What are noroviruses?
Noroviruses is the name given to a group of related 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 appearance when viewed by electron microscopy. The name Norovirus (NoV) has recently been recognised as the official genus for this group of human caliciviruses.
What foods can be contaminated?
Noroviruses cannot be cultured in the laboratory and 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 detection in foods is extremely difficult and has only been successful in shellfish.
Food vehicles for noroviruses are thought to include sewage contaminated bivalve shellfish, foods 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.
How does it affect human health?
Noroviruses can cause illness in any age group, although the elderly and the immuno-compromised are particularly susceptible. Illness can occur at any time of year but in temperate climates is more common during the winter. Norovirus infections 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 cause illness. Symptoms first appear 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. Illness is typically short lived, lasting from 12 – 60 hours, although there are reports that symptoms in some individuals last for more than two 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 infected food handlers have been associated with foods prepared before the onset of symptoms.
How common is illness?
Norovirus outbreaks are very common, but there is little published information on the incidence of foodborne infection.
In the USA it is estimated that there are more than 21 million cases of acute gastroenteritis each year due to NoV infection, and that these viruses cause more than 50% of all foodborne disease outbreaks. There were 382 confirmed outbreaks (not necessarily foodborne) recorded in the USA during the period October to December 2006 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. In 2010 more than 11,500 confirmed cases were recorded, although there is no indication of the proportion that were foodborne. However it has been estimated that noroviruses cause 200,000 cases of foodborne illness annually in England and Wales, with many going unreported.
Contaminated water is the most common source of NoV outbreaks, some of which can be very large. 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 facility that was contaminated during flooding was associated with a large outbreak.
Foodborne outbreaks of NoV infections are frequently caused by infected food handlers. Foods associated with 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.
Large outbreaks have also been associated with raw or lightly cooked shellfish, in particular oysters, from sewage-contaminated water, and contaminated fresh produce, in particular salads and raspberries. 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.
Where does it come from?
Humans are the only known reservoir for noroviruses. It has been suggested that there may also be an animal reservoir, but 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. 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.
How is it affected by environmental factors?
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 have 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 and in shellfish for extended periods (possibly months). It is resistant to desiccation, and is reported to persist on dry surfaces 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.
Noroviruses have been shown to remain infective when held at 60°C for 30 min. The virus is able to survive some pasteurisation processes and has caused illness associated with steamed shellfish, but it is inactivated by boiling.
How can it be controlled?
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.
For food processors
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 EFSA has recommended the introduction of control measures to avoid faecal contamination in mollusc production areas.
For caterers and consumers
Caterers should implement the same controls as processors as far as food handlers are concerned and should obtain shellfish from reliable uncontaminated sources.
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.
Are there rules and regulations?
There is no specific legislation in the EU or in the USA regarding levels of enteric viruses, including noroviruses, in foods. However, a 2011 European Food Safety Authority (EFSA) report, Foodborne Viruses: occurrence and control, recommended the development of microbiological criteria for viruses in bivalve molluscs, unless they are labelled: “to be cooked before consumption.”
Where can I learn more?