Norovirus is one of the most contagious bugs anyone can catch, sweeping through nations and leaving anyone who is affected decidedly unwell. It is the leading cause of acute gastroenteritis when compared to all other viral pathogens in a global sense.
Norovirus is also known as the Winter Vomiting Bug which hints at its seasonal appearance, usually around Christmas but certainly earlier if news reports are to be believed.
The Norovirus Biology
The virus is a member of the Caliciviridae family.
Norovirus can be classified into genogroups based on the degree of amino acid diversity in the composition of various proteins and nucleotides within a specific enzyme called RNA-dependent RNA polymerase (Vinjé, 2015; Chhabra et al., 2019).
Most human norovirus (HuNoV) outbreaks are related to genogroup I (GI) and II (GII), and GII.4, GII.17, and GII.2. These are the main genotypes producing gastroenteritis (Hasing et al., 2019; Kuang et al., 2019).
Different genotypes appear in different parts of the world. For example in Korea, the main genotype os GII.4 (Kim et al., 2013; 2018).
Symptoms
Noroviruses are a major cause of vital foodborne disease and the symptoms are predominantly vomiting, mostly projectile with some watery diarrhoea, abdominal fevers and cramps, aching limbs, nausea and dehydration. The symptoms are known to last about 12 to 60 hours (about one to three days) with patients being infectious for another two days afterwards. Those unpleasant symptoms can last for another two weeks so it is not a disease for the faint hearted.
The virus particles are usually shed for up to ten or more days. The incubation period is 10 to 15 hours, usually 24 to 48 hours, and is dose dependent. You only need between 10 and 100 virus particles to cause the disease. It is known that humans, pigs and cattle are the reservoir for the virus although an FSA report in 2011 showed that 76% of oysters tested from 39 UK growing beds were contaminated as well. One particular type of virus, the Astroviruses are occasionally foodborne and have a four-day onset period.
Norovirus is present in vomit and the mode of transmission is from person to person in the form of fomites or through environmental contamination with about 10% being foodborne and waterborne. Work surfaces, handles, in fact any type of surface that a sufferer touches can pass on the virus. It is viable for many days on surfaces which is also a major issue and why hospitals in particular are so careful about hygiene procedures.
Most outbreaks peak in winter and there is very often a high secondary attack rate.
The virus always hits the stomach first but it must multiply in the small intestine to have any real impact. Irritation of the gut lining leads to sickness. It enters the cells lining the intestine, making copies of itself which kills the cells, then releasing more virus particles, and the process is repeated.
The immune system recognises that cells are ‘dying’ and as an immune response, antibodies travel to the small intestine and deactivate the virus. Experts say this is when the body will start to feel the effects of the virus – such as fever and nausea. It is usually the way the body deals with the virus but also the manner in which it secures transmission to other hosts. We become immune to the virus when antibody levels have achieved sufficient levels to deal with the virus but we are still highkly contagious.
To identify norovirus, doctors need specimens within 48 hours of the first symptoms. Stool specimens that are negative for bacteria, with the illness duration of 1 to 3 days are immediate signs. The average attack rate is around 45% but asymptomatic infection is also possible in about 5% of people. Many doctors recommend taking oral rehydration sachets such as Dioralyte when they first start to experience symptoms of the infection but drinking plenty of water is essential to reduce the weakness associated with the disease. The elderly and young are always vulnerable but at least the disease is not life-threatening.
No vaccine is available.
Process Treatments Are Not Effective
The virus thrives in cold conditions and is killed at temperatures above 60°C. Freezing has no effect. Heat treatment of food at 90°C for 90 seconds is recommended to destroy the virus in shellfish. Unfortunately such treatment turns mollusc texture rubbery. Depuration of shellfish is not feasible and does not work against viruses generally. Chlorine cleaning is also ineffective.
Cases Involving Norovirus
The first recorded outbreak of foodborne vital gastroenteritis was in the USA, in a school in Norwalk, Ohio in 1968. It was known for many years as Norwalk virus before it acquired the more general sobriquet of norovirus. It is highly problematic on cruise ships where an outbreak means on-one is allowed to leave or enter the ship – it being effectively quarantined at sea.
Between February and March 2009, about 500 diners at the three Michelin star restaurant, ‘The Fat Duck’ which is run by Heston Blumenthal suffered the illness when contaminated oysters were consumed. It was thought food handlers which were symptomatic for the disease were able to work which contributed to the prevalence of the illness. The role of oysters is particularly important. The first case of similar virus particles being detected in both the mollusc and the faecal specimens of ill people who had consumed the shellfish was made in Kingston-upon-Hull in 1980.
Control Measures
- Exclude symptomatic food handlers until 48 hours after the symptoms have disappeared.
- Extend to 10 days if they are shown to be unreliable in washing their hands properly when handling any food but notably high-risk foods.
- Don’t share towels or clothing such as food aprons, bedding or other contact materials
- Use disposable gloves if possible.
- Use reputable suppliers of molluscs, salad vegetables and fruit.
- Wash and blanch ready-to-eat raw fruits such as melons and berries like raspberries.
- Implement stringent staff training in hand washing
- Implement HACCP obviously
- Destroy contaminated food
- Thoroughly clean and disinfect public areas even to the point of steam cleaning carpets, furniture and soft furnishings.
- All contaminated food surfaces, hand-contact areas and toilets are scrubbed thoroughly clean and disinfected. Hypochlorite washing (500ppm) is said to be effective.
- Supply paper towels, alcoholic gels (which don’t actually kill the virus but encourage hand washing and cleaning, and a sense of urgency).
- Separate bivalves from any ready-to-eat food
In sensitive institutions such as hospitals, wards and even the hospital itself can be closed at least to the public to reduce the incidence of contamination. Schools too are not immune and are regularly closed whilst the outbreak takes hold.
In other instances, shellfish beds are closed down if the virus is linked to bivalves as a source. Incidentally if you are interested to read about other food borne diseases which are associated with poor hygiene practice and oysters for example then please read our article on Shigella and dysentery.
References
(2015). Advances in laboratory methods for detection and typing of norovirus. Journal of Clinical Microbiology, 53(2), pp. 373-381 (Article)
Leave a Reply