Norwalk Virus and Norwalk-like Viruses

What are Norwalk and Norwalk-like viruses?

These viruses, also known as small round structured viruses or caliciviruses, are an important cause of gastrointestinal illness throughout the United States, including Illinois. Members of this category of viruses, subsequently referred to as Norwalk-like viruses, are typically named for the location in which they were first identified, for example, Hawaii, Snow Mountain, Montgomery County and Oklahoma. The Norwalk virus is the prototype for this group of viruses – there are at least 11 other related viruses – hence the name “Norwalk-like virus.”

What are the symptoms of Norwalk-like viruses?

The signs and symptoms of Norwalk-like viruses are similar and usually occur between 24 hours and 48 hours after exposure. They include nausea, vomiting, diarrhea, abdominal pain, muscle aches, headache, tiredness and low-grade fever. Symptoms typically last 24 hours to 48 hours and subside on their own. There are no known long- term effects after recovery from this infection.

How common is Norwalk-like virus infection?

The U.S. Centers for Disease Control and Prevention estimates that more than 180,000 cases of Norwalk-like virus infections occur annually in the United States. Some studies indicate that more than 60 percent of the U.S. population is exposed to one or more of these viruses by the age of 50. When these viruses are recognized to cause illness, they usually are associated with an outbreak of gastrointestinal illness.

How do people come in contact with these viruses?

Humans are the only source for these viruses. These viruses do not multiply outside the human body. The viruses are present in the feces of infected persons and can be transmitted to others when hands are not thoroughly washed after having a bowel movement. When food that is not later cooked is handled by an infected person who did not wash hands after toileting, others who eat the food can become infected. Heating foods to cooking temperatures kills these viruses. People also can be infected by drinking water contaminated by sewage containing one of these viruses or by consuming ice made from contaminated water. Unless thoroughly cooked, shellfish (such as oysters) harvested from waters containing sewage can transmit the viruses. These viruses also are transmitted readily from person to person when hands are not washed after toileting. There is some evidence that the viruses can be transmitted by aerosolized vomitus.

How are these infections diagnosed?

Standard hospital laboratories and commercial laboratories usually are not equipped to detect Norwalk-like viruses. The specialized laboratories that can detect these viruses perform tests on stool specimens from an infected person and, in some cases, can identify evidence of infection by testing blood for antibody. In Illinois, only the Illinois Department of Public Health’s laboratories have the capability to confirm a diagnosis with one of these viruses. This laboratory service is reserved for testing a small number of people associated with recognized outbreaks and is not available for testing individual cases of gastrointestinal illness.

If I have had a Norwalk-like virus infection in the past, can I get it again?

Yes. Immunity is believed to last around 14 weeks, but long-term immunity may not occur. Detecting antibody against these viruses in the blood does not assure a person is immune.

How can these infections be prevented?

Food handlers should practice careful handwashing after having a bowel movement, especially when handling food that will not be cooked later. Water supplies should be protected from the risk of contamination by sewage. Plumbing in dwellings and business establishments should be constructed with no cross-connections to prevent sewage from entering the drinking water supply.

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Illinois Department of Public Health
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Springfield, Illinois 62761
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TTY 800-547-0466
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