In September 2011, a patient cluster with a rare Salmonella serotype - Strathcona - was identified in Denmark. An outbreak investigation was initiated to reveal the source in order to stop the outbreak. In addition to hypothesis-generating interviews, comparable analyses of patients' household shopping receipts were conducted. A matched case-control study with 25 cases and 56 population register controls was conducted to test the findings of the hypothesis-generating investigation. In total, 43 cases of Salmonella Strathcona were reported in Denmark. Additionally, 28 cases were reported from Germany, Italy, Austria and Belgium. The results of the investigation in Denmark showed that 8/10 cases had bought datterino tomatoes prior to disease onset. Illness was associated with a specific supermarket chain [matched odds ratio (mOR) 16·9, 95% confidence interval (CI) 2·2-130], and having consumed elongated small tomatoes (OR 28·1, 95% CI 2·6-302). Traceback investigation showed that the tomatoes came from an Italian producer. This outbreak, linked to tomatoes, underpins the growing recognition of the broad source range of Salmonella and the ability of fresh produce to cause multi-country outbreaks. It is important to strengthen the international cooperation between public-health and food-safety authorities in the European Union to investigate future multi-country outbreaks in order to prevent illness from ready-to-eat produce.
Keywords: Case-control study; Salmonella; outbreaks.