Norovirus Infections

What is a norovirus?

Noroviruses are a group of related viruses that cause acute gastroenteritis, sometimes called the stomach flu, in humans. In more technical terms, noroviruses are single-stranded RNA, nonenveloped viruses. Their genetic information is encoded in a single strand of RNA and they lack an envelope.

Why are they called noroviruses?

Noroviruses are named after the original strain "Norwalk virus" which caused an outbreak of gastroenteritis in a school in Norwalk, Ohio, in 1968. Norovirus was recently approved as the official genus name for this group of viruses. Several other names have been used in the past for noroviruses, including:

ïNorwalk-like viruses (NLV);

ïCaliciviruses (because they belong to the virus family Caliciviridae);

ïSmall round structured viruses (because of their morphologic features).

The caliciviruses are so named because of their "Star of David" shape with cup-shaped (chalice) indentations.

What symptoms are caused by norovirus infection?

Norovirus infection usually starts suddenly. The infected person often feels very sick with nausea and vomiting and watery non-bloody diarrhea with stomach cramps. Vomiting is more common in children than adults. Sometimes there is a low-grade fever. There may also be chills, headache, muscle aches, and a general sense of tiredness.

Studies with volunteers have shown that as many as 30% of norovirus infections may be asymptomatic. These people are infected with the virus but have no symptoms (asymptomatic). The role of asymptomatic infection in the transmission of norovirus is not well understood. It is possible that people who seem well can transmit the virus.

What is this disease with norovirus called?

Disease caused by norovirus infection goes by a number of names, including:

ïViral gastroenteritis -- the most common name for illness caused by norovirus. Gastroenteritis is inflammation of the stomach and intestines;

ïAcute gastroenteritis -- the acute refers to the sudden onset;

ïNon-bacterial gastroenteritis -- non-bacterial because it is viral ;

ïStomach flu -- which is not related to the flu (or influenza), a respiratory illness caused by the influenza virus;

ïFood poisoning -- although there are many other causes of food poisoning; and

ïCalicivirus infection -- referring to its family name.

How important is the burden of norovirus infection?

It is estimated that 23 million cases of acute gastroenteritis a year are due to norovirus infection. And it is now thought that at least 50% of all foodborne outbreaks of gastroenteritis can be attributed to noroviruses.

Among the 232 outbreaks of norovirus illness reported over a 3-year period in the US, 57% were foodborne, 16% were due to person-to-person spread, and 3% were waterborne; and in 23% of outbreaks, the cause of transmission was not determined. Common settings for outbreaks included restaurants and catered meals (36%), nursing homes (23%), schools (13%), and vacation settings or cruise ships (10%).

Most foodborne outbreaks of norovirus illness are likely to arise though direct contamination of food by a food handler immediately before its consumption. Outbreaks have frequently been associated with consumption of cold foods, including various salads, sandwiches, and bakery products. Liquid items (e.g., salad dressing or cake icing) that allow virus to mix evenly are often implicated as a cause of outbreaks.

Food can also be contaminated at its source, and oysters from contaminated waters have been associated with widespread outbreaks of gastroenteritis. Other foods, including raspberries and salads, have been contaminated before widespread distribution and subsequently caused extensive outbreaks.

Waterborne outbreaks of norovirus disease in community settings have often been caused by sewage contamination of wells and recreational water.

How serious is a norovirus infection?

Norovirus disease is usually not serious, although people with it may not agree at the time. They may feel very, very sick and vomit many times a day.

Some people with norovirus infection are unable to drink enough liquids to replace the liquids they lost because of vomiting and diarrhea. Dehydration is the most common complication of the disease, especially among the young and elderly and immunedeficient persons. The dehydration may require medical attention.

Symptoms of norovirus disease usually last 24 to 60 hours -- up to three days. And then the victims get better and recover.

People who have had norovirus have no known long-term health effects related to their illness. There is also no evidence to suggest that an infected person can become a long-term carrier of norovirus.

How do you become infected with noroviruses?

Noroviruses are found in the stool (feces) and the vomitus of infected people. People can become infected with the virus in several ways, including:

ïEating food or drinking liquids that are contaminated with norovirus;

ïTouching surfaces or objects contaminated with norovirus, and then placing their hand in their mouth;

ïHaving direct contact with another person who is infected and showing symptoms (for example, when caring for someone with illness, or sharing foods or eating utensils with someone who is ill).

Persons working in day-care centers or nursing homes should pay special attention to children or residents who have norovirus illness. This virus is very contagious and can spread rapidly throughout such environments.

