In 1999 West Nile virus infected its first American, in New York. By 2004, the disease had spread across the country. Researchers speculated that, for the disease to spread so far so quickly, birds must have helped. Some species could carry the disease and pass it on to mosquitoes, which would then infect humans. But a report published March 2nd in the journal Molecular Ecology [not available online as this episode was posted] says mosquitoes didn’t need a middleman. Or in this case, a middle bird.

Researchers [Jason Rasgon, Bloomberg School of Public Health, Johns Hopkins and Meera Venkatesan, Howard Hughes Medical Institute] noticed that West Nile virus didn't move north-south along migratory bird routes. Nor did it "leap frog" across large areas in the manner of a bird traveling far from where it was infected.

The scientists focused on a different suspect and obtained mosquito DNA from 20 sites in the western U.S. They found three clusters of a species known to carry West Nile. Surprisingly, they found extensive gene flow between these populations, which meant mosquitoes traveled between groups. In some cases, these mosquitoes covered a range just as large as that of the birds first fingered in the outbreak. So bird lovers take heart, your feathered friends are innocent avians.

—Adam Hinterthuer

[The above text is an exact transcript of this podcast.]