological safety.