A foodborne norovirus outbreak due to manually prepared salad, Austria 2006

Background: Norovirus is increasingly being recognized as leading cause of foodborne disease. Nevertheless, well-documented foodborne outbreaks due to norovirus are found in the literature. Material and Methods: A retrospective cohort study was conducted for identifying the source of a gastroenteritis outbreak. A total of 325 persons were identified as the at-risk group. Results: The overall attack rate was 56% (182/325). Of the four working days of possible foodborne exposure to norovirus (Monday till Thursday), Wednesday (risk ratio [RR]: 18.82; 95% CI 11.82-29.96) and Thursday (RR 2.14; 95% CI 1.65-2.79) turned out to be the most likely days on which infections with norovirus occurred. The day-by-day food specific cohort analyses yielded consumption of salad on Wednesday (adjusted RR 2.82; 95% CI 1.0-7.94) to be associated with highest risk of illness. The most likely source of food contamination is a kitchen assistant having prepared salad manually. She fell ill with symptoms of gastroenteritis on Wednesday during the early working hours and continued working. Human stool samples obtained from five out of six outbreak cases, including the sick kitchen assistant, were RT-PCR positive for norovirus genotype GGII. 7 (Leeds-like). Conclusion: This foodborne norovirus outbreak underlines the drastic consequences of neglecting the rules of basic kitchen hygiene. Food handlers working despite manifest diarrhea or vomiting - often in fear of job loss are a common cause of foodborne norovirus outbreaks.