Norwalk virus group

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Norwalk virus

Virus classification

Group:	Group IV ((+)ssRNA)
Family:	Caliciviridae
Genus:	Norovirus
Species:	Norwalk virus

Norwalk virus is the prototype virus of the genus Norovirus of the family Caliciviridae. Noroviruses contain a positive strand RNA genome of approximately 7.5 kb, 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 visulaized 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 USA in 2002.

1 Nature of acute disease
2 Nature of disease
3 Diagnosis of human illness
4 Associated foods
5 Cruise ships
6 Relative frequency of disease
7 Course of disease and complications
8 Target populations
9 Food analysis
10 Selected outbreaks
11 External links
12 References

[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 (most commonly in American English) stomach flu. The virus can cause symptoms such as stomach pain, diarrhea, and vomiting in humans, as well as general lethargy and weakness.

[edit] Nature of disease

The disease is self-limiting, mild, and characterized by nausea, vomiting, diarrhea, and abdominal pain. Headache and low-grade fever may occur. Based on volunteer studies the infectious dose has been estimated to be in the range of 10-100 virus particles. Symptoms may persist for several days.

[edit] Diagnosis of human illness

Specific diagnosis of norovirus is routinely made by broadly-reactive conventional RT-PCR assays which are recently more and more replaced by realtime RT-PCR assays which give results within a few hours. Furthermore, these assays are very sensitive and can detect as low concentrations as 10 virus particles per ml. 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] Associated foods

Norwalk gastroenteritis is transmitted by the fecal-oral route via contaminated water and foods. Secondary person-to-person transmission has been documented. Water is the most common source of outbreaks and 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 Norwalk virus. Foods other than shellfish are contaminated by ill food handlers.

[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. [1] 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.

[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 not involving 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.

[edit] Course of disease and complications

Norovirus causes acute gastroenteritis that usually develops 24-48 h after contaminated food or water is consumed and lasts for 24-60 hours. 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 occuring in the elderly.

[edit] Target 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] Food analysis

The virus has been identified in clams and oysters by radioimmunoassay. The genome of Norwalk virus has been cloned and development of gene probes and PCR amplification techniques to detect the virus in clinical specimens and possibly in food are underway.

[edit] Selected outbreaks

Literature references can be found at the links below.

In early December 2006 more than 380 passengers and crew aboard the world's largest cruise ship were sickened by a virus during a seven-day Caribbean cruise. [2]

In late November 2006 14 employees at the Bloomer Community Hospital and half of the Nursing Home residents in Bloomer, Wisconsin experienced an outbreak of norovirus.[3]

In mid November 2006 about two dozen residents of Rice Lake, Wisconsin and the San Francisco Bay Area experienced an outbreak of norovirus.[4]

In mid November 2006 the University of Saskatchewan experienced a Norwalk outbreak.[5]

In early November 2006 the Carnival Cruise Lines ship Liberty was overwhelmed with over 700 passengers reported as sick with a possible norovirus.[6]

On October 11, 2006 an estimated 80 - 100 students[7][8] at Mount Allison University in Sackville, New Brunswick fell ill with what was suspected to be a Norwalk-like virus.
- By October 15, 2006, 297 students had been affected by the illness[9]. The university administration canceled classes on Friday 13 and extra-curricular activities until the 17, on the advice of health officials. A similar virus has been reported at St. Francis Xavier University, in Antigonish, Nova Scotia affecting over 50 students and staff[10][11]. Other Univerisities, such as the University of King's College, have reported but not conifrmed cases of the Norwalk virus. Health advisories have been dispatched to other universities in the Maritimes, warning students against visiting these colleges so as to limit the spread of the virus among students.

On May 7, 2006, the last day of the English Premier League, Tottenham Hotspur FC had an outbreak of the Norovirus in their squad with up to ten players suffering symptoms. It was initially blamed on food poisoning from the Marriott hotel they were staying in. A request to postpone the game for 24 hours was denied and Tottenham eventually lost the game against West Ham 2-1, thus giving their London nemesis Arsenal FC the final Champions League place. On 18 May 2006, results confirmed that Norovirus, a form of viral gastroenteritis, had been identified in three samples taken from Spurs players, and it had been contracted prior to their stay at the hotel.

On March 17, 2006, New Zealand high school Nelson College was closed due to an outbreak of gastroenteritis among the boarding students that appears to be caused by a norovirus.

In March 2006, many schools had to be closed down across West Yorkshire (Northern England) due to outbreak of the disease.

Two outbreaks occurred in August 2004 in the College Park, Maryland area. Beginning on August 9, about 100 students attending a leadership conference at the University of Maryland fell ill with severe gastroenteritis. [12] During the week of August 23, another 100 people at the nearby American Center for Physics experienced the same symptoms. [13] In the latter case, the virus was found to have been carried to the building by a university faculty member attending an off-site retreat. Both incidents were later confirmed as norovirus outbreaks.

