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Norovirus (cont.)

Norovirus Infection Cause

After an individual has contracted norovirus, it first attaches to cells in the gastrointestinal tract. 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 in the laboratory, it has not been precisely determined exactly how the virus causes disease. 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.

Norovirus is mainly spread by the fecal-oral route person-to-person. The virus is contagious and is also spread by droplets in the air (usually by droplets associated with vomiting), and if the droplets land on food or other objects, the virus can be transmitted to the mouth when touched by the hands.

Unfortunately, the virus has been referred to by many names (for example, Norwalk virus, Norwalk-like virus or NLV, SRSV [small round structured viruses], and 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.

In January 2013, the U.S. Centers for Disease Control and Prevention (CDC) reported a new strain of Gll.4 that originated in Australia and had spread to the United States in 2012. During that time, there were 266 norovirus outbreaks; 53% of these outbreaks were due to this strain. The strain was associated with higher rates of hospitalization and death than other norovirus strains.

Recently, researchers have shown that human noroviruses can be carried by other animals such as dogs, which can transmit the viruses to humans. Consequently, some investigators consider norovirus infections to be a zoonotic disease (a disease transferred between animals and humans).

Medically Reviewed by a Doctor on 11/18/2014

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