Bird diversity may protect from West Nile Virus

North American scientists studying West Nile virus have shown that more diverse bird populations can help to buffer people against infection. Since the virus first spread to North America it has reached epidemic proportions and claimed over 1,100 human lives.

“This is an important example of the links between biodiversity and human health,” observed Dr. Stuart Butchart, global research and indicators coordinator for BirdLife International, speaking Wednesday in Cambridge on the eve of a West Nile Virus conference today in Savannah, Georgia.

The study, by biologist John Swaddle and then undergraduate student, Stavros Calos ‘08, at William and Mary University, found that areas which have a more diverse bird populations show much lower incidences of West Nile virus infection in the human population.

The authors highlighted the “increasing evidence for economically valuable ecosystem services provided by biodiversity.”

West Nile virus mainly affects birds but can be transferred to humans via mosquitoes. Found in the United States since 1999, WNV human, bird, veterinary or mosquito activity have been reported from all states except Hawaii, Alaska, and Oregon. Since first introduced, the virus has reached an epidemic scale with over 28,000 human cases resulting in more than 1,100 deaths.

“We don’t yet know the precise mechanism that drives this pattern, but it’s likely to be due to diverse areas having relatively few of the bird species that are particularly competent hosts and reservoirs for the virus,” said Swaddle, an associate professor of biology and director of the Environmental Science and Policy Program at William and Mary.

Host competence, he explains, refers to a set of qualities that make aparticular species of bird best able to contract the disease and pass it on through a vector. The highest levels of host competence are found in crows, jays, thrushes and sparrows, the very birds that tend to thrive when avian biodiversity is reduced.

Over 300 species act as hosts, although Butchart says the American robin, Turdus migratorius, a migratory songbird of the thrush family, has been named as largely responsible for transmission from birds to humans.

“West Nile virus may compound existing pressures - like habitat loss - to increase the risk of extinction for species,” he said.

For example, the yellow-billed magpie, Pica nuttalli, which is found only in California, appears to have declined by almost 50 percent in the last two years as a consequence of the disease, Butchart said.

Scientists studying the virus looked at U.S. counties east of the Mississippi River and compared their avian diversity with the number of human cases. They found that high bird diversity was linked with low incidence of the virus in humans. They reported that about half of the human incidences of West Nile virus could be explained by the differences in local bird populations. 

Posted by Dennis on 02/27 at 09:11 PM