Food Protection Connection: Understanding Norwalk Virus

(reprinted from Dietary Manager, April 2002)

What does the flu have in common with foodborne illness? According to CDC officials, more than you might guess. If you define "flu" as one of those viral gastrointestinal illnesses everyone dreads, you may be talking about Norwalk virus. About one-third of gastrointestinal virus cases in adults and children over age two are caused by the Norwalk virus.

The symptoms follow one or two days after exposure and are probably familiar: nausea, vomiting, diarrhea, abdominal pain, and sometimes fever and headache as well. As illnesses go, Norwalk gastroenteritis is mild and self-limiting; it goes away on its own within a few days. It's uncommon for a person to require hospitalization or develop complications.

We hear a lot about Salmonella, E. coli, Shigella, Listeria, and other pathogens. All are serious concerns. But in sheer numbers, Norwalk is surprising. Today, CDC officials estimate that Norwalk virus is the number-one cause of foodborne illness in the U.S.

Because Norwalk is so common, about half of adults actually have some immunity to the virus. The immunity does not last a lifetime, though, and re-infection can occur. The fact that immunity takes time to develop reinforces another idea Ð that the very young and those with compromised immune systems (among others) are at heightened risk of developing foodborne illness. In developing countries where sanitation is less advanced, a majority of people have immunity to Norwalk, and immunity takes hold at a young age.

Exactly where does Norwalk virus come from? As foodservice operators, we tend to focus on problem foods. Norwalk virus does have its own list, which includes: contaminated shellfish (oysters and clams), salad ingredients washed or handled in contaminated water, impure water itself, and ice made from impure water. Many of the largest outbreaks have been traced to undercooked oysters. Impure water has caused outbreaks on cruise ships, at cocktail parties, and in entire communities.

Norwalk virus is passed through the feces of someone who is ill with the virus. Shellfish become infected as harvesters or recreational boaters dump raw sewage into the coastal waters where shellfish are later harvested.

People Pass It

Also critical, though, is the role of foodservice workers in transmitting the virus. In 1999, the National Advisory Committee for Microbiological Criteria for Foods (NACMCF) reviewed the role of people in passing illness through food, in order to develop sound guidelines for control. They found that people do contribute a great deal to the transmission of foodborne illness, especially Hepatitis A and Norwalk virus.

Generally, a foodservice worker is infected, fails to practice good hygiene, and sheds viruses onto food. Next, a customer consumes the food and becomes ill. Norwalk is a fecal-oral pathogen. This means it passes to feces and later, through poor hygiene, reaches the mouth of someone else.

According to the NACMCF, three practices can bring the problem under control. These are:

Excluding or restricting employees as recommended by the FDA Food Code
Washing hands
Avoiding bare-hand contact with ready-to-eat foods.

Interestingly, the evaluation says that one of these three alone is not quite enough to control the hazard of foodborne viruses. The NACMCF says that it actually takes all three to control food safety reliably. Consider this example: An employee who is ill with Norwalk virus comes to work anyway and starts preparing salads. The employee conscientiously washes hands thoroughly before starting work, and after each use of the restroom.

Of course, handwashing does not sterilize hands (destroy all germs). So the employee still has a few viruses left on the skin. With Norwalk, a few may be enough to make someone ill. Scientists say the virus has a very low infective dose. So here is an example where handwashing alone may not be adequate. However, if you also control the hazard by asking this employee to use tongs or gloves during salad prep, you may be getting closer. In this example, it's easy to see why excluding the ill employee in the first place would be the ideal protection.

With Norwalk virus, there's a good chance that ill employees will know they have "a bug." If you enforce a policy of excluding ill employees from work (e.g. if they are experiencing diarrhea), you are already doing much to control Norwalk virus. Unfortunately, Hepatitis A is more challenging. Employees may be infected and carry the virus for weeks before experiencing symptoms. Thus, you can't always know an employee presents a risk to customers. In this scenario, you count on the second two NACMCF recommendations (together) to control the hazard.

Basis for No Bare Hands

The FDA cites the NACMCF as one of its reasons for discouraging contact between bare hands and ready-to-eat foods. As a concerned dietary manager, you can help carry out this practice. Remember that ready-to-eat foods do not ordinarily carry the virus at all. When Norwalk shows up in salads, it generally got there through the hands of a foodservice worker.

In addition, ready-to-eat foods will not undergo cooking that could destroy the virus. This is why ready-to-eat foods require special protection from the germs a foodservice worker may carry.

A few simple tips can help:

Enforce handwashing guidelines.
Insist that staff use tongs, utensils, deli paper, or other sanitary tools for handling food.
If employees wear gloves, make sure they wash hands first, and change gloves as often as they would wash hands.
Enforce your policy of excluding or restricting ill employees, and encourage employees to report important symptoms.
Follow shellfish identification rules, and be sure to cook all shellfish to recommended time and temperature standards.
Use only potable water in your operation.
Train your staff about the "people factor" in foodborne illness, and ask for their commitment in helping to control it.

With some special attention, you and your work team may be among the first to defy statistics and make Norwalk virus a rare occurrence.

By Sue Grossbauer