Side View Mosquito Portrait

The westward spread of West Nile virus across the United States may have been the doing of mosquitoes. “In the past, people assumed that birds played the primary role in the spread of West Nile,” said senior author Jason Rasgon. “However, the rapid spread of West Nile did not follow a leap-frog pattern or move north to south along migratory bird routes like we would expect.” (Courtesy: iStockphoto)

JOHNS HOPKINS (US)—Mosquitoes, not birds as previously thought, may be to blame for West Nile virus’s rapid westward spread across the United States.

Researchers at Johns Hopkins University reach that conclusion in a study in the March 2 edition of Molecular Ecology.

The potentially fatal West Nile virus was first detected in the United States in 1999 in New York. Between 2001 and 2004, it spread rapidly, making its jump across the Mississippi River and into the Great Plains by 2002. In 2009, according to the Centers for Disease Control and Prevention, human cases were reported in 34 of the 48 contiguous states and mosquito, bird, or animal infections in all but one of the others.

Birds are known hosts of the virus; they transmit it to certain mosquitoes, like Culex tarsalis, who can then pass the virus to humans they bite.

“In the past, people assumed that birds played the primary role in the spread of West Nile,” says senior author Jason Rasgon, assistant professor with Bloomberg School of Public Health’s Malaria Research Institute and the W. Harry Feinstone Department of Molecular Microbiology and Immunology.

“However, the rapid spread of West Nile did not follow a leap-frog pattern or move north to south along migratory bird routes like we would expect,” he says. “When you see such rapid movement, one of the main questions we ask is: ‘What are the factors that mediated this jump?’ Our study shows mosquitoes are a likely candidate.”

For the study, Rasgon and his coauthor, Meera Venkatesan, a former graduate student at the Bloomberg School and current postdoctoral researcher with the Center for Vaccine Development and Howard Hughes Medical Institute, analyzed DNA from mosquitoes collected from 20 sites across the western United States.

Genetic analysis detected three distinct clusters of C. tarsalis populations. The researchers found extensive gene flow between the populations, which indicated widespread movement by the mosquitoes.

Gene flow, however, was limited in certain regions, such as Arizona’s Sonoran desert, the eastern Rocky Mountains, and the High Plains plateau—all three of which appear to have blocked mosquito movement. The researchers found that the pattern of genetic clustering was congruent with the pattern of West Nile virus infection across the country.

“People have this idea that mosquitoes don’t move very far. For certain mosquitoes, that is true,” Rasgon says. “But the range of this particular mosquito is as great as the range of the birds that were originally thought to move the virus.”

Research funding was provided by the National Institutes of Health and the Johns Hopkins Malaria Research Institute.

Johns Hopkins health news: www.jhsph.edu/publichealthnews/