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Norwalk virus

Transmission electron micrograph of Noroviruses. The bar = 50 nm
Virus classification
Group: Group IV ((+)ssRNA)
Family: Caliciviridae
Genus: Norovirus

Norovirus, a genus of RNA virus in the family Caliciviridae, causes around 50% of all epidemic gastroenteritis (stomach pain, diarrhea, and vomiting) around the world.[1] It is the most important group of viruses associated with this condition. Norovirus affects people of all ages. They are often transmitted by faecally contaminated food or water and are, therefore, subject to control by public health measures. Immunity to the virus is not complete or long-lasting. Outbreaks of norovirus disease often occur in closed or semi-closed communities, such as hospitals, prisons and cruise ships where people from different locations are brought together for the first time. Norovirus is rapidly inactivated by chlorine-based disinfectants but is less susceptible to alcohols and detergents. This is because the virus does not have a lipid envelope.


History of identification

The Norwalk virus was named after Norwalk, Ohio, where an outbreak of acute gastroenteritis occurred among children at an elementary school in November 1968. In 1972, immune electron microscopy on saved stool samples identified the virus, which was called Norwalk virus, and was initially grouped as "Small Round-Structured Viruses". 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.

Synonyms of disease

Common names of the illness caused by Noroviruses are winter vomiting disease, viral gastroenteritis, acute non-bacterial gastroenteritis, food poisoning, and (in American slang) stomach flu.

Nature of disease

The disease is usually self-limiting, and characterized by nausea, vomiting, diarrhea, and abdominal pain. General lethargy, weakness, muscle aches, headache, and low-grade fever may occur. Symptoms may persist for several days and may become life-threatening in the young, the elderly, and the immune-compromised if dehydration is ignored or not treated.

Diagnosis of human illness

Specific diagnosis of Norovirus is routinely made by broadly reactive conventional polymerase chain reaction (PCR) assays which more recently were replaced by real-time PCR assays, which give results within a few hours. Furthermore, these assays are very sensitive and can detect concentrations as low as 10 virus particles per assay. The disease is painful and also highly contagious.

Less technically demanding assays such as ELISA assays based on antibodies against a cocktail of different virus-like particles are commercially available but have been reported to lack both specificity and sensitivity.

Prevention and infection control

Hand washing remains an effective method to reduce the spread of Norovirus pathogens. Norovirus can be aerosolized when those stricken with the illness vomit. Surface sanitizing is recommended in areas where the Norovirus may be present on surfaces.

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.

Associated foods

Norwalk gastroenteritis is transmitted by the fecal-oral route via contaminated water and foods. A CDC study of eleven 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 Norwalk outbreaks. Ingestion of raw or insufficiently steamed clams and oysters poses a high risk for infection with the Norwalk virus. Foods other than shellfish are contaminated by ill food handlers.

Relative frequency of disease

Only the common cold is reported more frequently than viral gastroenteritis as a cause of illness in the U.S. Although viral gastroenteritis is caused by a number of viruses, it is estimated that Norwalk viruses are responsible for about 1/3 of the cases over the 6-to-24-month age group. In developing countries the percentage of individuals who have developed immunity at an early age is very high. In the U.S. the percentage increases gradually with age, reaching 50% in the population over 18 years of age. Immunity, however, is not permanent and reinfection can occur. There is some evidence that blood types B and AB confer partial protection against symptomatic infection.[2][3]

Course of disease and complications

Norovirus causes acute gastroenteritis that develops between 24 and 48 hours after exposure with a median of 33-36 hours, and lasts for 24-60 hours.[1] Severe illness is rare: although people are frequently treated in emergency rooms/A&E, they are rarely admitted to the hospital. The number of deaths from norovirus in the US is estimated to be around 300, with most of these occurring in the very young, elderly and persons with weakened immune systems.

Susceptible populations

All individuals who ingest the virus and who have not (within 24 months) had an infection with the same or related strain, are susceptible to infection and can develop the symptoms of gastroenteritis. Disease is more frequent in adults and older children than in the very young.

