|Transmission electron micrograph of noroviruses. The white bar = 50 nm|
|Group:||Group IV ((+)ssRNA)|
Noroviruses are a genetically diverse genus of single-stranded RNA, non-enveloped viruses in the Caliciviridae family. The known viruses in the genus are all considered to be the strains of a single species called Norwalk virus. The viruses are transmitted by fecally contaminated food or water; by person-to-person contact; and via aerosolization of the virus and subsequent contamination of surfaces. Noroviruses are the most common cause of viral gastroenteritis in humans, and affect people of all ages.
Norovirus infection is characterized by nausea, forceful vomiting, watery diarrhea, and abdominal pain, and in some cases, loss of taste. General lethargy, weakness, muscle aches, headache, coughs, and low-grade fever may occur. The disease is usually self-limiting, and severe illness is rare. The virus affects around 267 million people and causes over 200,000 deaths each year; these deaths are usually in less developed countries and in the very young, elderly and immuno-suppressed.
Winter vomiting bug is a common term for noroviruses in the UK, because the virus tends to cause vomiting and to spread more easily in winter, when people tend to spend more time indoors and near to each other.
After infection, immunity to norovirus is usually incomplete and temporary. Outbreaks of norovirus infection often occur in closed or semiclosed communities, such as long-term care facilities, overnight camps, hospitals, prisons, dormitories, and cruise ships, where the infection spreads very rapidly either by person-to-person transmission or through contaminated food. Many norovirus outbreaks have been traced to food that was handled by one infected person.
Norovirus is rapidly inactivated by either sufficient heating or by chlorine-based disinfectants, but the virus is less susceptible to alcohols and detergents, as it does not have a lipid envelope.
The genus name Norovirus is derived from Norwalk virus, the only species of the genus. The species causes approximately 90% of epidemic nonbacterial outbreaks of gastroenteritis around the world, and may be responsible for 50% of all foodborne outbreaks of gastroenteritis in the United States.
Specific diagnosis of norovirus is routinely made by polymerase chain reaction (PCR) assays or real-time PCR assays, which give results within a few hours. These assays are very sensitive and can detect as few as 10 virus particles.
Noroviruses are transmitted directly from person to person and indirectly via contaminated water and food. They are extremely contagious, and fewer than twenty virus particles can cause an infection. Transmission occurs through ingesting contaminated food and water and by person-to-person spread. Transmission can be aerosolized when those stricken with the illness vomit, and can be aerosolized by a toilet flush when vomit or diarrhea is present; infection can follow eating food or breathing air near an episode of vomiting, even if cleaned up. The viruses continue to be shed after symptoms have subsided and shedding can still be detected many weeks after infection.
Vomiting, in particular, transmits infection effectively. In one incident, a person who vomited spread infection right across a restaurant, suggesting that many unexplained cases of food poisoning may have their source in vomit. 126 people were dining at six tables in December 1998; one woman vomited. Staff quickly cleaned up, and people continued eating. Three days later others started falling ill; 52 people reported a range of symptoms, from fever and nausea to vomiting and diarrhea. The cause was not immediately identified. Researchers plotted the seating arrangement: more than 90% of the people at the same table as the sick woman later reported becoming ill. There was a direct correlation between the risk of infection of people at other tables and how close they were to the sick woman. More than 70% of the diners at an adjacent table fell ill; at a table on the other side of the restaurant, the rate was still 25%. The outbreak was attributed to a Norwalk-like virus (norovirus). Other cases of transmission by vomit were later identified.
In one outbreak at an international scout jamboree in the Netherlands, each person with gastroenteritis infected an average of 14 people before increased hygiene measures were put in place. Even after these new measures were enacted, an ill person still infected an average of 2.1 other people. A CDC study of 11 outbreaks in New York State lists the suspected mode of transmission as person-to-person in seven outbreaks, foodborne in two, waterborne in one, and one unknown. The source of waterborne outbreaks may include water from municipal supplies, wells, recreational lakes, swimming pools and ice machines.
