Norwalk virus group

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? Norwalk virus
Virus classification
Family: Caliciviridae Genus: Norovirus Species: Norwalk virus

Norwalk virus is the prototype virus of the genus Norovirus of the family Caliciviridae. Norovirus is an emerging virus and recent scientific findings reveal that it causes around 50% of all gastroenteritis (stomach pain, diarrhea, and vomiting) around the world. It is therefore now the most important virus associated with this condition. Noroviruses contain a positive strand RNA genome of approximately 7.5 kbp, encoding a major structural protein (VP1) of about 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. 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.

Historically, noroviruses have been named after the places where the outbreaks occurred, e.g., Norwalk, Ohio, where in November 1968 an outbreak of acute gastroenteritis occurred among children at an elementary school. In 1972, immune electron microscopy on saved stool samples identified the virus, which was called Norwalk virus. Numerous outbreaks with similar symptoms have been reported since and after initially being grouped as "Small Round-Structured Viruses". 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.

Norovirus classification is based upon numbering genogroups with Roman numerals (I, II or IV for human strains) and genotypes with numbers. For example, virus strains of genogroup II, genotype 4, are classified as a GII.4. GII.4 viruses 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 Australiasia. Last year (2006) there was another large increase in NoV infection around the globe. Two new GII.4 variants caused around 80% of those norovirus associated outbreaks and they have been termed 2006a and 2006b.

Contents 1 Nature of acute disease 2 Nature of disease 3 Diagnosis of human illness 4 Prevention and infection control 5 Associated foods 6 Relative frequency of disease 7 Course of disease and complications 8 Susceptible populations 9 Detection of norovirus in foods 10 Cruise ships 11 Notes 12 References 13 External links

[edit] Nature of acute disease

Common names of the illness caused by noroviruses (previously called Norwalk-like viruses) are winter vomiting disease, viral gastroenteritis, acute nonbacterial gastroenteritis, (incorrectly) food poisoning, and (in American slang) stomach flu. The virus can cause symptoms such as stomach pain, diarrhea, and vomiting in humans, as well as general lethargy, weakness, and muscle aches.

[edit] Nature of disease

The disease is usually self-limiting, and characterized by nausea, vomiting, diarrhea, and abdominal pain. 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.

[edit] Diagnosis of human illness

Specific diagnosis of norovirus is routinely made by broadly reactive conventional PCR assays which more recently were replaced by real-time (RT)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. 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.

[edit] 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.

Though alcohol has been proven to be an effective sanitizer against other viruses, the CDC recommends using alcohol-based sanitizers in conjunction with hand washing to reduce the spread of norovirus. In healthcare environments, the prevention of nosocomial infections involve routine and terminal cleaning. Nonflammable alcohol vapor in CO2 systems or sodium hypochlorite are frequently used to sanitize rooms occupied by patients with norovirus symptoms.

[edit] 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 the often overlooked culprit, 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.

[edit] 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^[1][2].

[edit] Course of disease and complications

Norovirus causes acute gastroenteritis that develops between 24 and 48 hours after exposure with a median of 33-36 h, and lasts for 24-60 hours.^[3] Severe illness is rare: although people are frequently treated in emergency rooms, 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.

[edit] 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.

[edit] Detection of norovirus in foods

Routine protocols to detect norovirus (norovirus RNA) in clams and oysters by RT-PCR are being employed by most governmental laboratories such as FDA. However, routine methods to detect the virus on other food items are not readily available due to the variable nature of different food items regarding concentration and extraction of the virus with a special emphasis on removing PCR inhibitory factors.

[edit] 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 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' Liberty, one of the world's largest cruise ships.

In December 2006, 384 passengers of Freedom of the Seas, the largest cruise ship in the world contracted the norovirus. At the same time, 97 passengers and 6 crew members of 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 QE2^[5] 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 cancelled their tickets because of the illness.

[edit] Notes

• Norwalk virus is also known as Norvovirus.

[edit] References

• "Sea Sick -- Infection Outbreaks Challenge the Cruise Ship Experience" Water and Health, Winter 2004.

• "Students' Sickness Is Blamed On Virus" by Amit R. Paley and Jamie Stockwell, The Washington Post, August 11, 2004, retrieved August 22, 2006

• "Outbreak Sickens About 100 at College Park Retreat" by Amit R. Paley, The Washington Post, August 28, 2004, retrieved August 22, 2006

• Washing your hands video - Norwalk Prevention

The above was originally based on a page from the public domain Bad Bug Book.

[edit] External links

• CDC Viral Gastroenteritis FAQs: Center for Disease Control and Prevention of Food Illness Fact Sheet
• Loci index for genome Norwalk virus. Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.
• Norwalk Virus and its Outbreaks in Japan
• "Norovirus in Healthcare Facilities Fact Sheet", CDC, released December 21, 2006