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BACKGROUND: Unpasteurized milk consumption increases the risk of gastrointestinal illness including Shiga toxin-producing E. coli O157:H7 infection (STEC O157:H7). Retail sale of unpasteurized milk is illegal in Tennessee, however, recent legislative changes have enabled consumers to obtain unpasteurized milk through cow-share programs. In October 2013, the Tennessee Department of Health (TDH) and Knox County Health Department (KCHD) investigated a STEC O157:H7 outbreak among children who consumed unpasteurized milk from Dairy A.
METHODS: Epidemiologic and environmental investigations were conducted including an on-farm environmental assessment at Dairy A. Information about milk collection, storage, and distribution was collected as well as bulk tank milk and bovine manure samples. Case-case comparisons were performed. A cohort study was conducted among cow-share participants <10 years of age. Dairy A cow-share participants were interviewed about demographics, illness, and food and animal exposures. Available milk from cow-share participants was collected. A case was defined as a person with ≥2 days of diarrhea since October 1. Clinical specimens, and environmental, and food samples were tested. Isolates were molecularly subtyped.
RESULTS: Of the 248 households reported as participating in Dairy A’s cow-share program, participants from 88 households were interviewed. Nine cases were identified among children aged 1 to 7 years, 3 (33%) reported bloody diarrhea, 5 (56%) were hospitalized, and 3 (33%) developed hemolytic uremic syndrome. Four culture-confirmed cases were molecularly indistinguishable. One of two Dairy A milk samples collected from cow-share participant households was Shiga toxin 2 positive by polymerase chain reaction but culture-negative. All bulk tank milk samples collected from Dairy A were negative for Shiga toxin and culture negative. Eight (100%) of 8 manure samples yielded STEC O157:H7 isolates indistinguishable from the outbreak subtype. Case-case comparisons identified consumption of raw milk as a statistically significant signal in the initial cluster of cases. Risk factor analysis among the cohort of cow-share participants <10 years identified significant differences among ill children who reported consuming any raw dairy (p=0.04) and consuming raw dairy from Dairy A (p=0.04) compared to well children, but no differences in consumption of beef, lettuce, or spinach, or visiting a farm.
CONCLUSIONS: An outbreak of STEC O157:H7 associated with consumption of unpasteurized milk from Dairy A initiated a multi-jurisdictional investigation. Epidemiologic, environmental, and laboratory investigations were used to identify unpasteurized milk as the cause. Education was provided to cow-share participants during interviews and to Dairy A regarding the inherent risks of consuming unpasteurized dairy products.