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.
"American robins have even been termed the 'super-spreader' by a couple of people," he said.
His analysis showed that increased numbers of robins corresponded to increased cases of West Nile infection. "But it was not as significant as overall diversity," he added.
Greater numbers of bird species in a given area may decrease the relative number of good hosts to poor hosts, reduce the total number of good hosts, or reduce the probability that a mosquito will spread the virus from one good host to another. But which of these possibilities is the dominant factor is not yet known.
The work fits in to a bigger overall body of research showing increased biodiversity reduces disease transmission to humans. The idea first gained wide attention in 2000, when Felicia Keesing of Bard College in Annandale, N.Y., and colleagues reported that human Lyme disease infection correlated to reduced diversity of the small mammals that Lyme-spreading ticks feed on.
As with bird hosts for West Nile Virus, certain mammals, like mice, are better hosts for Lyme disease than others, like deer, Keesing said.
"There's little that would motivate people to protect biodiversity as much as protecting their health would," Keesing said. "From my point of view our default position should be protecting diversity because it's going to reduce disease transmission for us and other species."
Planning developments so that they maintain large, common green areas, rather than having individual parcels of land with disconnected patches of trees is one strategy for maintaining diversity of birds or mammals, she said.
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