Shiga Toxin-Producing Escherichia coli O157:H7 Outbreak at Multiple Catered Events Associated with Celery

BACKGROUND: In July 2014, a physician called the Minnesota Department of Health to report that 5 individuals had been treated in the hospital emergency department for bloody diarrhea after attending a catered picnic on July 11. An investigation was initiated, and patients subsequently tested positive for Shiga toxin-producing Escherichia coli (STEC) O157:H7.

METHODS: A list of events from July 5-17 was provided by Caterer A. Cases were identified through routine laboratory surveillance, and interviews with event attendees. Stool specimens were cultured for STEC O157:H7. A case was defined as an individual who attended an event catered by Caterer A and subsequently developed diarrhea (≥3 loose stools in a 24-hour period) that was either bloody or ≥3 days in duration, or who had STEC O157:H7 isolated from stool. A case-control study including attendees of 5 catered events was conducted. Food samples from Caterer A were tested for STEC O157:H7, and food items were traced back.

RESULTS: A total of 199 individuals were interviewed; 57 (29%) cases from 5 events were identified. Twenty-seven laboratory-confirmed cases representing 3 different catered events were identified. Five closely related PFGE patterns of STEC O157:H7 were identified. The median case age was 62 years (range, 4 to 85 years), and 65% of cases were female. Nine cases were hospitalized; no HUS cases or deaths occurred. Raw celery was served at all 5 events: 3 events served the same batch of potato salad that contained celery; 1 event served a vegetable tray with celery sticks; and, 1 event had a chopped celery garnish on the salad bar. Consumption of celery (46 of 52 cases vs. 55 of 95 controls; odds ratio, 5.6; 95% CI, 2.2-14.3; p<0.001) was significantly associated with illness. Leftover potato salad collected from the caterer was positive for STEC O157 with 2 PFGE patterns indistinguishable from case isolates, and 2 closely related patterns. The celery was traced back to a field in California located adjacent to a defunct dairy operation. Five environmental samples were collected on August 13; all were negative for STEC O157:H7. Inspectors reported that grazing cattle are occasionally present in the adjacent field.

CONCLUSIONS: This was an outbreak of STEC O157:H7 infections associated with consumption of celery at catered events. To our knowledge, this is the first reported STEC O157:H7 outbreak associated with celery. Cattle were the most plausible source of contamination, but this was not confirmed.