A LARGE COMMUNITY OUTBREAK OF SALMONELLOSIS CAUSED BY INTENTIONAL CONTAMINATION OF RESTAURANT SALAD BARS

Context.-This large outbreak of foodborne disease highlights the chall enge of investigating outbreaks caused by intentional contamination an d demonstrates the vulnerability of self-service foods to intentional contamination. Objective.-To investigate a large community outbreak of Salmonella Typhimurium infections. Design.-Epidemiologic investigatio n of patients with Salmonella gastroenteritis and possible exposures i n The Dalles, Oregon. Cohort and case-control investigations were cond ucted among groups of restaurant patrons and employees to identify exp osures associated with illness. Setting.-A community in Oregon. Outbre ak period was September and October 1984. Patients.-A total of 751 per sons with Salmonella gastroenteritis associated with eating or working at area restaurants. Most patients were identified through passive su rveillance; active surveillance was conducted for selected groups. A c ase was defined either by clinical criteria or by a stool culture yiel ding S Typhimurium. Results.-The outbreak occurred in 2 waves, Septemb er 9 through 18 and September 19 through October 10. Most cases were a ssociated with 10 restaurants, and epidemiologic studies of customers at 4 restaurants and of employees at all 10 restaurants implicated eat ing from salad bars as the major risk factor for infection. Eight (80% ) of 10 affected restaurants compared with only 3 (11%) of the 28 othe r restaurants in The Dalles operated salad bars (relative risk, 7.5; 9 5% confidence interval, 2.4-22.7; P<.001). The implicated food items o n the salad bars differed from one restaurant to another. The investig ation did not identify any water supply, food item, supplier, or distr ibutor common to all affected restaurants, nor were employees exposed to any single common source. In some instances, infected employees may have contributed to the spread of illness by inadvertently contaminat ing foods. However, no evidence was found linking ill employees to ini tiation of the outbreak. Errors in food rotation and inadequate refrig eration on ice-chilled salad bars may have facilitated growth of the S Typhimurium but could not have caused the outbreak. A subsequent crim inal investigation revealed that members of a religious commune had de liberately contaminated the salad bars. An S Typhimurium strain found in a laboratory at the commune was indistinguishable from the outbreak strain. Conclusions.-This outbreak of salmonellosis was caused by int entional contamination of restaurant salad bars by members of a religi ous commune.