Use of sequential case-control studies to investigate a community salmonella outbreak in Wales

Study objective-To establish the source of a community outbreak of Salmonella typhimurium definitive type 124. Design-Two stage case-control study. Setting-Three districts in south east Wales. Subjects-Cases of salmonella food poisoning and community controls. Main results-An initial case-control study identified an association between illness and eating ham (odds ratio 4.50, 95% confidence intervals 1.10, 21.8) and also found a possible association between illness and food bought from delicatessen stores (odds ratio 5.03, 95% confidence intervals 1.01, 32.3). However, only after a second stage case-control study was a single common ham producer identified as the source (odds ratio 25.0, 95% confidence intervals 2.33, 1155). Conclusion-Sequential case-control studies are an important and underused tool in the investigation of community outbreaks.