Norovirus

(Norwalk-like Virus, Small Round Structured Virus/SRSV)

Norovirus is the most common cause of infectious gastroenteritis in England and Wales. Although relatively mild, norovirus illness can occur at any age because immunity to it is not long-lasting. The disease was historically known as 'winter vomiting disease' due to its seasonality and typical symptoms. Outbreaks of norovirus gastroenteritis are common in semi-closed environments such as hospitals, nursing homes, schools and cruise ships.

Current situation (January 2008)

This season we have seen an increase in reports of norovirus cases, almost double the number reported for the same period last year.

Several factors are thought to have contributed to the increase in cases reported already this year. The norovirus season has started uncharacteristically early compared to other years. There is also thought to be an increased level of awareness of the infection and symptoms by both the public and physicians, hence more cases are being identified. In addition, improvements have been made in diagnostics at regional laboratories and the increased sensitivity in testing may have resulted in more cases being diagnosed.

Numbers of norovirus fluctuate each year with laboratory reported cases representing only a small fraction of the number of cases that actually occur. The self-limiting infection usually only lasts a few days hence the majority of cases are not reported to the clinician.

Norovirus is highly contagious, however, one of the ways to protect against the infection or to help prevent yourself or others becoming infected, is by practising good hygiene. This includes thorough hand washing especially after using the toilet, and any contaminated surface should be thoroughly disinfected after an episode of illness. Food preparation should also be avoided until 48 hours after symptoms have disappeared.

Unfortunately there is no specific treatment for norovirus apart from letting the illness run its course, therefore it is important to drink plenty of fluids to prevent dehydration especially in the very young or elderly.

It is not unusual to see outbreaks occurring in hospitals, as the virus quickly spreads in confined environments.

Taking action early in an outbreak by closing a ward to new admissions can help control outbreaks. A study by the Health Protection Agency has shown that outbreaks of norovirus are shortened when control measures at healthcare settings are implemented quickly, such as closing wards to new admissions within 4 days of the beginning of the outbreak and implementing strict hygiene measures.

Clinical information – Norovirus

Incubation period

Usually 24 to 48 hours.

Common clinical features

Vomiting, diarrhoea, fever.

Reservoir

Gastrointestinal tract of man.

Transmission

Person to person by the faecal oral route; risk of infection from aerosols of projectile vomit. Environmental contamination, especially of toilets; gloves should be used by cleaners. Contaminated food and water, especially bivalve molluscs.

Other relevant features

Infectivity lasts for 48 hours after resolution of symptoms. The infective dose is extremely low.