Clues on West Nile Sought at Local Bird Paradises

The virus is transmitted by Culex pipiens mosquitoes, which appear as soon as the days warm in spring. The mosquitoes buzz everywhere by midsummer.

People can be infected only if they are bitten by virus-carrying mosquitoes, "not by contact with virus-carrying robins, an important point," said virologist Roger Nasci of the Division of Vector-Borne Diseases of the Centers for Disease Control and Prevention.

Scientists are still working to explain the large outbreaks of West Nile virus in the mid-Atlantic states. Kilpatrick, Daszak and others published results of a study on the patterns of transmission in the April issue of the journal PLoS Biology. Their findings showed that shifts in mosquito-feeding behavior from robins to humans explain why the number of West Nile cases spikes in the East in late summer and early fall.

In May and June, American robins, which made up just 4.5 percent of the bird population at the study sites, accounted for more than 50 percent of the animals that were bitten, Kilpatrick and colleagues found. "But as summer wears on and robins leave their breeding grounds to fly south, the probability that humans will become mosquito targets increases sevenfold," said ornithologist Peter Marra of the Smithsonian Migratory Bird Center in the District, who also took part in the study.

In the June issue of the British journal Proceedings of the Royal Society B, the biologists showed that mosquitoes would rather feed on robins than any other birds, including crows and house sparrows. They don't know why.

"We need exactly this kind of research -- studies not only of West Nile virus, but of the environment in which it exists -- to help us predict outbreaks and to figure out ways of preventing its movement into uninfected regions," the CDC's Nasci said.

Kilpatrick, Daszak and others also found that the virus is most likely to spread to new areas on airplanes carrying infected mosquitoes. "In fact," Daszak said, "West Nile virus probably originally arrived in the U.S. from Europe or Africa on a plane." That finding will be reported in the July issue of the journal Conservation Biology.

The researchers developed a computer model to predict the risks of human infection. The model indicates that cases in people will peak from late July to mid-August, then decline toward the end of August. "The actual number and pattern of human West Nile virus illnesses is strikingly similar" to what the model predicted, said infectious-disease scientist Laura Kramer of the New York State Department of Health in Albany, N.Y., lead scientist on the project.

While the virus is most common in the East, it has also spread west to California and Colorado. "A late-summer spike in West Nile virus infections happened there, too," Kramer said, "suggesting a continent-wide phenomenon."

Kramer is concerned that people may blame neighborhood robins for an increased risk of West Nile virus, but, she said, "If you removed the robins, something else would take their place.

"The mosquitoes would feed on another animal, and in late July and August, they obviously do: us."

Anne Brown, who lives one block from the 26th Street dog park, also worries about robins getting a bad rap. "People who live in nearby townhouses have gotten mixed up," Brown said. "They think they can get West Nile virus from a robin. 'I don't want any robins in my yard,' they'll say, 'they're going to give me West Nile.' "

If the scientists are right, those homeowners couldn't be more wrong.

"Robins are protecting us from virus-carrying mosquitoes lurking in the trees," Winograd, the neighbor, said. "Instead of robin red-breasts, maybe they should be called Robin Hoods."