Noroviruses are transmitted primarily through the fecal-oral route, either by consumption of fecally contaminated food or water or by direct person-to-person spread. Environmental and fomite contamination may also act as a source of infection.

Good evidence exists for transmission due to aerosolization of vomitus that presumably results in droplets contaminating surfaces or entering the oral mucosa and being swallowed. No evidence suggests that infection occurs through the respiratory system.

When do symptoms appear?

The symptoms of norovirus disease usually start about 24 to 48 hours after the virus is contracted, but they can appear as early as 12 hours after exposure.

The average incubation period -- the time between contracting the virus and the appearance of symptoms -- is 33 to 36 hours. Let's call it a day and a half.

Are noroviruses contagious?

Yes. Noroviruses are very highly contagious and can spread easily from person to person. Both stool (feces) and vomit are, as mentionned, infectious. Particular care should be taken with young children in diapers who may have diarrhea.

It is thought that an inoculum of as few as 10 viral particles may be sufficient to infect an individual. During outbreaks of norovirus gastroenteritis, several modes of transmission have been documented; for example, initial foodborne transmission in a restaurant, followed by secondary person-to-person transmission to household contacts.

How long are people contagious?

People infected with norovirus are contagious from the moment they begin feeling ill to at least 3 days after recovery. Some people may be contagious for as long as 2 weeks after recovery. Shedding of the virus usually begins with the onset of symptoms and may continue for 2 weeks after recovery.

Therefore, it is particularly important for people to use good handwashing and other hygienic practices after they have recently recovered from norovirus illness.

Who gets norovirus infection?

Anyone can become infected with these viruses. There are many different strains of norovirus, which makes it difficult for a person's body to develop long-lasting immunity. Therefore, norovirus illness can recur throughout a person's lifetime. In addition, because of differences in genetic factors, some people are more likely to become infected and develop more severe illness than others.

What is the treatment for norovirus infection?

There is currently no antiviral medication that works against norovirus and there is no vaccine to prevent infection. Norovirus infection cannot be treated with antibiotics.

Norovirus illness is usually brief in healthy individuals. When people are ill with vomiting and diarrhea, they should drink plenty of fluids to prevent dehydration. Dehydration among young children, the elderly, the sick, can be common, and it is the most serious health effect that can result from norovirus infection. By drinking oral rehydration fluids (ORF), juice, or water, people can reduce their chance of becoming dehydrated. Sports drinks do not replace the nutrients and minerals lost during this illness.

Can norovirus infection be prevented?

Yes. You can decrease your chance of coming in contact with noroviruses by following these preventive steps:

ïFrequently wash your hands, especially after toilet visits and changing diapers and before eating or preparing food.

ïCarefully wash fruits and vegetables, and steam oysters before eating them.

ïThoroughly clean and disinfect contaminated surfaces immediately after an episode of illness by using a bleach-based household cleaner.

ïImmediately remove and wash clothing or linens that may be contaminated with virus after an episode of illness (use hot water and soap).

ïFlush or discard any vomitus and/or stool in the toilet and make sure that the surrounding area is kept clean.

Persons who are infected with norovirus should not prepare food while they have symptoms and for 3 days after they recover from their illness. Food that may have been contaminated by an ill person should be disposed of properly.

How is norovirus diagnosed in the laboratory?

The diagnosis of norovirus now usually is done using a technique called reverse transcriptase polymerase chain reaction (RT-PCR). RT-PCR can be used to test stool and emesis samples, as well as to detect the presence of noroviruses on environmental swabs in special studies. Identification of the virus can be best made from stool specimens taken within 48 to 72 hours after onset of symptoms, although good results can be obtained by using RT-PCR on samples taken as long as 5 days after symptom onset. Virus can sometimes be found in stool samples taken as late as 2 weeks after recovery.

Sequencing of noroviruses found in clinical samples has helped in conducting epidemiologic investigations by linking cases to each other and to a common source and by differentiating outbreaks that were mistakenly connected.

After infection, is there immunity to norovirus?

The mechanisms of immunity to norovirus are unclear. It appears that immunity may be strain-specific and last only a few months. Therefore, given the genetic variability of noroviruses, individuals are likely to be repeatedly infected throughout their lifetimes. This may explain the high attack rates in all ages reported in outbreaks. Recent evidence also suggests that susceptibility to infection may be genetically determined, with people of O blood group being at greatest risk for severe infection.

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