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Overview :

Noroviral infection

Noroviruses are a major cause of viral gastroenteritis'an inflammation of the linings of the stomach and small and large intestines that causes vomiting and diarrhea. Viruses are responsible for 30-40% of all cases of infectious diarrhea and viral gastroenteritis is the second most common illness in the United States, exceeded only by the common cold.

Anyone can become infected with norovirus. During norovirus outbreaks there are high rates of infection among people of all ages. There are a large number of genetically-distinct strains of norovirus. Immunity appears to be specific for the norovirus strain and lasts for only a few months. Therefore norovirus infection can recur throughout a person s lifetime. Because of genetic (inherited) differences among humans, some people appear to be more susceptible to norovirus infection and may suffer more severe illness. People with type O blood are at the highest risk for severe infection.

Infected individuals are contagious from the first onset of symptoms until at least three days after full recovery. Some people may remain contagious for as long as two weeks after recovery.


Gastroenteritis often is referred to as the stomach flu even though the flu is a respiratory illness caused by an influenza virus. Other common names for viral gastroenteritis include:

  • food poisoning
  • winter-vomiting disease
  • non-bacterial gastroenteritis
  • calicivirus infection.

The U.S. Centers for Disease Control and Prevention (CDC) estimate that noroviruses are responsible for some 23 million cases of acute gastroenteritis in the United States every year. Epidemiologists estimate that about 50,000 Americans are hospitalized annually and about 400 die as a result of norovirus infection. In developing countries noroviruses are a major cause of human illness.

Gastroenteritis caused by infection with a norovirus is rarely a serious illness. Typically an infected person suddenly feels very ill and may vomit many times in a single day. The symptoms, although quite unpleasant, usually last only 24-60 hours.


Noroviruses are ubiquitous in the environment. They are highly contagious and are considered to be among the most infectious of viruses. The reasons for this include:

  • Only a small number of viral particles'fewer than 100'are required for infection.
  • Although noroviruses cannot reproduce outside of their human hosts, they can remain viable for weeks or even months on objects and surfaces.
  • Human immunity to norovirus is short-lived and strain-specific.

Noroviruses are transmitted among people by a fecal-oral route, either by ingestion of food or water contaminated with feces or by contact with the vomit or feces of an infected person. Norovirus infection can occur by:

  • consuming contaminated food or liquids
  • hand contact with contaminated objects or surfaces, followed by hand contact with the mouth
  • contact with an infected person, including caring for the sick person or sharing food or utensils
  • aerosolized vomit that is swallowed or contaminates surfaces.

Environmental contamination or contact with infected clothing or linen also may be a source of transmission. Although there is no evidence that norovirus infection can occur via the respiratory system, the sudden and violent vomiting of noroviral gastroenteritis can lead to contamination of the surroundings and of public areas. Particles laden with virus can be suspended in the air and swallowed.

FOODBORNE TRANSMISSION. Noroviruses account for at least 50% of food-related outbreaks of gastroenteritis. A European study found that between 1995 and 2000 noroviruses were responsible for more than 85% of all foodborne non-bacterial gastroenteritis outbreaks. Restaurant or catered foods are common sources of norovirus transmission, with subsequent infection of household members. The majority of norovirus outbreaks occur via contamination by a food handler immediately before the food is consumed.

Foods that frequently are associated with norovirus outbreaks include:

  • foods that are eaten without further cooking, including sandwiches, salads, and bakery products
  • liquids such as salad dressing or cake icing in which the virus becomes evenly distributed
  • food that is contaminated at its source, including oysters and clams from contaminated waters and raspberries irrigated with sewage-contaminated water
  • food that becomes contaminated before distribution, including salads and frozen fruit.
  • Shellfish, including oysters and clams, concentrate norovirus from contaminated water in their tissues. Steaming shellfish may not completely inactivate the virus.

WATERBORNE TRANSMISSION. There is widespread norovirus contamination of rivers and seas, often with more than one strain of the virus. Waterborne outbreaks of norovirus have been associated with:

  • sewage-contaminated wells
  • contaminated municipal water systems
  • stream and lake water
  • swimming pools and spas
  • commercial ice.


Norovirus infection can spread rapidly through daycare centers, schools, prisons, hospitals, nursing homes, camps, and other confined spaces. About 40% of group- or institutionally-related outbreaks of diarrhea are caused by norovirus. Outbreaks usually peak during the winter months.

Between July of 1997 and June of 2000, 232 norovirus outbreaks were reported to the CDC. It was determined that 57% of these outbreaks were due to foodborne transmission, 16% were spread by human contact, and 3% were due to waterborne transmission. The mode of transmission could not be determined in 23% of the outbreaks. Restaurants or catered food accounted for 36% of the outbreaks, 23% occurred in nursing homes, 13% in schools, and 10% at resorts or on cruise ships. Outbreaks also have occurred at large family gatherings.

Cruise ships have become notorious for norovirus outbreaks among passengers and staff. Cruise ships and naval vessels are at increased risk for contamination when docking in regions that lack adequate sanitation and where contaminated food or water may be brought onboard. Outbreaks on cruise ships are exacerbated by close living quarters and the arrival of new, susceptible passengers every one to two weeks. Norovirus outbreaks have been reported to continue through more than 12 successive cruises on a single ship.

A study of 12 calicivirus outbreaks on cruise ships in 2002 found that 11 of the outbreaks were caused by noroviruses and seven of these were due to a previously unreported strain. In the same year, 10 out of 22 land-based outbreaks were attributed to this new strain.

Outbreaks of norovirus appear on the increase. In 2005 the CDC reported that norovirus outbreaks were increasing in hospitals, daycare centers, nursing homes, and schools across the country. The International Council of Cruise Lines reported that, although less than 1% of passengers become infected with norovirus each year, outbreaks on cruise ships also were on the increase. In the summer of 2004 an outbreak at Yellowstone National Park sickened 134 people. More than 1,100 people became ill in early 2004 after a norovirus outbreak at Las Vegas hotels. The following autumn more than 1,200 people became sick from a norovirus outbreak at a single Las Vegas hotel-casino.

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