We conducted a matched case-control study using a questionnaire that included exposures previously associated with STEC infections (e.g. ground beef), and exposures reported by >=50% of cases during hypothesis generation to identify exposures associated with infection in the 7 days before illness onset. Our analyses included 22 cases from Missouri that were identified as of November 2, 2011. Controls were individuals who did not have diarrhea in the month preceding the interview and were geographically matched to cases using a reverse telephone number directory of landline telephones. A list of 90-110 neighborhood control phone numbers was generated for each case, and we aimed to recruit three controls per case. Controls were proportionally recruited by age category to ensure similar distributions of cases and controls in the following age categories: <18 years old, 18-50 years old, >50 years old. Controls were interviewed about their exposures during the third week of October to match the exposure period of cases. All data were analyzed using SAS 9.3 (SAS Institute, Cary, NC) and exact conditional logistic regression models. Matched odds ratios (mOR) with exact 95% Wald confidence intervals are reported.
The case-series study evaluated all cases, including those identified after the completion of the case-control study. Cases were interviewed by state and local health departments with a questionnaire that included exposures previously associated with STEC infections and exposures reported by >=50% of cases in the case-control study. Reported food exposures among cases were compared with the Foodborne Diseases Active Surveillance Network (FoodNet) Population Survey Atlas of Exposures, 2006-2007 to determine whether foods were eaten more frequently by cases compared to the FoodNet population. Ten state health departments comprised the 2006-2007 FoodNet sites which captures 15% of the United States population and assessed all food exposures in the seven days prior to interview of a randomly chosen subset of individuals who live in the FoodNet catchment area. We compared frequency of consumption of each food among patients in the case series to the FoodNet population using the binomial test. All data were analyzed using SAS 9.3 (SAS Institute, Cary, NC).
Overall, 58 cases of STEC in 10 states matching the outbreak strain were identified: Arizona (1), Arkansas (2), Georgia (1), Illinois (9), Indiana (2), Kansas (2), Kentucky (1), Minnesota (2), Missouri (37), and Nebraska (1). Onset dates ranged from October 9 through November 7, 2011 (Figure 1). The median age of cases was 28 years (range 1-94 years); 61% were female. Among 50 cases with information available, 34 (68%) were hospitalized and three cases of 47 with information available (6.4%) developed hemolytic uremic syndrome (HUS). No deaths were reported. Figure 2a depicts the XbaI and BlnI pattern combinations associated with the outbreak investigation. Figure 2b depicts representative MLVA patterns associated with the outbreak investigation.