E-mail survey as a rapid tool to confirm frozen strawberries as the vehicle of the largest foodborne Norovirus outbreak ever reported in Germany, 2012
Background: A norovirus outbreak (>10,950 reported cases) occurred in fall 2012 in Eastern Germany. More than 390 schools and kindergartens served by one caterer reported outbreaks starting mainly September 25th-28th, with many facilities closing for vacation on September 28th. As part of multiple investigations to identify the vehicle of infection, we conducted an e-mail survey in a Berlin school which had reported 40 cases.
Methods: We developed an interactive Adobe Acrobat Pro(TM) questionnaire based on the caterer’s menu, forwarded it to approximately 900 parents after approval by the school and automatically entered e-mailed responses into a database. We defined cases as schoolchildren with self-reported diarrhoea or vomiting during 24th-28th of September comparing them to healthy school children in a case-control design. We examined associations using exact logistic regression.
Results: Overall, 14/86 participants met the case definition. After 24 hours we had received 28, after 48 hours 61 and after 96 hours 86 completed questionnaires. Within 24 hours we detected associations between being a case and attending school meals (Odds Ratio (OR) 14.6; 95% confidence interval (CI) 1.8-∞) and exposure to frozen strawberries among those attending school-meals (OR 9.8; 95%CI 0.6-706.2), the latter became statistically significant after 48 hours (OR 15.9; 95%CI 1.5–881.2). Finally, among the 39 school-meal attendants 12/14 case patients and five of 25 control persons had been exposed to frozen strawberries (OR 21.4; 95%CI 3.3–257.9).
Conclusion: This e-mailed survey allowed obtaining partial evidence implicating frozen strawberries as the vehicle of the outbreak within 24 hours. Results at 96 hours along with other epidemiological investigations with concordant evidence conducted by our department lead to a timely recall of more than 30 tons of frozen strawberries.
Key words: Norovirus, outbreak, epidemiology, data collection, questionnaires