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Q What is the likelihood of West Nile virus affecting the UK blood supply?

A West Nile virus (WNV) is an arthropod-borne virus transmitted by mosquitoes, the principal host being wild birds. Humans and other animals can be infected via mosquito bites and then become 'incidental hosts' for short periods.

The incubation period in humans is three to 15 days. Most cases are either asymptomatic or result only in mild flu-like symptoms with full recovery following within three to six days. In around one in 150 cases however, a severe form of the disease can develop which may prove fatal in elderly or immuno-suppressed patients. The risk of transmission by blood transfusion, relates to donations given following the first few days after infection.

In the US epidemic in 2000-1, 23 patients were later confirmed to have acquired WNV via the transfusion of blood components.

Although cases of WNV have been reported in Europe, the risk of it in the UK is not considered to be serious, with no reported cases here, due to the UK climate and the 'herd immunity' in the UK's resident bird population.

There are two risks for the UK blood services: imported plasma from the US; and UK donors returning from high incidence areas who may be incubating the disease.

In the case of imported plasma, 'methylene blue' treatment has been introduced to inactivate WNV to make the risk negligible in returning travellers. Stringent donor deferral policies and safety tests have now been introduced so any prospective donors returning from high-risk areas are carefully screened.

Barry Hill is chief biomedical scientist, Blood Transfusion Department, Wigan Royal Infirmary

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