November 30, 2012 7:57 pm

Norovirus: coming soon to an intestine near you

The virus affects up to a million people in the UK each year, giving it the dubious distinction of being Britain’s commonest stomach bug

There are some illnesses that your doctor doesn’t want to see. Norovirus is one of them. Hospitals, like cruise ships, are full of vulnerable captives; GP surgeries beg the afflicted to stay away. Norovirus is the most ill that some of us are ever likely to feel at home. The information leaflets we give patients confidently describe the symptoms of the winter vomiting bug as “mild”, which is not the description used by anyone who’s ever actually had it. “I thought I was going to die” is usual, followed by the patient telling you how much weight they lost in a week.

For something that makes you feel so ill, norovirus is surprisingly pretty. Each particle consists of a single-stranded RNA molecule enclosed in a geometric capsule. With a relatively small amount of nucleic acid, the virus is limited in the proteins it can produce, so its capsule follows an “icosahedral” model, using repeated shapes tessellated together like the casing of a football. This is cheaper, in protein terms, than a covering composed of lots of different structures, such as humans inhabit, with our complicated noses and fingernails. Polioviruses, adenoviruses and herpesviruses use the same principle; the results are often beautiful. In an electron microscope, norovirus looks like a Christmas bauble, or something in Star Trek.

The effects are less attractive. You don’t need to ingest many norovirus particles to become infected – as few as a hundred are enough. Once inside you, the particles multiply in the favourable environment of the small intestine. During this time you know nothing – this is the latent period. (You can, however, shed virus particles during this time, and so infect other people.) As the virus multiplies, it invades the cells lining the gut wall and damages them, temporarily deforming their structure. At this point your poor invaded intestine is no longer able to maintain its usual fine balance between absorbing fluid and excreting it: torrential vomiting and watery diarrhoea are the result.

As an enterovirus, or “gut virus”, norovirus is usually transmitted by the faecal-oral route: the viral particles are shed in stool, and you need to swallow them to become infected. It can also travel through the air, once “aerosolised”, as the phrase has it, through forceful ejection. But anything that allows the virus to get on to your skin and from there into your digestive tract can give you the infection. Norovirus is hardy – it can survive for quite a time outside its human host (unlike, say, HIV), making it an example of the fabled infection that you can actually catch from the toilet bowl.

Though it creates modern plagues – outbreaks in hospitals can close wards for days at a time, and it can be dangerous for anyone already weakened by illness – norovirus is controllable by old-fashioned measures: it doesn’t require particularly advanced techniques to prevent it spreading. Alcohol-based hand gels are not very good at killing norovirus, even those that promise to kill “99.5 per cent” of germs. On the other hand, washing your hands with hot water and soap is very effective, as is cleaning surfaces with bleach. Not mixing with other people when you’re sick is also important.

But despite precautions, norovirus affects up to a million people in the UK each year, giving it the dubious distinction of being Britain’s commonest stomach bug. This year, ominously, there have already been more cases than usual, and we haven’t reached peak season yet.

Sophie Harrison is a hospital doctor in South Yorkshire

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