The viruses

Noroviruses are single-stranded RNA viruses and belong to the family Caliciviridae. There are three genogroups of noroviruses. New variants emerge every few years.

Epidemiology

Route of spread

Noroviruses most frequently spread by the ingestion or inhalation of vomit. Patients frequently have no prior warning that they are about to vomit, which results in environmental contamination.

Noroviruses are also transmitted by contaminated food (e.g. bivalve molluscs, such as cockles and oysters, contaminated by human sewage in sea water). Symptomatic food handlers can also contaminate food, resulting in outbreaks.

Prevalence

Norovirus infection is common and 90% of adults have been infected at some time in their lives. Immunity lasts for less than a year, and reinfection can occur with the same or different strains.

Incubation period

24–48 hours after contact with a contaminated environment or eating contaminated food.

Infectious period

From onset of symptoms to 48 hours after symptoms stop.

At-risk groups

All ages.

Clinical

Symptoms

Noroviruses are associated with diarrhoea and vomiting (especially projectile vomiting).

Outbreaks

Noroviruses cause large outbreaks in hospitals, cruise ships and in the community, especially in schools and nursing homes. Outbreaks occur more frequently in the winter (winter vomiting disease), but when new variants emerge outbreaks in the summer occur.