General characteristics

Norwalk virus is the prototype of a family of unclassified small round structured viruses which may be related to the caliciviruses. 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. Other strains have been identified in other countries. Their serological relationships remain to be determined.

Disease symptoms

Common names of the illness caused by the Norwalk and Norwalk-like viruses are viral gastroenteritis, acute nonbacterial gastroenteritis, food poisoning, and food infection.

The disease is self-limiting, mild, and characterized by nausea, vomiting, diarrhoea, and abdominal pain. Headache and low-grade fever may occur. The infectious dose is unknown but presumed to be low.

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 hospitalisation is very rare.

Diagnosis

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 very specialised techniques.

Only the common cold is reported more frequently than viral gastroenteritis as a cause of illness in the Western developed world. 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.

Associated foods

Norwalk gastroenteritis is transmitted by the faecal-oral route via contaminated water and foods. Some person-to-person transmission has been documented, but is not common. Water is the most common source of outbreaks and may include water from municipal supplies, wells, recreational lakes, swimming 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.

Prevention

The viruses are heat-sensitive and will be killed by thorough heating (over 70 °C). Raw or undercooked foods are the main causes of infection.

Risk 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.