a1 Gastrointestinal, Emerging and Zoonotic Infections Department, Health Protection Agency, London, UK
In March 2008, the Health Protection Agency in England conducted a retrospective case-control study to investigate the cause of 179 cases of the newly recognized, fully antimicrobial-sensitive Salmonella Typhimurium PT U320. Forty-three symptomatic laboratory-confirmed case-patients and 84 asymptomatic location-matched controls were interviewed by telephone about exposures in the 3 days prior to illness or interview. Multivariate logistic analysis indicated consumption of pre-packaged egg sandwiches (odds ratio 3·29, 95% confidence interval 1·19–9·09) was independently associated with illness. Eight of the 15 case-patients who consumed egg sandwiches did so from retail chain A (53·3%) whereas none of the eight controls consumed similar sandwiches (χ2=7·20, P≤0·01). A review of the pre-packaged egg sandwich ingredients suggested this outbreak was probably caused by exposure to an ingredient common to pre-packaged sandwiches and prepared salads but we established a definitive epidemiological link with only the former. Short shelf-life, product diversity and investigation lag hinder epidemiological investigations of such popular products, providing continued challenges for food safety enforcement of freshly prepared produce.
(Accepted December 15 2010)
(Online publication January 24 2011)