Clues on West Nile Sought at Local Bird Paradises

By Cheryl Lyn Dybas
Special to The Washington Post
Monday, June 26, 2006

Dawn mist settled onto trees and bushes in the dog park on 26th Street NW, obscuring signs for Canal Road and the Whitehurst Freeway. The drizzle hid the antics of American robins chasing wormy early-morning meals. It also cloaked a threat more menacing than the fogged-in rush hour: mosquitoes carrying West Nile virus.

Robins foraging in seemingly tranquil places such as the park often pay a steep price: a bite from a mosquito laden with the lethal virus.

Foggy Bottom near the Watergate Hotel is one of a number of urban hot spots for mosquitoes carrying West Nile virus -- and for American robins. Other hot spots include the areas around the Hirshhorn Museum, the National Gallery of Art and the National Museum of Natural History. Not to mention Bethesda and Ridgeley's Delight, a posh neighborhood in Baltimore near Orioles Park at Camden Yards.

"We'd like to find out why these locations are hot," said biologist Peter Daszak of the Consortium for Conservation Medicine in New York, "and what we can do to decrease the impact of the virus on wildlife and humans." As they search for answers, scientists from the consortium are conducting studies on the links between changes in the environment and threats to wildlife and human health.

From May through September -- "picnic season," Daszak calls it -- infectious-disease ecologist Marm Kilpatrick and a team of biologists catch robins and other birds in fine-mesh nets strung between upright poles and snare mosquitoes in insect traps hung from tree limbs. The traps are baited with "stinky water" -- bits of grass, rabbit food and yeast mixed in water and left in a sealed jug in the sun for a week: mosquito manna.

"It's CSI: Infectious Diseases," said David Winograd, a neighbor passing by who paused to watch the research team's work.

Kilpatrick, who is with the consortium, collects thousands of mosquitoes at hot spots and samples those that have just fed. He sequences the DNA in their blood meals to identify the bird species the mosquitoes bit and takes blood samples from robins and other birds to test them for West Nile virus.

Robins, it turns out, appear to be taking the hit for humans, getting sick and dying as did thousands of crows that were infected in the first wave of West Nile virus after it arrived in North America. Thanks to the robins, humans who frequent the 26th Street dog park and similar areas have a lower chance of contracting the virus, at least in spring and early summer months. The reason? To mosquitoes, robins are far more tempting meals.

Then the scene changes.

"Robins begin to migrate south in late July and August," Kilpatrick said, "leaving mosquitoes on the hunt for blood from another source."

That source turns out to be Homo sapiens . The number of human infections with the virus shoots up come the dog days of August. Then it's mosquito vs. man or woman, instead of mosquito vs. robin.

Since it arrived in the United States in 1999, West Nile virus has become the major "vector-borne" disease here, with 826 reported deaths, 22,000 illnesses and "almost certainly a million people infected," Daszak said. Recent data suggest that the virus will cause 2,000 to 10,000 new U.S. cases a year, according to consortium reports.

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."