Over 40 people got infected with norovirus after attending birthday parties on April 16, 2005 at a Victorville, California Chuck E. Cheese's, including a 6-year-old whose parents filed a lawsuit against CEC Entertainment, Inc., the parent company of Chuck E. Cheese's. County health officials confirmed that the cause of the outbreak was failure to properly sanitize and disinfect a sky tube after an individual vomited inside the sky tube.

MMWR 48(11):1999 During August 27-September 1, 1998, 99 (12%) of 835 soldiers in one unit at a U.S. Army training center in El Paso, Texas, were hospitalized for acute gastroenteritis (AGE). Their symptoms included acute onset of vomiting, abdominal pain, diarrhea, and fever. Review of medical center admission records for AGE during the previous year indicated that fewer than five cases occurred each month.

MMWR 44(2):1995 A report on the multistate outbreak of viral gastroenteritis associated with consumption of oysters from Apalachicola Bay, Florida, December 1994-January 1995.

MMWR 43(24):1994 During November 20-30, 1993, four county public health units (CPHUs) of the Florida Department of Health and Rehabilitative Services (HRS) in northwestern Florida conducted preliminary investigations of seven separate outbreaks of foodborne illness following consumption of raw oysters. On December 1, the HRS State Health Office initiated an investigation to characterize the illness, examine risk factors for oyster-associated gastroenteritis, and quantify the dose-response relation. In four specimens, small round-structured viruses were detected by EM; in one specimen, a Norwalk-like genome was confirmed by RT-PCR.

MMWR 42(49):1993 On November 17, 1993, the state health departments of Louisiana, Maryland, and Mississippi notified CDC of several outbreaks of gastroenteritis occurring in their states since November 12. Preliminary epidemiologic investigations identified consumption of oysters as the primary risk factor for illness. On November 16, the Louisiana Department of Health and Hospitals (LDHH) had identified the Grand Pass and Cabbage Reef harvesting areas off the Louisiana coast as the source of oysters associated with outbreaks in Louisiana and Mississippi. Tagged oysters associated with outbreaks in Maryland were traced to the same oyster beds. Small round structured viruses or Norwalk-like viruses were detected by EM and confirmed by RT-PCR in 13 of 26 stool specimens from ill persons in Louisiana, Maryland, Mississippi, and North Carolina.

MMWR 37(5):1988 An outbreak of diarrhea occurred among the 331 participants in an outing held at a South Dakota campground on August 30 and 31, 1986. A biotin-avidin immunoassay performed at CDC yielded a fourfold rise in antibody titer to Norwalk virus in seven of 11 paired human serum specimens. No pathogenic bacterial or parasitic agents were identified from stool samples. Illness was strongly associated with the consumption of water or the reconstituted powdered soft drink made with water.

Snow Mountain virus was implicated in an outbreak in a retirement community in California (1988) which resulted in two deaths. Illness was associated with consumption of shrimp probably contaminated by food handlers.

Preliminary evidence suggests that large outbreaks of gastroenteritis which occurred in Pennsylvania and Delaware in September, 1987, were caused by Norwalk virus. The source of both outbreaks was traced to ice made with water from a contaminated well. In Pennsylvania, the ice was consumed at a football game, and in Delaware, at a cocktail party. Norwalk virus is also suspected to have caused an outbreak aboard a cruise ship in Hawaii in 1990. Fresh fruits were the probable vehicle of contamination.

MMWR 35(23):1986 Three outbreaks of gastroenteritis occurred on two Caribbean cruise ships between April 26, and May 10, 1986. More than 1,200 persons developed gastrointestinal illness; no deaths were reported. At least one of the outbreaks appears to be associated with Norwalk virus. An outbreak of gastroenteritis caused by the Norwalk virus recently occurred in Tate, a rural community in north Georgia. An investigation implicated the community water system as the source of infection. (MMWR 31(30):1982 Aug 06)

Foodborne outbreaks of gastroenteritis caused by Norwalk virus are often related to consumption of raw shellfish. Frequent and widespread outbreaks, reaching epidemic proportions, occurred in Australia (1978) and in the state of New York (1982) among consumers of raw clams and oysters. From 1983 to 1987, ten well documented outbreaks caused by Norwalk virus were reported in the U.S., involving a variety of foods: fruits, salads, eggs, clams, and bakery items.

For more information on recent outbreaks see the CDC Morbidity and Mortality Weekly Reports.

Ongoing outbreaks have also plagued the Vancouver, Washington and Adelaide, Australia areas.

[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.
Staying healthy on cruise ships. Tips and considerations for stomach ailments, norovirus, and other concerns.