BACKGROUND: On June 28, 2016, Chicago Department of Public Health (CDPH) received five reports of Shiga toxin-producing Escherichia Coli (STEC) illness. Interviews revealed a common exposure to food items from Restaurant A, with two locations in Chicago, prompting an investigation.
METHODS: A case control study was conducted to determine risk factors for STEC infection. A confirmed case was a person with illness onset June 3–July 23, with isolation of STEC with an outbreak-associated pulsed-field gel electrophoresis (PFGE) pattern. Probable cases had clinically compatible symptoms without laboratory confirmation. Controls were identified by interviewing case meal companions and patrons who ordered meals on-line. Controls were frequency matched 4:1 to primary (restaurant-exposure) confirmed cases by meal date and restaurant location. Bivariate associations between illness and individual food items were assessed. Multivariable logistic regression was performed to evaluate age- and gender-adjusted associations. Food handler interviews and environmental inspections were performed. Food handler rectal swabs and food samples were collected and cultured for STEC.
RESULTS: Sixty-nine confirmed and 37 probable cases were identified. Among the confirmed, 55 were primary cases, four were secondary (household contacts), and 10 had incomplete data or no identifiable epidemiologic linkage. Among 40 food handlers tested, 38 consumed Restaurant A food items, none reported symptoms, and 16 (40%) yielded STEC isolates. Ten distinct PFGE patterns were identified among primary cases, and an additional four among food handlers. Primary confirmed case illness onset dates ranged from June 19-July 3. Consumption of raw cilantro (adjusted odds ratio [aOR] 4.6, 95% confidence interval [CI]: 1.8–12.0), salsa fresca (aOR 2.8, CI: 1.3–6.0), and lettuce (aOR 2.5, CI: 1.2–5.2) were independently associated with illness. A composite variable representing consumption of either raw cilantro or salsa fresca, which also contained raw cilantro, explained 95% of cases (aOR 6.9, CI: 2.0–24.0). No STEC was isolated from cilantro collected from the restaurant or the restaurant’s distributor.
CONCLUSIONS: Cilantro was the most likely food vehicle causing this STEC outbreak, based on the strong association of raw cilantro consumption with illness, and the high percentage of cases explained by cilantro consumption. The large number of associated PFGE patterns was suggestive of a heavily contaminated food item as the outbreak source. Additionally, the large proportion of food handlers found to have asymptomatic STEC shedding might have amplified the outbreak. Testing and restriction of positive food handlers during restaurant-associated STEC outbreaks can be an important outbreak control measure.