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How do you solve a problem like - Norovirus?

15 May 2014

In his first regular column for Cleaning Matters, scientific director at Chemex International Seán Derrig sets the record straight on Norovirus

I get more questions about norovirus than any other organism. What is it? Where has it come from – we never used to get it? What kills it? 

This season’s official numbers are down – but these are principally lab-confirmed outbreaks in hospitals and so do not give the whole picture. In the community the 2013/14 season has dragged on, not helped by rainfall generally and flooding specifically. 

‘Norovirus’ is actually a group of similar viruses and is the biggest cause of viral gastroenteritis worldwide with 250-300 million infections annually. Symptoms begin 24-48 hours after exposure including vomiting, diarrhoea, abdominal pain and low fever. Most people recover within two to three days. And it has been around for ages but we’ve only had microscopes powerful enough to see it since the early ‘70s; it’s only 30nm across. For comparison visible light has wavelengths between 400-750nm. So, smaller than the wavelengths of light by an order of magnitude. That’s pretty small. Before the virus was discovered we called the illness ‘gastric flu’ or ‘winter vomiting bug’. 

Some strains are nastier than others; in the UK it’s pretty unpleasant while you have it but not usually dangerous. It’s a killer in the developing world, though: ~250,000 deaths annually, principally the very young and very old. As with any diarrhoeal illness dehydration is the biggest risk factor. 

  • Not only is norovirus a prolific and contagious illness, it mutates quickly so we don’t build up immunity – that’s why it’s possible to have it more than once in a ‘season’
  • It’s pretty much impervious to most chemicals 
  • It also has a very low infectious dose of under 10 virus particles and – here’s the nasty bit – after recovering you can be ‘stool positive’ for two months. 

Norovirus is principally transmitted via the ‘faeco-oral’ route – poor hand hygiene after using the lavatory – but less commonly it’s possible to pick it up by touching vomit from an infected person or breathing in virus particles in aerosolised vomit. It can survive on surfaces and can be deactivated by 6-10,000ppm chlorine or some trick chemicals such as peroxymonsulphate compounds – but hand washing (not alcohol etc gels – they don’t kill it) is the best prevention. 

Great care needs to be taken with food handlers who have had the virus. The FSA publishes a good guide (http://www.food.gov.uk/multimedia/pdfs/publication/fitnesstoworkguide09v3.pdf) but hand washing is key due to this organism’s infectivity and because you can still shed virus particles for so long after symptoms subside.