A new report by the CDC on norovirus outbreaks in the food industry revealed that the virus is a more important cause of foodborne illness than previously thought and ill food workers are a significant cause of outbreaks. The report underscores the importance of hand hygiene and the challenges of increasing compliance in restaurant and other food service settings.
A surprising finding of the report has been that noroviruses, not bacteria like Salmonella, are the leading cause of foodborne illness. According to the CDC, about 20 million people are sickened by norovirus each year, through contact with infected people or by eating contaminated food. Most cases are not formally diagnosed by a physician, who may suspect norovirus, but gives a diagnosis of gastroenteritis or “stomach flu.” Thus, in spite of the widespread nature of the illness, there is not much awareness of it as a problem, especially in the food supply.
The CDC found that between 2001 and 2008, there were about 365 foodborne outbreaks of norovirus per year, resulting in 156 hospitalizations and one death, on average. Leafy vegetables were the most common food culprit in the outbreaks, followed by fruits and nuts, and mollusks. However, between 53 and 82 percent of outbreaks were attributable to infected food handlers.
The source of an outbreak can be very difficult to identify. Human noroviruses cannot be cultured in a cell line, so identification in foods is carried out by polymerase chain reaction, or PCR, analysis. That analysis confirms the presence of norovirus ribonucleic acid, but does not give information on the infectivity of the virus. A positive identification of norovirus in a food only proves that the food has been contaminated at some point, but not whether that food was actually the cause of an outbreak.
“Norovirus is just an extremely contagious virus,” says Aron Hall, an epidemiologist in the CDC division of viral diseases. Hall leads the norovirus epidemiology program at CDC. “It takes just a few viral particles to make someone sick. Somebody that’s infected is shedding billions of virus particles. Even with a little bit of hand hygiene or occasional lapse, there could still be enough virus on someone’s hands to spread the infection.”
Unlike many bacteria, human noroviruses are not transmitted through animals and manure. Contamination of food, therefore, is most likely to occur either before harvest through irrigation water contaminated with feces, or through handling by infected food workers. Once an outbreak has occurred, it can be nearly impossible to trace the source.
Features of norovirus that facilitate transfer include low infective dose, rapid onset of gastroenteritis symptoms, high load of virus particles in the feces and vomit, asymptomatic early disease, and long persistence outside the human host. Contaminated surfaces are a very efficient mode of transmission of the virus.
Contamination of foods and surfaces occurs through viruses on the hands or aerosolized vomit, and infection of susceptible individuals occurs through hand-to-mouth contact, so the role of hand hygiene in preventing outbreaks becomes crucial.
Some food workers mistakenly believe that a continuous cold chain of storage for foods will prevent transmission or destroy norovirus, as is effective with bacteria. According to University of Helsinki scientist Maria Ronnqvist, however, that is not true at all. “The fact that keeping a continuous cold chain for foods does not destroy viruses but rather preserves them has to be kept in mind in the food industry, for if the virus is transferred to the final, ready-to-eat [RTE] food product in the preparation phase, it could easily stay infective in the product until consumed,” she says.
Meticulous hand hygiene is the most effective way to prevent norovirus contamination. Even gloves are not fully effective for preventing transmission, although they lower the amount of virus that can be transferred.