Researchers John Swaddle and Stavros Calos have found that high bird diversity is linked with low incidence of the West Nile virus in humans. Their study can be found online.
Called the "dilution effect," the link between biodiversity and disease rates is not completely clear, but scientists believe that increased diversity within an ecosystem reduces the proportion of suitable hosts for a disease, and therefore reduces transmission rates.
The West Nile virus spread to North America in 2002, and since then more than 28,000 human cases have been reported, including more than 1,000 deaths. Western states have been particularly hard hit by the virus -- and avian diversity may be a factor.
Swaddle and Calos looked at U.S. counties east of the Mississippi River and compared their avian diversity with the number of human cases. The researchers report that half of the human cases could be explained by the differences in local bird populations.
More than 300 species act as hosts, although American Robin Turdus migratorius has been singled out as the bird primarily responsible for transmission to humans. WNV may exacerbate the threat to vulnerable birds and increase the risk of extinction, say the researchers. The numbers of Yellow-billed Magpies, found only in California, appear to have declined by almost half since 2007 as a result of WNV.
In related news, scientists at Colorado State University are investigating how mosquitoes survive dengue fever, yellow fever, Japanese encephalitis and WNV -- when the viruses kill humans who are millions of times larger. Apparently the blood-sucking mosquito creates an immediate immunity not strong enough to kill the virus, but strong enough to save the mosquito's life.
In the future, scientists may genetically modify mosquitoes that are immune to the viruses and release them into the wild -- the fittest mosquitoes would replace the ones run down by carrying viruses in their bodies. (Although why humans want more mosquitoes, even "fit" ones, is a question...)
"You can't fight evolution," one of the researchers told the Denver Post. "You've got to figure out how to manipulate it."