The West Nile Virus May Have Been Spread by Mosquitos, Not Birds

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Phoenix, AZ, June 16, 2010 — According to a study at Johns Hopkins Bloomberg School of Public Health, mosquitoes, not birds, may have played a primary role in the spread of West Nile Virus westward across the United States. The study, published in Molecular Ecology, is among the first to examine the role of mosquitoes in the spread, which was originally suspected to be caused by birds.

First detected in the United States in 1999 in New York, the virus spread rapidly across the U.S. between 2001 and 2004. Between 2001 and 2002 it made a large jump across the Mississippi River and into the Great Plains. Birds, known as a host of the disease, have long been considered the suspect in transporting the virus across the continent. The birds can transmit the virus to certain mosquito breeds, such as Culex tarsalis, which then pass the virus to humans through bites.

The reason why birds were reconsidered as the primary cause of the West Nile Virus spread is the rapid movement of the spread. It did not follow the traditional leap-frog pattern or move across migratory bird routes as would be expected if the birds were in fact the cause of the spread.

For the study, DNA was analyzed from mosquitoes collected from 20 sites across the western U.S. Three distinct clusters of C. tarsalis populations were detected from the genetic analysis. Extensive gene flow between populations were found, which indicated widespread movement by mosquitoes. However, certain regions experienced limited gene flow, such as the Sonoran desert in Arizona, the eastern Rocky Mountains and the High Plains plateau. All three appear to have blocked mosquito movement. Researchers have also found the pattern of genetic clustering matched the pattern of West Nile Virus infection across the U.S.

As mosquitoes have been identified as a primary cause of the West Nile Virus spread, more efforts are being made to reduce the breeding cycles, thus reducing the spread and rate of infection. Mosquito traps available to the general population can interrupt the mosquito breeding cycle for up to one acre an entire season long. The traps work by releasing CO2 and attractant into the air continuously to lure mosquitoes. Once near the trap, they are vacuumed into a net where they dehydrate and die. By running continuously, the breeding cycle of mosquitoes is interrupted and the mosquito population is greatly reduced in the area.