Mosquitoes may have carried West Nile virus across the United States

Mosquitoes — not birds, as was previously suspected — may have carried West Nile virus westward across the United States, according to research from the Johns Hopkins Bloomberg School of Public Health.

The researchers wrote that after the initial appearance of the virus in the United States in New York in 1999, it was assumed that birds carried it across the Mississippi River and into the Great Plains over the span of four or five years. The assumption was that birds transmitted the virus to certain mosquitoes, including Culex tarsalis, which then transmitted it to humans.

The researchers analyzed mosquito DNA that had been collected from 20 sites across the western United States. Three distinct clusters of C. tarsalis populations were observed among these mosquito populations. The pattern of genetic clustering was congruent with the pattern of West Nile virus infection across the country, according to the researchers.

“People have this idea that mosquitoes don’t move very far,” Jason L. Rasgon, PhD of Bloomberg School’s Malaria Research Institute, said in a press release. “For certain mosquitoes, that is true. But the range of this particular mosquito is as great as the range of the birds that were originally thought to move the virus.”

Extensive gene flow between the populations also was observed, indicating the widespread migratory patterns of the mosquitoes. Despite this, gene flow was limited in certain regions, including the Sonoran Desert in Arizona, the eastern Rocky Mountains and the High Plains plateau. Mosquito movement likely was blocked by these obstacles, the researchers said.

Venkatesan.M. Molecular Ecology. 2010; DOI: 10.1111/j.1365-294X.2010.04577.