West Nile Mosquitoes Prefer Robins, Study Finds

Nicholas Bakalar
for National Geographic News

September 25, 2006

Mosquitoes that carry West Nile virus prefer the blood of certain birds to others—in particular the red-breasted American robin, scientists say.

The phenomenon is described in two recent studies—one conducted in the Washington, D.C., area, the other in birds in Connecticut.

In the most recent study, A. Marm Kilpatrick, a senior research scientist at the Consortium for Conservation Medicine in New York, worked with colleagues at five sites in Maryland and Washington, D.C., from May to September 2004.

The research, which was funded by the National Institute of Allergies and Infectious Diseases, is described in the June issue of the journal Proceedings of the Royal Society B.

In the study, scientists trapped birds and tested their blood for antibodies to the West Nile virus.

(See related photo gallery: "The Next Killer Flu.")

The researchers then collected two species of mosquitoes—Culex pipiens and Culex restuans—from each of the five study sites to see which birds the insects fed on.

Culex mosquitoes are the principal transmitters of West Nile virus to humans.

The team found that while robins constituted about 3.7 percent of the birds at the study sites, their blood accounted for 43.4 percent of mosquito feedings.

In other words, the insects fed on robins about 17 times more often, on average, than would be expected if the mosquitoes had no dining preference.

Greater West Nile Threat?

By examining the concentration of virus in the bird blood samples, scientists were also able determine how efficiently the birds reproduce and transmit the disease.

"The concentration of virus in the blood reveals that there is 'host competence,'" Kilpatrick explained. "If robins were fed on often but didn't reproduce the virus, it wouldn't matter."

But in fact, the virus does very well in robin blood. As a result, more than half of West Nile virus-infected mosquitoes may have become infected by feeding on robins.

This has two implications for humans, according to Kilpatrick.

First, by concentrating on a single bird species, mosquitoes transmit the virus more efficiently than if they spread their biting among several species. And because robins arrive early in the spring, epidemics start earlier in the year.

More ominously, Kilpatrick said, "When robins are around, they're fed on a lot. When they leave, the mosquitoes switch to people."

Why Culex mosquitoes prefer some birds to others is unclear, according to Kilpatrick. But he suggests various possibilities:

• larger birds produce more heat and carbon dioxide and are easier for mosquitoes to find,
• some birds have more exposed skin,
• birds have different odors or roosting locations that may help a mosquito find a bird,
• some birds may defend themselves against the bugs by shaking or biting them off.

Kilpatrick's work helps quantify the implications of mosquitoes' preference for robins that was first reported by Goudarz Molaei and his colleagues at the Connecticut Agricultural Experiment Station in the March issue of the journal Emerging Infectious Diseases.

According to Kilpatrick, the same effect has now been observed elsewhere as well.

"In Tennessee, the phenomenon has also been noticed," he said. "It looks like these mosquitoes love robins across a large area."

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