Hazard Identification

What are noroviruses?

Noroviruses is the name given to a group of related non-enveloped, single stranded RNA viruses that have recently been classified in the family Caliciviridae , genus Norovirus. These highly infectious enteric viruses are a major cause of acute gastroenteritis in humans (the infection is often called viral gastroenteritis). Although many cases are caused by person-to-person spread, the ingestion of contaminated food or water also plays a significant part in their transmission.

Noroviruses were first described following an outbreak of gastroenteritis in a school in Norwalk, Ohio in 1968. For many years they were known as the Norwalk group, as Norwalk-like viruses (NLV), or as 'small round structured viruses' (SRSVs), because of their morphological characteristics. However, the name Norovirus (NoV) has recently been recognised as the official genus for this group of human caliciviruses. NoV strains are named after the location from which they were first associated, e.g. Norwalk virus, Southampton virus, Snow Mountain virus and Mexico virus.

Occurrence in foods

Noroviruses are non-culturable in the diagnostic laboratory and there is no known animal model. Until relatively recently they could only be detected when present in high numbers using electron microscopy. Recent technological advances have enabled noroviruses to be detected and characterised by molecular methods, but the detection of these viruses in foods is extremely difficult and has only been successful in shellfish.

Food vehicles for noroviruses are thought to include sewage contaminated bivalve shellfish, foodstuffs that are contaminated by an infected handler, fruits and vegetables contaminated during irrigation or washing, and water (including drinking water and ice).

Infected food handlers can contaminate any foodstuff, and outbreaks of NoV infections can be associated with any food that is handled and will be eaten without a further cooking step. Contamination can occur during the preparation of foods as well as during the harvesting of fresh produce such as soft fruits.

Hazard characterisation

Effects on health

Noroviruses can cause illness in any age group, although the elderly and the immunocompromised are particularly susceptible. Recent evidence suggests that susceptibility to NoV infection could be genetically determined, and people with blood group O seem more likely to develop a severe infection. Illness can occur at any time of year but in temperate climates is more common during the winter months. Noroviruses are very contagious, however the illness is usually mild and self limiting.

The infective dose is low, and as few as 10 virus particles may be sufficient to infect an individual. Signs of infection first appear from between 10 - 50 hours, typically 24 - 28 hours, after ingestion of the virus. The onset of illness is abrupt and typical symptoms are vomiting (often projectile), diarrhoea, abdominal pains, nausea, headache, stomach cramps and occasionally low grade fever. The illness is typically relatively short, lasting from 12 - 60 hours, although there are reports that symptoms in some individuals last for more than 2 weeks. Recovery is usually complete with no long lasting effects.

During the illness high numbers of the virus are generated in the vomit of affected individuals as well as being shed in their faeces. Virus shedding appears to occur before symptoms start and continue for up to two weeks after symptoms have ceased. Outbreaks associated with an infected food handler have been associated with foods prepared before the onset of symptoms.

Incidence and outbreaks

Norovirus outbreaks are very common, but there is little published information on the incidence of foodborne infection. In the USA 382 confirmed outbreaks (not necessarily foodborne) were recorded in the period from October to December 2006 alone and rising incidence is thought to be linked to the appearance of new strains of the virus. In the UK, the incidence of norovirus infections has also been rising steadily since the 1980s, and in 2006, nearly 4,500 confirmed cases were recorded, although there is no indication of the proportion that were foodborne.

Contaminated water is the most common source of a NoV outbreak and has caused very large outbreaks of viral gastroenteritis. Outbreaks have been linked to water from wells, municipal water supplies, swimming pools, lakes and water stored on cruise ships. In the USA, commercially prepared ice from a production facility that was contaminated during flooding was associated with a widespread outbreak.

Foodborne outbreaks of NoV infections are frequently caused by infected food handlers. Foods associated this source of contamination are cold, ready-to-eat foods such as prepared salads, fresh cut fruits, sandwiches and bakery products. Large outbreaks have been caused when liquid foods such as icings or salad dressings have become infected during preparation and then mixed leading to widespread distribution of the virus.

