West Nile virus spread linked to specific bird species

The spread of the West Nile virus in North America has been found to be related to only a few key species of bird hosts.

After initially appearing in New York in 1999, West Nile virus spread quickly and is now well established in the United States. The mosquitoes that transmit it and the birds that are critical hosts for it are abundant in areas modified by human habitats.

"The virus has had an important impact on human health in the United States partly because it took advantage of species that do well around people," Marm Kilpatrick, a biologist at the University of California - Santa Cruz, said.

Although West Nile virus is believed to be capable of infecting more than 300 species of birds and 60 species of mosquitoes, as well as mammals, reptiles and amphibians, it is now thought that only a few key species of birds and mosquitoes are critical for its continued transmission.

"We now know that in any given location, only one or two species of mosquitoes play a big role, and only a handful of birds appear to be important in overall transmission rates," Kilpatrick said.

Kilpatrick recently reviewed more than a decade of research on the ecology and evolution of the West Nile virus to reach the conclusion that the familiar American robin has played an integral role in spreading the illness across North America.

It is not the abundance of the birds that is behind their role, but the feeding patterns of the mosquitoes that transmit the virus.

"Robins are more important in transmission than their abundance alone would suggest," Kilpatrick said. "The peculiar feeding habits of the vectors play a really important role in transmission, and this idea applies to many different diseases. It's one of the really interesting things we've learned from the past decade of research on West Nile virus."

The results of Kilpatrick’s analysis were recently published in the journal Science.