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Mosquitoes May Have Been Main Carriers Of West Nile Virus

March 2, 2010

West Nile virus set the country abuzz when it rapidly spread from coast to coast just a few years after arriving in the United States. Most experts assumed birds were responsible for moving the virus across the country, but a paper published March 1 in the journal Molecular Ecology finds that smaller wings may be to blame.

“This is one of the first studies to suggest that mosquitoes may have played a greater role in the rapid movement of West Nile virus,” said Jason Rasgon, a professor at the Johns Hopkins Bloomberg School of Public Health and lead author of the study.

The virus first appeared in the United States in the summer of 1999, when it infected more than sixty people in the New York metropolitan area and killed seven. By the fall of 2004, West Nile virus had dispersed throughout the contiguous U.S.

“When you see such rapid movement, one of the main questions we ask is: What are the factors that mediated this jump?” Rasgon said. “In the past, people assumed that birds played the primary role in the spread of West Nile.”

The new study shows the actual pattern of West Nile movement across the Great Plains to the West coast more closely mimics the flight of mosquitoes than the haphazard leaps and bounds of resident birds.

In particular, the blame lies with Culex tarsalis, a mosquito found in the western U.S. where West Nile virus is particularly problematic. The mosquitoes are known to travel up to several miles a day in search of food, and can potentially transmit the virus to every human and bird they bite along the way.

“People have this idea that mosquitoes don’t move very far. For certain mosquitoes that’s 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,” Rasgon said.

In the recent report, Rasgon and colleague Meera Venkatesan collected C. tarsalis from twenty locations in the Midwest and western U.S., and performed genetic analyses to determine how mosquitoes in different areas of the country were related. Their results revealed the movement of West Nile virus across the western U.S. closely matched the path travelled by C. tarsalis from 2002 to 2004.

The findings are a reminder that even in the U.S, where many mosquito-borne illnesses have been eradicated, insects still play an important role in the spread of viral disease. “It sounds counter-intuitive, but the role of mosquitoes is often overlooked in the spread of mosquito-borne diseases,” Rasgon said. “It’s very interesting to figure out the factors that mediated West Nile invasion in the U.S., but we hope that these concepts can be extended in a more general way to understand the role of mosquitoes in the emergence of new pathogens.”

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