Infection with norovirus causes a person to develop a rapid onset of nausea and vomiting, diarrhea, and occasionally stomach cramping (all symptoms ofgastroenteritis). In addition, the person may feel tired, have muscle aches, headache, and a low-grade fever (less than 101 F) with chills. The symptoms last about one to two days. Although no long-term problems persist or develop with this viral infection, dehydration (loss of body water) may be significant enough to require medical treatment.
Norovirus was probably first noticed by Dr. J. Zahorsky in 1929 and termed "winter vomiting disease." In 1968 in Norwalk, Ohio, there was an outbreak of gastroenteritis thought to be caused by a virus. The virus was termed the Norwalk agent (also termed particle or virus). It was classified as a "small round virus" and later, after genetic studies, classified as a member of the family Caliciviridae, with a single strand of RNA for its genome. The name of the genus, Norovirus, was approved in 2002 by an international committee.
Researchers suggest that norovirus is responsible for about 50% of all outbreaks of gastroenteritis in the U.S. and about 90% of epidemic nonbacterial gastroenteritis outbreaks worldwide. Most outbreaks or epidemics occur in places where people come together in close contact (for example, dorms, hospitals, prisons, cruise ships, schools, and nursing homes). Norovirus infections are transmitted person to person, by contaminated food and water, and by viruses on contaminated surfaces.
The symptoms and signs of norovirus infection usually occur within 12-48 hours of contact with the virus and often are first noticed within a cluster of people that are gathered together in groups (for example, military or school dorms, cruise ships, and nursing homes). Groups of people (although occasionally individuals) rapidly develop nausea, vomiting, diarrhea, and abdominal discomfort or cramping. Some people may develop low-grade fever (less than 101 F), headache, weakness, muscle aches, and loss of taste.
The diarrhea may be frequent (many watery stools in 12-24 hours); for children, the elderly, or immunocompromised patients, this symptom can lead to dehydration (body water loss). The norovirus infection is usually self-limiting and resolves in about one to three days, but people with severe dehydration can develop complications (for example, electrolyte imbalances, coma, or infrequently, death).
After an individual has contracted norovirus, it first attaches to gastrointestinal cells. The virus enters the cells, triggering the gastrointestinal tract to cause vomiting and preventing good fluid adsorption, which results in diarrhea. Because the virus is very difficult to cultivate, many of the specific mechanisms of its pathogenesis and replication have not yet been elucidated and are currently under investigation.
However, studies done to date suggest that the capsid (external coat) helps attach virus to cells, avoid the body's immune defenses, and targets people with specific blood groups (especially group O). At least five genogroups (genetically related groups GI, GII, GIII, GIV, GV) of norovirus, with at least 31 genetic clusters (genetic subgroups) have been identified. One genogroup, GII subgroup 4, is responsible for about 80% of recent (2002-2008) outbreaks.
Unfortunately, the virus has been referred to by many names (for example, Norwalk virus, Norwalk-like virus or NLV, SRSV, meaning small round structured viruses, Snow Mountain virus). Many of these names often arise from the area or region where an outbreak occurs, like Toronto virus, Hawaii virus, or Bristol virus. Common names like "winter vomiting virus" or "stomach flu" are also used.
"Stomach flu" is not related to any type of influenza; the term was likely coined because of the frequent watery diarrhea norovirus produces which is like the frequent clear or "watery" nasal discharge of the flu. This loose terminology can be confusing but it is understandable because by the time the norovirus is identified as the causative agent, usually the brief outbreak is ending or is over.
Drawings and electron microscopy pictures of the norovirus can be seen by going to the last two Internet sites listed below.
There is no specific treatment or medication for norovirus. It is important that infected individuals remain well hydrated, drinking plenty of fluids. Fluids containing electrolytes and sugars should be encouraged. Over the counter antidiarrheal medications may be used according to directions but should be avoided in cases with severe abdominal painor fever. Some researchers advise people to avoid these medications if possible as they may prolong the disease.
What are possible complications of a norovirus infection?
Although most cases of norovirus infection are mild, complications may occur. Complications are related to the degree of dehydration. People who cannot keep up with fluid losses may require hospitalization for intravenous fluids.
Approximately 10% of infected people seek medical attention. Very young children and infants are at high risk for dehydration because they cannot communicate their symptoms and because dehydration may occur rapidly. Pregnant women should pay particular attention to keeping up with fluid losses.
Although norovirus infection is not fatal, it can contribute to mortality by causing underlying illnesses to become worse. Elderly, debilitated people are especially at risk for complications related to dehydration, including kidney failure.
What is the prognosis of a norovirus infection?
Norovirus infection is a self-limited illness that lasts two to three days in most people. As discussed above, complications are usually related to dehydration or underlying illnesses. In some countries where poor hydration already may exist in children, many may die from dehydration if IV replenishment is not available.
In 2006, the World Health Organization recommended a new prepackaged oral rehydration salts (ORS) formula that can be shipped to underdeveloped countries and simply poured into clean water that can prevent dehydration in many patients. This approach has improved the prognosis for many children in underdeveloped countries.