Shellfish, in particular oysters, from sewage-contaminated water, when eaten raw, or lightly cooked, have also caused large outbreaks of NoV illness.

Contaminated fresh produce, in particular salads and raspberries, has been associated with large foodborne outbreaks of NoV infections. These foods may be contaminated either from irrigation water, during washing or spraying, or during harvesting by infected handlers. In recent years frozen raspberries have caused extensive foodborne outbreaks in Canada and in Europe. The viruses are able to survive the freezing process and frozen fruits are often exported to other countries resulting in the wide distribution of the virus.

Sources

Humans are the only known reservoir for noroviruses. It has been hypothesised that there may also be an animal reservoir, but, although related caliciviruses have been found in many animal species, there have not been any documented cases of cross-species transmission.

Faeces or vomit from infected individuals can lead to the environmental contamination of soil, water and surfaces. Airborne droplets produced during vomiting are a particularly effective method of distribution for viruses.

Noroviruses can accumulate and concentrate in the guts of bivalve molluscs, such as oysters and mussels, growing in sewage-contaminated waters. Depuration processes designed to reduce the bacterial contamination of these shellfish are ineffective for removing viruses. Faecal contamination of water supplies can be a potential source of noroviruses. Live viruses have even been detected in commercially available bottled mineral water, although cases of illness have not yet been traced to this possible source of infection.

Growth and survival characteristics

Viruses, including noroviruses, are unable to multiply outside of the host. Although noroviruses cannot grow in food or water, they can survive in many environments for significant periods. The virus can remain infective when held at ambient, chilled and freezing temperatures. In chilled and frozen environments survival can be measured in months or even years. Noroviruses are resistant to acid and can survive gastric acid at pH 3 - 4. They have also been shown to still be infective when exposed to a pH of 2.7 for 3 hours at ambient temperature. The virus can survive in water environments and in shellfish for extended periods (possibly months). It is resistant to drying, and is reported to persist on environmental surfaces, such as carpets, for up to 12 days.

Noroviruses can survive exposure to up to 10 ppm free chlorine, and can therefore survive the usual chlorination processes used to treat public water supplies.

Thermal inactivation

Noroviruses have been shown to remain infective when held at 60 ^°C for 30 mins. The virus is able to survive some pasteurisation processes and has also caused illness after it was steamed in shellfish. It is inactivated by boiling.

Control options

To reduce the risk of foodborne transmission of noroviruses, controls should focus on ensuring the use of potable water for food processing, strict hygiene control, and using shellfish from approved waters.

Processing

Food handlers or fruit pickers suffering from viral gastroenteritis should not return to work for at least 48 - 72 hours after symptoms have ceased. Effective training in adequate personal hygiene practices is essential. Thorough cleaning with an effective sanitiser should follow any episode of vomiting in a food processing environment.

Shellfish should be gathered from approved harvesting waters and should be thoroughly cooked prior to consumption.

Product use

Consumers should be advised not to eat raw shellfish and to ensure these products are thoroughly cooked prior to consumption. In addition consumers should be advised to thoroughly wash all fruits and vegetables that will be eaten raw or lightly cooked in potable water.

Legislation

There is no specific legislation in the EC or in the USA regarding levels of enteric viruses, such as noroviruses, in foods.

Sources of Further Information

Published

Koopmans, M., Duizer, E.
Foodborne viruses: an emerging problem.
International Journal of Food Microbiology, 2004. 90, 23 - 41.

Koopmans, M., von Bonsdorff, C-H., Vinjé, J., de Medici, D. and Monroe, S.
Foodborne viruses.
FEMS Microbiology reviews, 2002. 26, 187 - 205.

Lopmann, B.A., Brown, D.W., Koopmans, M.
Human caliciviruses in Europe.
Journal of Clinical Virology, 2002. 24, 137 - 160.

On the web

Risk profile: Norwalk-like viruses in mollusca (raw). Institute of Environmental Science and Research Limited. (January 2003).
http://www.nzfsa.govt.nz/science/risk-profiles/norwalk-like-virus-in-raw-mollusca.pdf