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.