Norovirus causes nearly 60% of all foodborne illness outbreaks. Norovirus is transmitted primarily through the fecal-oral route, with fewer than 100 norovirus particles needed to cause infection. Transmission occurs either person-to-person or through contamination of food or water. CDC statistics show that food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling a food directly before it is eaten.
Infected individuals shed the virus in large numbers in their vomit and stool, shedding the highest number of viral particles while they are ill. Aerosolized vomit has also been implicated as a mode of norovirus transmission. Previously, it was thought that viral shedding ceased approximately 100 hours after infection; however, some individuals continue to shed norovirus long after they have recovered from it, in some cases up to 28 days after experiencing symptoms. Viral shedding can also precede symptoms, which occurs in approximately 30% of cases. Often, an infected food handler may not even show symptoms. In these cases, people can carry the same viral load as those who do experience symptoms.
Common settings for norovirus outbreaks include restaurants and events with catered meals (36%), nursing homes (23%), schools (13%), and vacation settings or cruise ships (10%). Proper hand washing is the best way to prevent the spread of norovirus.
The good news about norovirus is that it does not multiply in foods as many bacteria do. In addition, thorough cooking destroys this virus. To avoid norovirus, make sure the food you eat is cooked completely. While traveling in in areas that have polluted water sources, raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice.
Shellfish (oysters, clams, mussels) pose the greatest risk and any serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one. Shellfish become contaminated when their waters become contaminated—e.g., when raw sewage is dumped overboard by recreational or commercial boaters). Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission. Some researchers suggest that norovirus monitoring in shellfish areas could be a good preventive strategy as well. Waterborne norovirus outbreaks are ubiquitous, but difficult to recognize. Improved analysis of environmental samples would have the potential to significantly improve the detection for norovirus in shellfish waters.
Finally, and as briefly mentioned earlier, outbreaks of norovirus infections have become synonymous with cruise ships. Healthcare facilities also experience a high incidence of norovirus outbreaks. The CDC has published information regarding the prevention of norovirus outbreaks on cruise ships and in healthcare facilities on its website. Once a case has occurred, even more stringent hygienic measures than normal are required to prevent an outbreak, particularly on an enclosed space such as a cruise ship.