for National Geographic News
Mosquitoes that carry West Nile virus prefer the blood of certain birds to othersin particular the red-breasted American robin, scientists say.
The phenomenon is described in two recent studiesone 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 mosquitoesCulex pipiens and Culex restuansfrom 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.
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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.
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