Bird Diversity Helps Protect Humans From West Nile Virus

"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.

Biologist John Swaddle examines a young bluebird. (Photo courtesy William and Mary)

"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.

A flock of cedar waxwings in Savannah, Georgia. This species is on the Centers for Disease Control list for West Nile Virus mortalities. (Photo by Grandma Tina)

The findings also suggest that diverse bird communities lowered human case numbers even when the epidemic was underway.

The way in which biodiversity and disease rates are linked has been dubbed the dilution effect. Although the exact mechanisms are not currently clear, scientists believe that increased diversity within an ecosystem reduces, or dilutes, the proportion of suitable hosts for a disease, and therefore reduces transmission rates.

The dilution effect has previously been studied through another infection, Lyme disease, but this new research suggests that it may be more widely applicable. If so, it could be a valuable tool for public health and safety plans.

Swaddle said the research indicates that very small changes in land management could attract more bird species, with the increase in biodiversity paying off in the form of lower human infection rates during outbreaks of West Nile or other diseases in the bird population, such as avian flu and bubonic plague, that can be transmitted to humans.

"Biodiversity is giving us a public health service that people have rarely considered," he said, "and the value of this service should be considered when developing land and managing bird populations in the future."

Entitled "Increased avian diversity is associated with lower incidence of human West Nile Infection: observation of the dilution effect," the paper was published in June 2008 in the peer-reviewed open access journal "PLoS ONE."

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