Detection of norovirus in foods

Routine protocols to detect norovirus (norovirus RNA) in clams and oysters by RT-PCR (reverse transcription polymerase chain reaction) are being employed by governmental laboratories such as the FDA in the USA. However, routine methods to detect the virus on other food items are not readily available due to the variable nature of different food items affecting concentration and extraction of the virus and presence of factors that make PCR (Polymerase chain reaction) analysis techniques ineffective.


Noroviruses can genetically be classified into 5 different genogroups (GI, GII, GIII, GIV, and GV) which can be further divided into different genetic groups 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.

Norwalk viruses 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-90s, Farmington Hills virus associated with outbreaks in Europe and the United States in 2002 and in 2004 Hunter virus was associated with outbreaks in Europe, Japan and Australasia. In 2006 there was another large increase in NoV infection around the globe. In December, 2007 there was an outbreak at a country club in northern California where around 80-100 people got sick. Two new GII.4 variants caused around 80% of those Norovirus associated outbreaks and they have been termed 2006a and 2006b.[citation needed]


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.

Norovirus on cruise ships

Norovirus continues to be a problem on cruise ships. In 2002, there were 25 reported outbreaks, with 2,648 passengers becoming ill from the virus.[4] Outbreak investigations by the U.S. Centers for Disease Control and Prevention have shown that transmission among cruise ship passengers is almost wholly person-to-person. Cruise ship water supplies have never been implicated.

  • In November 2002 about 100 passengers aboard the Holland America Line cruise ship Amsterdam came down with a severe stomach virus which had been spreading through its cruise ships.[5]
  • In December 2002 over 100 passengers and crew on board the P&O Cruises cruise ship Oceana caught the Norwalk vomiting virus.[6]
  • In Feb 2003 almost 250 passengers and crew on board the Princess Cruises cruise ship Sun Princess were taken ill during a two week trip around Hawaii.[7]
  • In November 2003 about 400 holidaymakers on the P&O Cruises cruise ship Aurora were affected by the highly contagious norovirus stomach bug.[8]
  • In June 2006 the Fred. Olsen Cruise Lines cruise ship Black Prince returned to port carrying scores of passengers suffering from a highly contagious vomiting virus.[9]
  • In November 2006, more than 700 passengers and crew members aboard a trans-Atlantic cruise fell ill with vomiting and diarrhea. The outbreak, believed to be norovirus, struck people aboard the Carnival Cruise Lines' Carnival Liberty, one of the world's largest cruise ships.[10]
  • In December 2006, 384 passengers of Freedom of the Seas, of Royal Caribbean International, the largest cruise ship in the world contracted the Norovirus. At the same time, 97 passengers and 6 crew members of the Princess Cruises Sun Princess cruise ship appeared to have Norovirus symptoms. Weeks earlier, 97 passengers of the same Freedom of the Seas had also contracted the virus.
  • In January 2007, several cases were discovered aboard the cruise-liner Queen Elizabeth 2[11] of Cunard Line in what the Center for Disease Control (CDC) called an "unusually large outbreak." Seventeen percent (276 out of 1,652) of the passengers on board reportedly fell ill. 28 crew members were also ill. The ship was boarded and investigated by members of the CDC while the ship was docked in Acapulco, Mexico. Because of the intensified sanitation efforts of the crew (even going so far as to sanitize poker chips in the ship's casino), the outbreak was considered well under control. The ship continued its voyage and no passengers canceled their tickets because of the illness.[12]
  • On its 1 April 2007 voyage, the Princess Cruises ship Caribbean Princess had at least 79 passenger and 4 crew member cases of Norovirus, or Norwalk disease.
  • In late May 2007, nearly 200 passengers and 10 crew members aboard Royal Caribbean's newest ship, Liberty of the Seas, contracted Norovirus.
  • In late August 2007 over 500 passengers and an unknown number of crew members on the Celebrity Cruises ship Millennium contracted Norovirus before the end of a 12-day (Aug. 17-29, 2007) cruise from Barcelona to Venice. Mostly older people fell ill, an unknown but considerable number of them were hospitalized in the ship's hospital, and after disembarkation around 10 were sent home, mostly to the U.S., on stretchers. A smell of sewage was reported to be everpresent on the lower decks (1 and 2) of the ship. According to an anonymous crew member this odor had been there for at least five months. The outbreak was officially acknowledged and identified as Norwalk-like by the ships's captain, Zisis Taramas, in a newsletter to passengers. Crews were ordered to disinfect the ship.
  • In mid-November 2007 state health officials identified a Norwalk virus outbreak affecting about 220 passengers aboard Norwegian Cruise Line's Pride of Hawaii. The virus infected 9 percent of the ships 2,500 passengers. Passengers who felt sick and their cabinmates were asked to remain in their rooms for 24 hours. Norwegian said it was giving them a $200 on-ship credit.
  • In early December 2007 the virus infected about 10% of the passengers and crew aboard Norwegian Cruise Line's Pearl out of Miami. They also were given an on-ship credit.
  • In late December 2007 the virus infected about 80 of 3000 passengers during the second voyage of Cunard Line's cruise liner MS Queen Victoria.[13]

