An outbreak is defined as the occurrence of disease cases in excess of normal expectancy within a particular area and a given time. Foodborne outbreaks caused by gastrointestinal bacteria such as Salmonella Typhimurium are among the most commonly reported and most extensively investigated. The classic outbreak investigation follows a series of well-defined steps which lead to a faster confirmation of the source and hopefully preventing of further cases. These steps are ideally undertaken using a One Health cross-sectorial collaboration approach involving partners from public health, food safety, and the veterinary and environmental sectors. In order to firmly identify the source of the outbreak, descriptive epidemiology is often combined with more robust evidence from analytical epidemiology such as a case-control study. A case-control study assesses whether a specific exposure is associated with illness, firstly by identifying cases (persons known to have been ill) and controls (persons who have not been ill, used as a reference group), and then retrospectively through interviews determining specific exposures for all persons. This information ultimately leads to the calculation of an odds ratio (see Note 3) which indicates the strength of the association between specific exposures and the outcome (illness or no illness). A well-conducted case-control study may substantiate or form core evidence as to the vehicle of a foodborne outbreak and is often a very important investigation tool, particularly in situations where microbiological proof cannot be obtained.
Keywords: Case-control; Clusters; Epidemiology; Foodborne; Gastrointestinal bacteria; Odds ratio; One health; Outbreak investigation; Outbreaks; Salmonella; Salmonella Typhimurium.