More Bird Species Means Fewer West Nile Cases

June 25, 2008 -- Here's proof that biodiversity is good for your health: Having more bird species present in a given area reduces the incidence of West Nile virus infection in humans.

That's the finding of a new study analyzing every U.S. county east of the Mississippi River where human cases of West Nile virus infection have been documented, and comparing each with a neighboring county where human West Nile cases have not been reported.

Applying statistical analysis while controlling for socioeconomic factors and how urbanized each county is, John Swaddle and Stavros Calos at the College of William and Mary in Williamsburg, Va., found that bird diversity and other factors related to the bird population could explain 50 percent of the variation in West Nile cases.

"We were surprised by how high it was," Swaddle said.

Bird diversity alone can explain about 20 to 25 percent of the variation, he said. The study was published online yesterday in the journal PLoS ONE.

Birds are the hosts for the West Nile virus, which is spread by mosquitoes. When the density of infected birds is high enough, the likelihood that a human will be bitten by an infected mosquito increases.

Most humans do not fall ill with West Nile virus, even if bitten by an infected mosquito; those over 50 are at greater risk of experiencing severe symptoms.

Part of the reason that increased diversity reduces West Nile cases, Swaddle said, is that some birds are better carriers for the virus than others.

The American robin, for instance, appears to be a good host for the disease.