Outbreak of Shigella flexneri linked to salad prepared at a central commissary in Michigan

In August 1992, the Michigan Department of Public Health was notified of a cluster of persons with Shigella flexneri infections, all of whom had eaten at different outlets of a single restaurant chain. The chain prepared many foods at a central kitchen. A matched case-control study to determine risk factors for illness among patrons of the restaurant chain was undertaken. An inspection of the commissary and a review of commissary inspection and employee records were conducted. Of the 46 patients identified, 44 had eaten tossed salad versus 33 of the 71 controls (matched odds ratio = 56.9; 95 percent confidence interval 5.0, 648.1). Improper salad preparation techniques were used, and the salad preparation area had not been inspected in several years. Some salad preparers had been ill shortly before the outbreak but continued to work. The greater use of central kitchens could lead to larger outbreaks of illness related to improper food handling. Raw vegetables are a potential vehicle for transmission of shigellosis. Coordination of all agencies responsible for inspecting commissaries should be assured. Assuring restriction of ill food handlers will require management to take an active role in identification of ill employees and in the development of incentives to report illness.