Thursday, October 7, 2010

"Delhi" Belly hits Com Game Swimmers - betting on Norovirus

Commonwealth Games reports are dominated by accounts of "Delhi" Belly among swimmers today.  My bet is that norovirus is the culprit - a highly infectious gastrointestinal bug that is easily spread from person to person.  Fingers are pointing at the swimming pool but it could be clustered among swimmers due to social contact or sharing the same facilities.  Norovirus is killed by chlorine but maybe not quickly enough if there is massive contamination of the pool and swimmers are swallowing the water immediately after the water is contaminated.   From the media reports it appears that some have been swimming in the pool within days of experiencing gastro when they could still be shedding virus - even though they have no symptoms. Additionally, there are reports of swimmers having to leave the pool due to illness - suggesting they may be actively contaminating the pool.

A single person with diarrhoea can contaminate an entire olympic pool very easily, in fact following an outbreak of norovirus associated with oysters in the Gulf of Mexico it was estimated that one litre of diarrhoeal stool could contaminate at least a square mile of seabed! 

So, is it norovirus? If at least of diarrhoea cases have vomiting and there is roughly a 24 to 48 hour incubation period then its very likely noro.  Norovirus is fairly resistant to chlorine but you can be sure there is now heaps of chlorine in the implicated pools.  Hyperchlorination will wipe it out fairly quickly but not instantly.  The virus needs some contact time with the chlorine to be inactivated.  It will be difficult to stop the outbreak if swimmers with active diarrhoea continue to contaminate the water. Even swimmers who appear to have recovered will still be excreting virus in their stool for weeks and any residue on their body will contaminate the pool.

So what to do?  Testing the pool water for norovirus is not a routine laboratory test and may be unproductive.  The PCR test for norovirus is not 100% sensitive so it cannot rule out contamination of the water.  Additionally, there will likely be continued contamination events.  Only an epidemiological analysis, not a laboratory analysis, can rule the pool in or out. While there may be lots of finger pointing at the Delhi pool maintenance team, it will be interesting to see if swim coaches are willing to exclude their recently ill athletes from the water to prevent further transmission - probably the most important intervention at this stage.  How long should they be excluded from the pool?  We don't know how long people are infectious after the symptoms resolve. Could be anything from 24 hours to 2 weeks, but  probably 48 to 72 hours after their last episode of diarrhoea would be a reasonable pool exclusion period.  Maybe the greatest challenge for coaches will be keeping recently recovered swimmers out of the pool.  Now there is a challenge!

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