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Noroviruses belong to the family Caliciviridae. They are a group of non-enveloped, single-stranded RNA viruses that primarily cause acute gastroenteritis in humans. Noroviruses were originally called Norwalk-like viruses because they were first identified as a cause of gastroenteritis in school children at Bronson Elementary School in Norwalk, Ohio, in 1968 The name was officially changed to norovirus by the International Committee on Taxonomy of Viruses in 2002.

Noroviruses are classified into ten genogroups (GI-GX) and 48 genotypes. Frequent recombination between strains contributes to genetic diversity. Most noroviruses that infect humans belong to Genogroups GI, GII, and GIV.

According to the Centers for Disease Control and Prevention, there are 21 million cases of acute gastroenteritis due to Norovirus each year in the United States, 109,000 hospitalizations, and 900 deaths. Children between the ages of 6 months and 18 months and adults older than 65 years of age are at highest risk of developing severe disease.

Norovirus can survive on surfaces for weeks and is easily transmitted from person-to-person via the fecal-oral route. Transmission most commonly occurs when a person touches their mouth after direct contact with an infected person or contaminated surfaces. Norovirus can also be transmitted by ingesting food that has been handled by an infected person. A less common mode of transmission is exposure to aerosolized water after an infected person flushes a toilet.

The most effective preventive measures are thorough hand washing and cleaning potentially contaminated surfaces with bleach-based products. Alcohol-based hand sanitizers are not effective against norovirus.

Noroviruses are an important cause of food-borne outbreaks of gastroenteritis, especially after the ingestion of shellfish. This is particularly true of oysters since they are filter feeders that concentrate particulate matter from the environment. Noroviruses are also frequent causes of travelers' diarrhea. Dual infections with enterotoxigenic E. coli are common in this setting.

Viral shedding can occur at extremely high levels of up to 7.7 x 10(10) viruses/gm of stool and persist for weeks after symptoms have resolved. Norovirus can be spread by an asymptomatic infected person for up to 2 weeks after symptoms have resolved. Clinical illness can be caused by transmission of relatively low doses of virus. The median infectious dose is only 18 viruses

Norovirus outbreaks are most common between November and April and usually peak during February and March in the United States. Symptoms usually develop within 12 to 48 hours after exposure and include non-bloody diarrhea, nausea, vomiting, and abdominal cramping. Clinical illness usually lasts for 24 to 72 hours. The most common complication is dehydration.

Norovirus outbreaks are characterized by high secondary attack rates. On average, one infected person will infect 2 to 7 other people. Consequently, norovirus outbreaks have been reported in diverse settings including nursing homes, long-term care facilities, retirement centers, restaurants, hospitals, schools, day care centers, and cruise ships. Noroviruses have also been responsible for large outbreaks among disaster refugees such as after hurricane Katrina. They have also caused outbreaks among military personnel and athletic teams.

After infection, a person develops immunity to the same norovirus genotype which persists for 6 months to 2 years. However, this immunity does not fully protect a person against reinfection with a different norovirus genotype. For this reason, individuals can get norovirus illness many times during their lifetime.

Stool specimens collected within the first 72 hours of symptom onset are preferred for laboratory diagnosis of norovirus illness. Stool specimens can be stored refrigerated in Cary Blair medium.

TaqMan-based RT-PCR assays use different oligonucleotide primer sets to detect the RNA of genogroup I, genogroup II, genogroup VIII and GIX noroviruses. PCR is very sensitive and can detect as few as 10 to 100 norovirus copies. Several commercial platforms can detect multiple gastrointestinal pathogens including norovirus. They have approximately the same sensitivity as RT-PCR.

Rapid commercial enzyme immunoassays (EIAs) that detect norovirus antigen in stool samples are available but are not recommended because of their poor sensitivity (50 to 75%).

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