In early January 2012, Olmsted County Public Health Services (OCPHS) received reports of illness from people who had dined at a restaurant located in Rochester, Minnesota. The implicated meal date was December 27. The Minnesota Department of Health (MDH) foodborne illness hotline also received illness complaints. An outbreak investigation was conducted.

Patrons identified from credit card receipts and foodborne illness complaints were interviewed by OCPHS staff to obtain information on food/beverage consumption and illness history. Fifty-four patrons were interviewed; 17 (31%) met the case definition. One secondary case was identified and excluded from analysis. Case meal dates were December 27 and December 31. Two patron stool specimens were tested at the MDH Public Health Laboratory. Both tested positive for norovirus GII.4 New Orleans. No one was hospitalized and no one died.

In a univariate analysis, consuming a pita entrée side salad (6 of 16 cases vs. 1 of 36 controls; odds ratio [OR], 21.0; 95% confidence interval [CI], 2.3 to 195.4; p = 0.002), a gyros/pita/fries lunch special (8 of 16 cases vs. 5 of 35 controls; OR, 6.0; 95% CI, 1.5 to 23.4; p = 0.01), or a pita entrée rice side (5 of 16 cases vs. 2 of 36 controls; OR, 7.7; 95% CI, 1.3 to 45.6; p = 0.02) were significantly associated with illness. Most, but not all, salads contained romaine lettuce. A composite variable including any salad and any romaine lettuce-containing item, which included most pitas and the gyros/pita/fries lunch special, explained a greater proportion of cases than controls (16 of 17 cases vs. 24 of 36 controls; OR, 8.0; 95% CI, 0.9 to 67.7; p = 0.04). A multivariate analysis was not attempted due to overlap in menu items and ingredients.

Three (23%) of 13 restaurant employees interviewed reported recent vomiting or diarrhea. Two submitted stool specimens that tested positive for norovirus GII.4 New Orleans. Two employees had illness onset on December 28 and one employee had illness onset on December 31. EH specialists reviewed food flow for romaine lettuce served at the restaurant. Gloves were used by food workers when chopping a case of whole heads of romaine; however, bare hands were then used to agitate the chopped lettuce in a water rinse bath, which was not changed between cases of lettuce.

Environmental and statistical evidence indicated that romaine lettuce was the most likely vehicle of transmission. The source of contamination was not identified, however, bare-hand contact with an ill or recently ill food worker likely contaminated the lettuce during preparation or serving.