Norovirus mass infections

Main article: Nokia water supply contamination

In late November 2007 Norovirus was one of several infective agents introduced to a municipal potable water supply. In the Kullaanvuori sewage treatment plant of City of Nokia, Finland the fluctuation of drinking water and sewage water was reversed due to a valve between the two systems being unintentionally left open by maintenance staff. The city of 30,000 inhabitants was incapacitated by lack of water supply, and thousands of citizens became sick. In some of the schools more than half of the students and staff were affected. According to analysed samples drinking water had been contaminated by, at least, Norovirus, Adenovirus and Campylobacter. Salmonella bacteria were detected in some samples. Cleansing of the water system was expected to take weeks. Another Wave of Nokia Residents Seeks Medical Care[14][15][16]

In January 2008, it was estimated by doctors that 100,000 people each week were developing symptoms of Norovirus infection in the United Kingdom. At least 100 hospital wards in the country had been closed due to outbreaks of the virus.[17]


  1. ^ a b Norovirus: Technical Fact Sheet. National Center for Infectious Diseases, CDC.
  2. ^ "Norwalk Virus Infection and Disease Is Associated with ABO Histo-Blood Group Type", The Journal of Infectious Diseases, 2002. Retrieved on 2007-02-05. 
  3. ^ "Binding of Norwalk Virus-Like Particles to ABH Histo-Blood Group Antigens Is Blocked by Antisera from Infected Human Volunteers or Experimentally Vaccinated Mice", Journal of Virology, 2002. Retrieved on 2007-02-05. 
  4. ^ Sea Sick — Infection Outbreaks Challenge the Cruise Ship Experience. Water Quality and Health Council.
  5. ^ BBC news Nov 2002 - Virus ravages US cruise ships
  6. ^ BBC news Dec 2002 - Vomiting virus outbreak on Oceana liner
  7. ^ BBC news Feb 2003 - 250 taken ill on P&O cruise
  8. ^ BBC news Nov 2003 - Bug-hit P & O liner Aurora heads for Gibraltar
  9. ^ BBC news June 2006 - Virus-hit liner returns to port
  10. ^ BBC news Nov 2006 - Virus-hit cruise ship ends voyage.
  11. ^ "Norovirus sickens hundreds aboard QE2",, 2007-01-24. Retrieved on 2007-01-24. .
  12. ^ BBC news Jan 2007 - Vomiting virus sweeps through QE2
  13. ^ Luxury liner hit by sickness bug. BBC News.
  14. ^ [1]
  15. ^ [2]
  16. ^ [3]
  17. ^ [4]
The above was originally based on a page from the public domain Bad Bug Book.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Norovirus". A list of authors is available in Wikipedia.
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