Large multistate outbreak of norovirus gastroenteritis associated with frozen strawberries, Germany, 2012

From 20 September through 5 October 2012, the largest recorded food-borne outbreak in Germany occurred. Norovirus was identified as the causative agent. We conducted four analytical epidemiological studies, two case-control studies and two surveys (in total 150 cases) in secondary schools in three different federal states. Overall, 390 institutions in five federal states reported nearly 11,000 cases of gastroenteritis. They were predominantly schools and childcare facilities and were supplied almost exclusively by one large catering company. The analytical epidemiological studies consistently identified dishes containing strawberries as the most likely vehicle, with estimated odds ratios ranging from 2.6 to 45.4. The dishes had been prepared in different regional kitchens of the catering company and were served in the schools two days before the peaks of the respective outbreaks. All affected institutions had received strawberries of one lot, imported frozen from China. The outbreak vehicle was identified within a week, which led to a timely recall and prevented more than half of the lot from reaching the consumer. This outbreak exemplifies the risk of large outbreaks in the era of global food trade. It underlines the importance of timely surveillance and epidemiological outbreak investigations for food safety.