A large, multiple-restaurant outbreak of infection with Shigella flexneri serotype 2a traced to tomatoes

Background. Foodborne outbreaks of Shigella infection are uncommon and tomatoes are an unusual vehicle. We describe a large, multiple-restaurant outbreak of Shigella flexneri serotype 2a infection that was associated with tomatoes. Methods. We conducted nationwide surveillance and a case-control study, collected fecal specimens for culture, and measured the survival of the outbreak strain of S. flexneri in tomatoes. Results. We interviewed 306 of 886 ill restaurant patrons and 167 control subjects. Matched univariate analysis showed that several food items were associated with illness, but only tomatoes remained significant in multivariate models. Illness peaked at each restaurant within 24 h after the arrival of hand-sorted bruised and overripe tomatoes from a new distributor; all patient isolates that were tested were indistinguishable by PFGE. Sliced tomatoes from the distributor were inoculated with the outbreak strain, and viable S. flexneri were recovered for 72 h. Conclusion. To prevent such outbreaks, persons with shigellosis should be excluded from handling food at all points along the distribution chain.