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Preventing Norovirus is in Hands of Food Workers
by Catherine Shaffer
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.
The FDA produces a document known as the Food Code that recommends best practices for safety in the food industry, including hand hygiene, according to Lee-Ann Jakus, a Food Science professor at North Carolina State University. “A huge issue in this country is compliance. You can tell people you should wash your hands this way for this amount of time, with soap and water, and you need to do it after you use the bathroom, but you have no way of ensuring people are actually doing that.”
The CDC recommends washing hands frequently with soap and warm water for at least 20 seconds, particularly after using the restroom. Also, workers should use utensils and single-use disposable gloves to avoid touching RTE foods with their bare hands. Kitchen surfaces and objects should be sanitized with a product approved by the EPA for use against norovirus. Fruits and vegetables should be carefully washed and shellfish should be cooked to at least 140 degrees.
Hand hygiene in food service is well below 100 percent in observational studies, according to Hall. In one study, it was only performed in about 20 percent of cases. “Clearly there’s room to improve in hand hygiene compliance,” says Hall.
There are some measures known to improve compliance. Training and certification of kitchen managers in food safety can improve compliance and decrease incidence of norovirus outbreak, Hall says.
However, perfect hand hygiene is difficult in a fast-paced food service environment. “There are so many instances in the chain where you have to wash your hands in a busy food service establishment—it becomes overwhelming,” says Jaykus. “It adds a level of complexity. This particular workforce in many instances is not as highly skilled as other workforces.”
Another intervention possibly even more powerful than hand hygiene is sick time.
One of the other major findings of the CDC report was that people shed virus in their fecal material before they show symptoms, and for most of the time after they show symptoms. Although it’s difficult to identify workers who are shedding virus, but not yet sick, excluding workers who are ill is an obvious way to minimize norovirus outbreaks.
Food service workers often don’t have paid sick time, and so are reporting to work when actively sick and contagious. In addition to handwashing, the food industry can prevent outbreaks by encouraging workers to stay home when ill.
Although norovirus is a clear and persistent problem in the food industry, there are not many options for intervening and preventing outbreaks. The CDC findings that fresh produce, mollusks, and RTE foods are most likely to be contaminated, and that food handlers are a significant source of outbreaks offer guidance as to where to focus prevention efforts. The CDC also advocated development and validation of more advanced analytic methods, particularly those capable of correlating virus levels with infection risk. More study is also needed on the role of food handlers, particularly those that are asymptomatic.
The CDC also advocates for better surveillance by public health agencies and development of vaccines. In the meantime, hand hygiene is potentially the most powerful front-line tool for controlling norovirus outbreaks. Increasing hand hygiene awareness and compliance can stop outbreaks at the source, whether that source is the farm or the point of service, since all human norovirus requires a human vector, and it requires little investment beyond time spent on training and awareness.
Shaffer is a freelance writer based in Ann Arbor, Mich. Reach her at firstname.lastname@example.org.