Across the world noroviruses cause over 685 million cases of acute gastroenteritis each year. For most people it causes a very unpleasant few days but isn’t life threatening. However, globally it does kill around 250,000 people annually – predominantly the very young and the very old, mainly in developing countries.
It spreads easily and rapidly via contaminated food, water, surfaces and person-to-person, and in closed environments such as ships, hospitals, nursing homes – and productions – its transmissibility means once it gets in, it’s extremely difficult to get it out again.
We are the only reservoir so person-to-person spread is the main risk, but some shellfish – especially oysters – can concentrate the virus if contaminated with sewage.
Key Facts
Clean Up
Cleaning up after a norovirus incident requires careful planning to prevent further spread.
This video will give you an idea of just how challenging this process is – and why keeping people with norovirus out of the workplace is so important.
- Isolate the area: keep others away from the contaminated area until it has been scrupulously cleaned and disinfected.
- Wear PPE: put on disposable gloves, a face mask, eye protection and a disposable apron to avoid direct exposure.
- Remove vomit carefully: ideally use an absorbent spill powder to immobilise it. These powders turn liquid and semi-solid waste into a gel which is much easier to remove safely.
- Avoid vacuuming; this can help aerosolise the virus. Dispose of spill powder and/or paper towels in a sealed plastic bag and double bag it before disposal.
- Clean surfaces: thoroughly clean all nearby surfaces with detergent, focusing on high-contact areas like handles, railings, and switches.
- Disinfect surfaces: few disinfectants kill norovirus but it is recommended to use bleach to disinfect surfaces. This needs to be a separate step from cleaning: the chlorine that gives bleach its disinfectant properties will react with dirt and be rendered ineffective: you can’t disinfect dirt.
- Dispose of contaminated items: throw away all disposable cleaning supplies and wash non-disposable items with hot water and bleach.
- Wash hands thoroughly: use soap and water immediately after clean up, as hand sanitisers are not effective against norovirus.
- Ventilate the area: if possible, open windows or doors to air out the space after cleaning.
- Launder all exposed fabrics: wash clothes, linens, and towels in hot water with detergent and bleach if possible, then dry them on the highest heat setting.
These steps reduce the chance of further contamination as norovirus can survive on surfaces for days if not properly cleaned. And don’t underestimate how easy it is to infect oneself when clearing up body wastes contaminated with norovirus.
About Norovirus
The noroviruses are a group of similar Caliciviridae which have been known as Norwalk-Type Virus, Winter Vomiting Bug, Viral Gastroenteritis, Hawaii virus, Snow Mountain virus, Mexico virus, Desert Shield virus, Southampton virus, Lordsdale virus, ‘stomach flu’ – the list goes on.
It was first identified by electron microscopy as the cause of a gastroenteritis outbreak at Bronson Elementary School in Norwalk, Ohio in November 1968. But the epidemiology of noroviruses remained poorly understood until relatively recently due to the lack of accurate diagnostic tests.
Advances in molecular genetics enabled scientists to group what appeared to be many different infections together, and identify that this RNA virus causes approximately 90% of acute gastroenteritis globally.
It is estimated that Norovirus infects 600,000 to 1,000,000 people in the UK every year, and infections tend to rise during the winter hence it often being called ‘winter vomiting bug’.
Norovirus spreads rapidly, is very easy to catch but very difficult to kill; this type of viruses is largely unaffected by hygiene chemicals, unlike influenza viruses or SARS-CoV-2.
How Do We Get It?
Norovirus can result from faecal contamination of food or water. Raw or undercooked oysters, clams, and mussels are high-risk foods because they are filter feeders and can accumulate the virus from sewage-contaminated water.
Oysters are a particular risk and have been associated with major outbreaks; each filters several litres of seawater daily and noroviruses bioaccumulate in them. It’s not just oysters: in Japan, outbreaks have been linked to nori (dried seaweed) because the virus survives the seaweed drying process.
Whatever the source, norovirus can easily spread person-to-person or by food handlers through poor personal hygiene.
Contained environments such as ships, hotels, hospitals, nursing homes, schools – and productions – are particularly susceptible because norovirus spreads so easily from person to person and can survive for many days outside the body in contaminated areas.
How Common Is It?
Very. Norovirus levels are currently the highest we have seen at this time of year in over a decade.
