The Norwalk virus family

Norwalk virus is the prototype of a family of unclassified small round structured viruses (SRSVs).

The disease is self-limiting, mild, and characterized by nausea, vomiting, diarrhea, and abdominal pain. Headache and low-grade fever may occur.

A few laboratories possessing reagents from human volunteer studies can only make specific diagnosis of the disease.

Identification of the virus can be made on early stool specimens using immune electron microscopy and various immunoassays.

The faecal-oral route via contaminated water and foods transmits Norwalk gastroenteritis.

Secondary person-to-person transmission also occurs. Water is the most common source of outbreaks and may include water from municipal supplies, well, recreational lakes, swimming pools, and water stored aboard cruise ships.

Shellfish and salad ingredients are often implicated in Norwalk outbreaks. Ingestion of raw or insufficiently steamed clams and oysters poses a high risk for infection with Norwalk virus. Ill food handlers contaminate foods other than shellfish

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, is not permanent and reinfection can occur.

Symptoms: 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. 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. Disease is more frequent in adults and older children than in the very young.