Sources of calicivirus contamination in foodborne outbreaks, Denmark, 2005-2011 - the role of the asymptomatic food handler

Background. Norovirus is the predominant cause of foodborne disease outbreaks. Virus contamination may occur during all steps of food processing from production to preparation and serving. The relative importance of these different routes of contamination is unknown.
Methods. The purpose of this study was to estimate the proportions of outbreaks caused by asymptomatic and symptomatic food handlers. Reported foodborne norovirus and sapovirus outbreaks (n=191) occurring over a seven-year period were extracted, reviewed, and categorized according to the available evidence for source of contamination.
Results. In 64 (34%) of the outbreaks, contamination from food handlers took place during preparation or serving of food. In the majority of these outbreaks (n=41; 64%), the food handlers were asymptomatic during food handling. Some had been in contact with ill household members before handling the food and remained asymptomatic; others developed symptoms shortly after or were post-symptomatic. In 51 (27%) of the outbreaks, contamination occurred during production of the food and in 55 (29%) of the outbreaks, contamination had supposedly occurred after serving via a guest at a self-serving buffet.
Conclusions. Guidelines regarding exclusion of food handlers where household members suffer from gastroenteritis could limit the number of outbreaks.