Outbreak of human listeriosis associated with tomme cheese in northwest Switzerland, 2005

During an eight week period in spring 2005, 10 cases of listeriosis were reported in a small area of northwest Switzerland (150,000 inhabitants). Eight cases were in older immunocompromised patients who became ill with bacteraemia (three deaths), and two cases were in pregnant women who had septic abortion. All cases were due to a serotype 1/2a isolate with one of two pulsovars found by PFGE. Patient interviews quickly revealed that a locally made and distributed soft cheese (known as 'tomme') was the food source responsible for the outbreak. Samples of this cheese, and of butter made in the same factory, revealed Listeria monocytogenes sv 1/2a of the same pulsovar in amounts of 1000-10 000 and 10-100 cfu/g, respectively. The prompt suspension of production, the market recall of the product, and a public alert terminated the outbreak. However, two cases of febrile gastroenteritis due to the same strains were reported within 10 days of product recall. The restricted distribution area of the contaminated cheese and the collaboration of local physicians, medical microbiologists and food health services all contributed to a rapid and successful investigation. This small outbreak of listeriosis reinforces the need for a laboratory-based surveillance system with rapid typing, as well as collaboration between physicians and microbiologists.