Norovirus outbreaks are becoming better recognized and are popping up in diverse scenarios.

Today’s issue of Eurosurveillance presents three different norovirus outbreaks, each with its own investigative twists and turns. Excerpts from the three reports are below.

Mayet et al., report that on April 13/11, the medical service of a French military parachuting unit reported an outbreak of acute gastroenteritis involving 147 persons among the military personnel. Meals suspected to have caused the outbreak (pasta and some raw vegetables) were tested for norovirus by PCR. The same norovirus (genogroup I) was found in some of the food items consumed by the cases and in a cook who prepared the meals.

At French military base canteens, meal items are routinely sampled and samples are kept for five days. We tested for norovirus the water of the drinking fountains and the food items served and sampled in the canteen on 11 and 12 April, which were suspected to be associated with the outbreak following the analytical study. The extracted RNA was tested for norovirus by real-time RT-PCR [3]. Pasta was tested by culture for Bacillus cereus which was initially suspected to have caused the outbreak by the physicians who treated the cases. In addition, water from the drinking fountain was tested by culture for coliform germs. For logistical reasons, no samples were requested from the cases, apart from a cook who had prepared the meals and who had fallen ill before the outbreak. The stool sample from the cook was tested for norovirus by PCR as described.

This norovirus-related food-borne disease outbreak involving 147 cases occurred during a parachuting exercise on the night of 12 April and affected significantly the activities of the military unit. It is interesting to note that another outbreak of acute gastroenteritis occurred between 10 and 12 April among residents of a retirement home in the same geographical area, in which the same cook involved in the outbreak in the military unit prepared food on 9 and 10 April. However, the outbreak in the nursing home was only suspected after interrogation of the ill cook; it had not been reported to the health authorities and consequently, it had not been investigated, but it is likely that it was also caused by norovirus considering that around 50% of acute gastroenteritis outbreaks in industrialised countries are related to this agent. Other norovirus outbreaks related to raw vegetables have been described in the past in other military units. The episode described here illustrates once more that food-borne disease outbreaks can easily occur in such settings and stricter hygiene measures may need to be considered.

Guzman-Herrador et al., report that 56 people were affected with gastroenteritis after attending a one-day meeting in a high-quality hotel in the centre of Oslo, Norway, at the end of January 2011. A complete outbreak investigation was carried out. The microbiological investigation confirmed that the outbreak was caused by norovirus. All participants at the meeting were invited by email to complete an online questionnaire asking for information on demographic data, symptoms and food consumption. The results of the epidemiological investigation of the food items served were inconclusive and the source and transmission route of this outbreak remains unclear. However, the environmental investigation highlighted several irregularities in the kitchen that may have enabled the spread of the virus. Specific cleaning procedures and rules were set up for the kitchen staff. As a consequence of this outbreak investigation, the hotel is planning to change its internal routine protocols, for example, samples of food items served at every meal during an event will be stored.

The irregularities that the Food Safety Authority’s inspection found in the kitchen may have enabled the spread of the virus. Handling of ready-to-eat foods by infected food handlers is commonly identified as a contributing factor in outbreaks caused by norovirus. However, the role of kitchen employees or food handlers in the outbreak reported here remains unclear since none of those in the hotel reported any symptoms to the Food Safety Authority and no information was available regarding the health status of the food handlers who produced some of the food items outside the hotel. The importance of identifying asymptomatic food handlers shedding the virus is also well described in the literature: such people can also be a contributing factor in norovirus outbreaks. We do not know if asymptomatic food handlers were involved in the spread of the virus in this outbreak as the employees were not asked to provide stool samples.

Finally, Nicolay et al., report that in March 2009, the Department of Public Health in Dublin, Ireland, was notified of a cluster of four gastroenteritis cases among people who attended a family lunch in a Dublin hotel. A retrospective cohort study was carried out. An outbreak case was defined as an attendee who developed diarrhoea and/or vomiting in the 60 hours following the lunch. Of 57 respondents, 27 met the case definition. Consumption of egg mayonnaise, turkey with stuffing or chicken sandwiches were each associated with increased risk of gastroenteritis. An environmental investigation established that before notification of the cluster, there had been unreported gastroenteritis among staff at the hotel. The earliest symptomatic person identified was a staff member who had vomited in the staff toilets but had not reported it. The sandwiches had most likely been contaminated by three asymptomatic kitchen food handlers who had used the same toilets. Stool samples were submitted by eight cases and 10 staff members. All eight cases and three asymptomatic food handlers on duty at the lunch tested positive for norovirus genogroup II.4 2006. Our analysis suggests that asymptomatic food-handlers can be responsible for norovirus transmission.