WNV is continuing to spread westwards across the continent. So far in 2002, 638 people have tested positive for the virus in 27 states stretching as far west as Texas. The only means of transmission was thought to be from the bite of an infected mosquito. But on Sunday, the Centers for Disease Control in Atlanta revealed it was investigating possible transmission through organ transplantation.

On Tuesday, James Hughes of the CDC said subsequent tests have provided "clear evidence that the organ transplant appears" to be the cause of West Nile encephalitis in three recipients, one of whom has died. The results of tests on a fourth sick recipient in Florida are not yet in.

All four received organs from a woman from Georgia who died of car crash injuries. She had been given 37 units of blood from 60 people before she died. She had not shown symptoms of West Nile prior to her death, so was most probably infected by blood transfusion or a mosquito bite shortly before she was injured, Hughes said.

Blood from the 60 donors has been recalled and officials are tracking down an estimated 12 other people who may have received their blood. There is currently no test for infection before symptoms appear. Diagnosis relies on investigations of the immune response to the virus, which can take 15 days.

Blood supply
 

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FDA

The three organ recipients with West Nile encephalitis live in Georgia and Florida, where cases of viral infection via mosquitoes have been reported in 2002. The CDC says it is not ruling out that these three people were coincidentally all bitten by infected mosquitoes, but they say it is unlikely. But it could be two weeks before any firm conclusion is reached, Hughes said.

"At the moment, there's no evidence the nation's blood supply has been contaminated," he added. "That's just one possibility right now. If I personally needed a blood transfusion, the last thing I would be worried about is West Nile virus infection."

But the development of a quick blood test for detecting infection before symptoms appear is a priority, says the US Food and Drug Administration.

Establishing why only a minority of infected people develop serious symptoms is also important, say researchers. About 20 per cent of infections lead to a mild flu-like illness, and less than one per cent cause potentially deadly brain inflammation. Most people develop symptoms within two weeks of infection.

Emma Young

This story is from NewScientist.com's news service - for more exclusive news and expert analysis every week subscribe to New Scientist print edition.