BACKGROUND: Flour is a raw, minimally-processed product intended to be cooked before consumption. Although several previous Shiga-toxin producing E. coli (STEC) outbreak investigations in the United States suspected contaminated flour as the source, it had never been proven. In February 2016, PulseNet, the laboratory network for foodborne disease surveillance, detected a cluster of STEC O121 infections in 12 states having the same rare genetic fingerprint. A multistate outbreak investigation identified additional STEC strains that were linked to the outbreak through epidemiologic and laboratory analysis.
METHODS: We defined a case as infection with an outbreak strain of STEC O121 or O26 occurring between December 21, 2015, and September 5, 2016, and interviewed case-patients about foods and other exposures in the week before illness onset. We performed univariable matched exact conditional logistic regression to identify exposures associated with case-status, using people with non-STEC enteric infections as a comparison group, matched on age, gender, and state of residence. Samples of suspected food products were collected and cultured for STEC. A common point of contamination was sought through traceback. Whole genome sequencing (WGS) was performed on selected clinical and food isolates.
RESULTS: Sixty-two cases of STEC O121 and one case of STEC O26 were identified in 24 states; seventeen people were hospitalized and no deaths were reported. Using one flour brand (OR 21.04, 95% CI 4.69‒94.37) and tasting unbaked homemade dough or batter (OR 36.02, 95% CI 4.63‒280.17) were both significantly associated with illness. Traceback identified a common flour production facility. Three illnesses were identified in children exposed to raw dough at multiple locations of a restaurant chain. STEC isolated from five open leftover flour samples collected from case-patients’ homes and additional product samples collected from the flour producer were analyzed by WGS and the outbreak strains were found to be closely related genetically.
CONCLUSIONS: This is the first investigation to definitively link an outbreak of STEC infections to raw flour. Nearly 250 products containing flour were recalled by the flour producer and companies that used the contaminated flour. This investigation demonstrates the risks associated with eating products made with raw flour, or playing with uncooked dough. Flour producers should consider adding prominent packaging labels to warn consumer not to eat undercooked or raw flour or play with uncooked dough. Foodborne illnesses associated with raw flour are likely preventable if appropriate control measures are taken in grain fields, production facilities, restaurants, and by consumers.