Shellfish and salad ingredients are the foods most often implicated in norovirus outbreaks. Ingestion of shellfish that have not been sufficiently heated poses a high risk for norovirus infection. Foods other than shellfish may be contaminated by infected food handlers.
Noroviruses (NoV) are a genetically diverse group of single-stranded RNA, non-enveloped viruses belonging to the Caliciviridae family. According to the International Committee on Taxonomy of Viruses, the genus Norovirus has one species, which is called Norwalk virus. Serotypes, strains and isolates include:
- Norwalk virus;
- Hawaii virus;
- Snow Mountain virus;
- Mexico virus;
- Desert Shield virus;
- Southampton virus;
- Lordsdale virus;
- Wilkinson virus.
Noroviruses commonly isolated in cases of acute gastroenteritis belong to two genogroups: genogroup I (GI) includes Norwalk virus, Desert Shield virus and Southampton virus; and II (GII), which includes Bristol virus, Lordsdale virus, Toronto virus, Mexico virus, Hawaii virus and Snow Mountain virus.
Noroviruses can genetically be classified into five different genogroups (GI, GII, GIII, GIV, and GV), which can be further divided into different genetic clusters or genotypes. For example, genogroup II, the most prevalent human genogroup, presently contains 19 genotypes. Genogroups I, II and IV infect humans, whereas genogroup III infects bovine species, and genogroup V has recently been isolated in mice.
Most noroviruses that infect humans belong to genogroups GI and GII. Noroviruses from Genogroup II, genotype 4 (abbreviated as GII.4) account for the majority of adult outbreaks of gastroenteritis and often sweep across the globe. Recent examples include US95/96-US strain, associated with global outbreaks in the mid- to late-1990s; Farmington Hills virus associated with outbreaks in Europe and the United States in 2002 and in 2004; and Hunter virus which was associated with outbreaks in Europe, Japan and Australasia. In 2006, there was another large increase in NoV infection around the globe. Reports have shown a link between the expression of human histo-blood group antigens (HBGAs) and the susceptibility to norovirus infection. Studies have suggested the viral capsid of noroviruses may have evolved from selective pressure of human HBGAs.
A 2008 study suggests the protein MDA-5 may be the primary immune sensor that detects the presence of noroviruses in the body. Interestingly, some people have common variations of the MDA-5 gene that could make them more susceptible to norovirus infection.
A 2010 study suggested a specific genetic version of norovirus (which would not be distinguishable from other types of the virus using standard viral antibody tests) interacts with a specific mutation in the ATG16L1 gene to help trigger symptomatic Crohn's disease in mice that have been subjected to a chemical that causes intestinal injury similar to the process in humans. (There are other similar ways for such diseases to happen like this, and this study in itself does not prove norovirus causes Crohn's in humans).
Noroviruses contain a positive-sense RNA genome of approximately 7.5 kbp, encoding a major structural protein (VP1) of about 58~60 kDa and a minor capsid protein (VP2). The virus particles demonstrate an amorphous surface structure when visualized using electron microscopy and are between 27-38 nm in size. The most variable region of the viral capsid is the P2 domain, which contains antigen-presenting sites and carbohydrate-receptor binding regions.
The estimated mutation rate (1.21 x 10−2 to 1.41 x 10−2 substitutions per site per year) in this virus is high even compared with other RNA viruses.
When a person becomes infected with norovirus, the virus begins to replicate within the small intestine. After approximately one to two days, norovirus infection symptoms can appear. The principal symptom is acute gastroenteritis that develops between 24 and 48 hours after exposure, and lasts for 24–60 hours. The disease is usually self-limiting, and characterized by nausea, forceful vomiting, watery diarrhea, and abdominal pain, and in some cases, loss of taste. General lethargy, weakness, muscle aches, headache, coughs, and low-grade fever may occur.
Severe illness is rare; although people are frequently treated at the emergency ward, they are rarely admitted to the hospital. The number of deaths from norovirus in the United States is estimated to be around 300 each year, with most of these occurring in the very young, the elderly, and persons with weakened immune systems. Symptoms may become life-threatening in these groups if dehydration is ignored or not treated.
