"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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