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BACKGROUND: U.S. outbreaks of Escherichia coli O157:H7 (ECO157) have been associated with produce including leafy greens and sprouts, but not cucumbers. In October 2013, ECO157 cases with a history of consuming sandwiches from three locations of a sandwich restaurant chain (Chain A) prompted a public health investigation.
METHODS: All cases were defined as Colorado residents with onset of symptoms in October 2013 after dining at Chain A. Confirmed cases had a laboratory confirmed ECO157 infection with the outbreak multiple-locus variable number tandem repeat analysis pattern. Probable cases had post-diarrheal hemolytic uremic syndrome (HUS) with positive ECO157 serology. We conducted case finding, two case-control studies (Study I and II), an environmental investigation, produce traceback, and laboratory testing. Case finding occurred via routine public health surveillance methods. Study I assessed restaurant exposures; attempts were made to enroll three age-, sex-, and neighborhood-matched controls per case using sequential digit dialing. Study II assessed Chain A food exposures; online and phone order records were used to recruit controls who purchased food from the same locations on the same dates as cases. Environmental investigations were conducted at three Chain A locations and the common produce distributor. Produce records were obtained to conduct a traceback. The state public health laboratory tested food and environmental samples from Chain A and the produce distributor.
RESULTS: One probable and 8 confirmed cases were identified. Cases ate sandwiches at three Chain A locations on October 5th, 6th, 7th and 9th. Case median age was 23 years; 78% were female; 1 was hospitalized with HUS; all survived. Eight cases and 12 controls were enrolled in Study I; consuming food from Chain A was associated with illness (OR=undefined, p=0.00007). Eight cases and 112 controls were enrolled in Study II; consuming cucumbers from Chain A was associated with illness (OR=undefined, p=0.0016); no other food items were statistically associated with illness. The environmental investigations found no obvious cause of contamination. Produce traceback conducted by CDPHE determined that the three Chain A locations received a common lot of imported cucumbers; no other food items had a common source. ECO157 was not recovered in food or environmental samples; however, samples of the implicated cucumbers were not available for testing.
CONCLUSIONS: The likely cause of this outbreak was imported cucumbers served at Chain A. Cucumbers are a novel vehicle for ECO157 transmission. Public health and food safety agencies should be aware that cucumbers can be contaminated with ECO157.