Hand washing with soap and water is an effective method for reducing the transmission of norovirus pathogens. Alcohol rubs (≥62% ethanol) may be used as an adjunct, but are less effective than hand-washing, as norovirus lacks a lipid viral envelope. Surfaces where norovirus particles may be present can be sanitised with a solution of 1.5% to 7.5% of household bleach in water, or other disinfectants effective against norovirus.
In health-care environments, the prevention of nosocomial infections involves routine and terminal cleaning. Nonflammable alcohol vapor in CO2 systems are used in health care environments where medical electronics would be adversely affected by aerosolized chlorine or other caustic compounds.
Ligocyte announced in 2007 that it was working on a vaccine and had started phase 1 trials. As of 2011[update], a monovalent nasal vaccine had completed phase I/II trials, while bivalent intramuscular and nasal vaccines were at earlier stages of development. The two vaccines rely on using a virus-like particle that is made of the Norovirus capsid proteins in order to mimic the external structure of the virus. Since there is no RNA in this particle, it is incapable of reproducing and cannot cause an infection.
The norovirus can survive for long periods outside a human host depending on the surface and temperature conditions: 12 hours on hard surfaces, and up to 12 days on contaminated fabrics, and it can survive for months, maybe even years in contaminated still water. A study done in 2006 found the virus on several surfaces used for food preparation 7 days after contamination.
Detection in foods 
Routine protocols to detect norovirus (norovirus RNA) in clams and oysters by reverse transcription polymerase chain reaction are being employed by governmental laboratories such as the FDA in the USA.
Epidemiological data from developing countries about the importance of norovirus in pediatric diarrhea are limited. Recently, in Nicaragua, it has been observed that norovirus is responsible for 11% of the diarrhea cases occurring in children less than five years of age at community level and 15% of the moderate to severe cases requiring intravenous rehydration.
In the Guangdong province of the People's Republic of China, the Provincial Health Department said on December 17, 2010, that it had confirmed 429 cases of norovirus infection in the November 2010 outbreak in Conghua, Guangzhou, but no one died from it.
Human genetics 
A non-functional fucosyltransferase FUT2 provides high protection from the most common norovirus GII.4. Functional FUT2 fucosyltransferase transfers a fucose sugar to the end of the Histo-blood group ABO(H) precursor in gastrointestinal cells and saliva glands. The ABH antigen produced is thought to act as receptors for human norovirus. Homozygous carriers of any nonsense mutation in the FUT2 gene are called non-secretors, as no ABH antigen is produced. Approximately 20% of Caucasians are non-secretors due to the G428A and C571T nonsense mutations in FUT2 and therefore have strong although not absolute protection from the norovirus GII.4. Non-secretors can still produce ABH antigens in erythrocytes, as the precursor is formed by FUT1. Some norovirus genotypes (GI.3) can infect non-secretors.
The norovirus was originally named the "Norwalk agent" after Norwalk, Ohio, in the United States, where an outbreak of acute gastroenteritis occurred among children at Bronson Elementary School in November 1968. In 1972, electron microscopy on stored human stool samples identified a virus, which was given the name "Norwalk virus." Numerous outbreaks with similar symptoms have been reported since. The cloning and sequencing of the Norwalk virus genome showed that these viruses have a genomic organization consistent with viruses belonging to the family Caliciviridae. The name was shortened to "norovirus" after being identified in a number of outbreaks on cruise ships and receiving attention throughout the United States. The name "norovirus" (Norovirus for the genus) was approved by the International Committee on Taxonomy of Viruses (ICTV) in 2002. In 2011, however, a press release and a newsletter were published by ICTV, which strongly encourage the media, national health authorities and the scientific community to use the virus name Norwalk virus, rather than the genus name Norovirus, when referring to outbreaks of the disease. This is also a public statement of ICTV to the request from an individual in Japan to rename the Norovirus genus because of the possibility of negative associations for people in Japan and elsewhere who have the family name "Noro". Before this position of ICTV was made public, ICTV consulted widely with members of the Caliciviridae Study Group and carefully discussed the case.
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