Norovirus has quite a distinct clinical presentation, there are no treatments and it is generally self-limiting so few suspected cases are tested or lab-confirmed. But lab surveillance is the only data we have, and the graph below shows current lab-confirmed cases against the five year average.
Symptoms and Treatment
It is a prolific and contagious illness and its course can be quite unpleasant. Symptoms can begin 12 – 48 hours after exposure, and usually include abdominal pain, vomiting and diarrhoea, sometimes with headaches, low fever and tiredness. Symptoms usually last for one to three days.
Most cases are self-limiting and there is no specific treatment for the illness apart from supportive care – you just need to allow it to run its course of gastrointestinal carnage. Dehydration is the main risk, especially in the very young and the elderly.
Immunity after infection is short lived and strain-specific – this means it’s possible to be infected more than once in a ‘season’.
While the usual advice is that it is safe to return to work 48hrs after the symptoms have passed, an infected person can remain stool-positive for the virus for some weeks so scrupulous hand hygiene remains vital.
How Contagious Is It?
Very. It is transmitted mainly via the faeco-oral route but also by touching vomit from an infected person. And vomiting spews norovirus into the air where it can infect anyone who inhales the aerosolised droplets or touches anything the aerosolised vomit lands on.
The virus has a very low infectious dose (you only need to ingest a few virus particles), and someone with it will be shedding an awful lot of virus particles.
Prolific viral shedding, low infectious dose – and its incredible resistance to hygiene chemicals – are why it’s so difficult to get rid of it if it gets in to a workplace or production. It often provokes rolling waves of infections that just never seem to end.
More Information
Different noroviruses have been detected in cats, dogs, pigs, sheep, cattle, sea lions, rodents, porpoises and bats, as well as humans.
Microbiology
For such a nuisance of a virus it’s incredibly small. It’s about 30nm across (that’s 30 millionths of a millimetre). This makes it about a quarter of the size of flu or coronaviruses and about a thousandth of the size of the cells it infects. Put another way, light is a wave. You could place 20 norovirus particles side-by-side along one wavelength of orange light.
There are 10 known genogroups of norovirus, which further branch into numerous genotypes.
There are two human genogroups: genogroup I include Norwalk virus, Desert Shield virus and Southampton virus; and genogroup II, which includes Bristol virus, Lordsdale virus, Toronto virus, Mexico virus, Hawaii virus, and Snow Mountain virus. Other animals have their own genogroups.
Different strains have different morbidity and mortality with Genogroup 2 Genotype 4 more likely to cause hospitalization and death. Unfortunately any immunity you get is limited to a specific genotype only.
Blood Type
On the surface of epithelial cells and red blood cells, as well as floating in secretions like saliva, are molecules called histo-blood group antigens (HBGA). The structure of these molecules determines your blood type according to the ABO system. It turns out HBGA expression in the wall of the small intestine is important for the virus to initiate infection.
But biology is messy. A 2005 study suggested that presence of HBGA type B correlated with a lack of susceptibility to norovirus, specifically genotype 1, though a 2020 meta-analysis suggested that blood types A, B and AB do not affect susceptibility. However, in that same analysis, blood type O was associated with increased risk.
But whether or not a person gets infected will be a complex interaction between the host and the virus. Relying on your blood type for protection really isn’t a winning strategy.
Interestingly, oysters have histo-blood group antigens resembling those of humans which can be bound by noroviruses. This is how noroviruses accumulate in oysters and the reason eating raw oysters is so closely associated with foodborne outbreaks in humans.
Microbiome
Given norovirus is a gastrointestinal pathogen, it’s not really surprising that the gut microbiota seem to have a role to play too. Notably, some gut bacteria produce molecules on their surface that look a lot like HBGA. This seems to have implications for viral replication and protection from environmental factors like heat stress.
Bile acids have also been linked to norovirus replication and facilitate binding to HBGA. Moreover, interactions between the microbiota and host can help shape the immune response to modulate norovirus infection dynamics and duration.
Vaccines
There isn’t a vaccine for norovirus yet but there are several candidates in the pipeline. These include an mRNA vaccine and a virus-like particle vaccine which have shown promise for preventing moderate to severe cases of acute gastroenteritis from norovirus infection in phase 2 clinical trials.
Vaccines would be particularly beneficial for those who are vulnerable to serious infection such as the young, the immunocompromised and the elderly.
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Article last updated on Nov 3rd, 2024
