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Escherichia coli O157:H7 is the most common serotype of E. coli in the United States. Approximately 50 infections with E. coli O157:H7 are reported in Tennessee each year. Infection typically results in diarrhea, bloody stool, and abdominal cramps. Five to fifteen percent of cases develop hemolytic uremic syndrome (HUS), a serious kidney disorder. Outbreaks of E. coli O157:H7 have been associated with exposure to ground beef, produce, and dairy products. We investigated an outbreak of E. coli O157:H7 in Tennessee with cases clustered in school settings.
METHODS:
A case was defined as an individual testing positive for E. coli O157:H7 PFGE Xbal pattern EXHX01.0248 with illness onset on or after April 15, 2012. Among the Tennessee cases, there were groups of cases that attended the same daycares and schools. A case-control study was conducted among cases from selected schools with controls matched by school and grade. The questionnaire included exposures that had been reported in at least 25% of cases during the initial health department hypothesis-generating interviews.
RESULTS:
Fifteen cases were identified in Tennessee with an additional 3 cases in other states. Cases ranged in age from 3 to 88 years with a median of 23 years. Seventy-eight percent (14) of cases were female. Six (33%) were hospitalized and 2 died. Onset dates ranged from April 29 to May 12. Case-control analyses identified a statistically significant association with consuming lettuce provided by school cafeteria (OR = 9.4; 95% CI = 1.4 - ∞). Traceback was conducted by obtaining facility invoices and menus. Bagged salad from a single manufacturer was identified in the epidemiological traceback. The implicated bagged salad product was traced back to a single production facility with one day of production explaining the majority of cases.
CONCLUSIONS:
This Shiga toxin-producing Escherichia coli O157:H7 outbreak was associated with a bagged salad product. Epidemiologic traceback identified a single manufacturer as the likely source of the contaminated product. Investigational details were presented to the producer and regulatory officials who conducted a separate traceback. Implicated growing areas were scheduled for heightened inspection for the upcoming growing season. A combination of surveillance, epidemiological traceback, and analytical epidemiological studies among subclusters of cases strongly implicated produce in this outbreak investigation.