Noroviruses (NoVs) account for about 95% of outbreaks of viral gastroenteritis, of which it has been estimated that about 40% may be due to foodborne transmission. Although the infective dose is unknown it is presumed to be low - perhaps just a few viral particles. Most foodborne NoV infections can be traced back to infected persons who manually handle food that is not heated or otherwise processed afterwards.
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Incubation Period, Symptoms & Duration
 Incubation Period
24 - 48 hours usually, but can be 12 - 60 hours.
Symptoms
The infected individual may feel perfectly well until the sudden onset of symptoms, which may include:
- (projectile) vomiting
- diarrhoea
- abdominal pain
- headache
- low grade fever, lasting for less than 24 hours usually
Symptoms are often mild (which can create problems when infected persons remain at work) but can be more serious due to dehydration being a common complication. Hospital admission is unusual. NoV infections have been known to hasten the death of debilitated patients.
Duration
1 - 3 days, although this can be longer in the elderly.
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Source & Spread
Sources/Associated Foods
- Human gastrointestinal tract
- Water or food contaminated by faecal material (via hands or inadequate foul drainage/disposal) or vomit (including aerosol droplets).
Foods implicated in outbreaks have included:
- filter-feeding bivalve shellfish (e.g. oysters, mussels, clams and cockles) grown and harvested in contaminated estuarine and coastal waters, their feeding mechanism actually concentrating the virus more than 100 times that in the surrounding water. Cockles, mussels and clams are only lightly cooked and frequently oysters are eaten raw.
- any food that is handled extensively, and which is either eaten raw/lightly cooked or which is not subject to further heating prior to consumption. Such foods include soft fruit (particularly berries), salad vegetables, desserts, bakery products, prepared salads and sandwiches.
Spread
The virus is shed in large numbers in faeces and vomit, with the former starting during the incubation period and lasting for 10 days or more. 30% of cases may still shed the virus in faeces up to 3 weeks after infection. Equally, it has been estimated that one projectile vomiting incident could release around 30 million NoV particles.
Contamination of foods may occur either at source in their growing and harvesting areas or during handling and preparation (most often by infected food handlers).
Spread is direct person-to-person via the faecal:oral route and occurs:
- when infected food handlers do not observe proper personal hygiene practices (e.g. not washing hands after using the toilet)
- when areas are not properly cleaned and disinfected, especially after a vomiting incident
- when sewage contamination of shellfish-growing areas occurs (compounded by insufficient purification procedures)
As NoVs can survive on a variety of inert substances for several days, transmission of the virus can occur via both contaminated food-contact surfaces and contaminated inanimate objects [fomites] such as (flush) handles, switches and door knobs.
NoV infections are highly contagious, with an average attack (infection) rate of 45-50% of contacts. Indeed, attack rates of up to 100% have been noted. Outbreaks due to NoV are therefore common in institutional or closed settings such as hospitals, residential/nursing homes, schools and cruise ships.
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Control
- Good standards of personal hygiene during food preparation. Hand washing should be thorough and frequent, and always undertaken after visiting the toilet. Wearing suitable food grade gloves may be considered.
- Thorough and regular cleaning and disinfection of surfaces, equipment, utensils etc.
- Thorough cleaning and disinfection after any vomiting incident. N.B. In one study NoVs were detected on hospital lockers and from hotel carpets supposedly cleaned after incidents of vomiting.
- Increasing the awareness and knowledge of all food handlers about the transmission of NoV, including its spread by:
- vomiting
- asymptomatic carriers
- those recovering from infection
- those with sick children in the household
- Thorough washing of all salad items, fresh fruit and raw vegetables prior to consumption.
- NoVs can be inactivated by normal cooking processes (but are not always inactivated in shellfish given only minimal heat treatment).
- Observing the legal requirement to use shellfish only from approved/classified waters.
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Exclusion/Return to Work Parameters
Cases in risk groups 1- 4 not to return until 48 hours after first normal stool.
[Group 1 - food handlers; Group 2 - health care workers; Group 3 - children under 5 in nurseries etc.; Group 4 - older children and adults who may find it difficult to implement good standards of personal hygiene]
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Additional Information
- Noroviruses were previously known as Norwalk-Like Viruses (NLVs) or Small Round Structured Viruses (SRSVs).
- NoVs cause harm by invading and damaging the gastrointestinal tract, causing lesions of the mucosa of the small intestine that disappear during the two weeks or so following infection.
- Characteristics that facilitate NoV spread in outbreaks include:
- they can survive well in the environment, being relatively resistant to heat, disinfection and pH changes. They can been shown to persist in the environment for many days (even weeks where adequate disinfection has not been carried out).
- the prolonged asymptomatic shedding of viruses (up to 2/3 weeks)
- the low infective dose, leading to high levels of person-to-person spread (in 86% of outbreaks)
- the great diversity of strains
- no induction of lasting immunity
- reinfection can therefore easily occur.
- Studies in the US and UK suggest that a substantial proportion of foodborne infections may be due to NoVs (estimates for the USA vary from 32 - 67%).
- In England and Wales the number of laboratory-confirmed outbreaks of Infectious Intestinal Disease (IID) due to NoV varies, but is generally between 130 to 250 outbreaks each year. NoV infections tend to increase over the winter months with the highest levels being in January - March.
- 79% of all outbreaks caused by NoV occur in healthcare settings (hospitals or residential care homes), 7% in hotels and 5% in schools. In such closed settings, the highly infectious nature of NoV results in a great number of secondary infections (in about 50% of contacts).
- 58% of foodborne outbreaks originate in food outlets and 24% in hotels.
- NoVs also cause numerous sporadic cases of gastroenteritis. One study indicated that 5% of patients with gastroenteritis who consult a doctor have NoV infection, compared with 4% for Salmonella. In addition, for every case of NoV gastroenteritis reporting to a GP, a 1999 study estimated that there are probably around 5 or more unreported cases in the community. On this basis, NoVs may account for at least 30% of all cases of sporadic gastroenteritis in the UK. This gross under-reporting is partly due to the mild nature of the infection and partly from the difficulty in detecting and identifying the virus.
- NoVs are acid stable and can survive food processing and preservation designed to produce the low pH that inhibits bacterial/fungal growth. They retain their infectivity after exposure to acidity levels below pH 3 and, for example, have been implicated in outbreaks associated with cockles pickled in brine and vinegar.
- NoVs are divided into 3 distinct genogroups, which in turn are divided into more than 15 genetic clusters. Multiple strains of NoVs circulate at any given time, although there is evidence that one strain or group of strains may occasionally predominate.
- As regards projectile vomiting as a means of transmission one study examined an outbreak following a meal in a large hotel during which one of the guests vomited. Interestingly, they found an inverse relationship between the attack rate per table and the distance from the person who threw up!
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