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Center for Food Safety & Applied Nutrition Foodborne Pathogenic Microorganisms and Natural Toxins Handbook |
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1. Name of the Organism: The Norwalk virus family |
Norwalk virus is the prototype of a family of unclassified small round structured viruses (SRSVs) which may be related to the caliciviruses. They contain a positive strand RNA genome of 7.5 kb and a single structural protein of about 60 kDa. The 27-32 nm viral particles have a buoyant density of 1.39-1.40 g/ml in CsCl. The family consists of several serologically distinct groups of viruses that have been named after the places where the outbreaks occurred. In the U.S., the Norwalk and Montgomery County agents are serologically related but distinct from the Hawaii and Snow Mountain agents. The Taunton, Moorcroft, Barnett, and Amulree agents were identified in the U.K., and the Sapporo and Otofuke agents in Japan. Their serological relationships remain to be determined. |
| 2. Name of Acute Disease: | Common names of the illness caused by the Norwalk and Norwalk-like viruses are viral gastroenteritis, acute nonbacterial gastroenteritis, food poisoning, and food infection. |
| 3. 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. The infectious dose is unknown but presumed to be low. |
| 4. Diagnosis of Human Illness: | Specific diagnosis of the disease can only be made by a few laboratories possessing reagents from human volunteer studies. Identification of the virus can be made on early stool specimens using immune electron microscopy and various immunoassays. Confirmation often requires demonstration of seroconversion, the presence of specific IgM antibody, or a four-fold rise in antibody titer to Norwalk virus on paired acute-convalescent sera. |
| 5. 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, well, recreational lakes, swiming pools,
and water stored aboard cruise ships.
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. |
| 6. 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 is very high at an early age. 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. |
| 7. Usual Course of Disease and Some Complications: | A mild and brief illness usually develops 24-48 h after contaminated food or water is consumed and lasts for 24-60 hours. Severe illness or hospitalization is very rare. |
| 8. 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. |
| 9. Analysis of Foods: | 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 under way. |
| 10. Selected Outbreaks: |
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.
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. 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. For outbreaks of Norwalk virus see MMWR 42(49):1993 Dec 17 and this MMWR 43(24):1994 Jun 24 as well. The multistate outbreak of viral gastroenteritis associated with consumption of oysters from Apalachicola Bay, Florida, December 1994-January 1995 is reported in MMWR 44(2):1995 Jan 20. |
| For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC. |
mow@cfsan.fda.gov
January 1992 with periodic updates
