Multistate outbreak of Escherichia coli O157:H7 infections associated with a national fast-food chain, 2006: a study incorporating epidemiological and food source traceback results

(Accepted 30 March 2010; first published online 30 April 2010)SUMMARYA multistate outbreak of Escherichia coli O157:H7 infections occurred in the USA inNovember–December 2006 in patrons of restaurant chain A. We identified 77 cases with chain Aexposure in four states – Delaware, New Jersey, New York, and Pennsylvania. Fifty-one (66%)patients were hospitalized, and seven (9%) developed haemolytic uraemic syndrome; none died.In a matched analysis controlling for age in 31 cases and 55 controls, illness was associated withconsumption of shredded iceberg lettuce [matched odds ratio (mOR) 8.0, 95% confidence interval(CI) 1.1–348.1] and shredded cheddar cheese (mOR 6.2, CI 1.7–33.7). Lettuce, an uncookedingredient, was more commonly consumed (97% of patients) than cheddar cheese (84%) and asingle source supplied all affected restaurants. A single source of cheese could not explain theregional distribution of outbreak cases. The outbreak highlights challenges in conducting rapidmultistate investigations and the importance of incorporating epidemiological study results withother investigative findings.Key words: Diarrhoea, Escherichia coli O157, lettuce, outbreak.INTRODUCTIONEscherichia coli O157:H7, a major cause of haemor-rhagic colitis and haemolytic uraemic syndrome(HUS), causes an estimated 73000 illnesses and 61deaths annually in the USA [1]. Of these illnesses,85% are estimated to be caused by foodbornetransmission [1]. Prevention of illnesses depends onidentifying food vehicles and understanding themechanisms of contamination in order to make con-tamination less likely in the future. Much of whatis known about sources of foodborne pathogens,including E. coli O157:H7, has been learned fromoutbreakinvestigations[2].Analyticalepidemiologicalstudies have been a powerful tool in this regard.However,resultsofepidemiologicalstudiesinvariablyneedtobereconciledwithotherinvestigationfindings,such as food supply information, or kitchen prep-aration and hygiene practices to ensure that the trueoutbreak source has been identified and control meas-ures have been properly targeted.