Mosquitoes and West Nile Virus

In the United States, West Nile virus is transmitted through the bite of infected mosquitoes (Acrobat PDF file 32K), primarily members of the Culex genus. In Fairfax County, the primary vector of WNV is Culex restuans. This mosquito feeds primarily on birds and serves to disseminate the virus in the bird population. The bridge vector of WNV in Fairfax County is Culex pipiens. This mosquito will feed upon both birds and humans and in so doing may transmit the virus to the human population.

These two species of mosquito are virtually impossible to distinguish with the naked eye. Both are small brownish mosquitoes, as seen below.


Culex Culex Mosquito laying eggs

Culex Mosquito laying eggs. Photograph courtesy of CDC .

Of growing concern is the Asian Tiger Mosquito, Aedes albopictus, a mosquito that has proven to be a severe nuisance species and will certainly become an unwanted pest once it is established near your home. This mosquito is a container breeder and is often closely associated with homes. This mosquito is small and may be identified by the striking black and white pattern on its legs and white stripe on its back, as seen below.

Picture of Asian Tiger Mosquito

Aedes albopictus,the Asian Tiger Mosquito. Photo courtesy of CDC

Because of the preferred breeding sites and the aggressive nature of this mosquito, its potential to transmit West Nile Virus is a great concern. However, the public health importance of this mosquito remains undetermined. The potential for Ae. albopictus to transmit certain pathogenic arboviruses indigenous to the United States has been proven in laboratory experiments; however, disease transmission by this mosquito in natural settings has not been documented. To date, there have been two positive Aedes albopictus pools collected in Fairfax County. Both occurred in 2003.

For information about other potential mosquito vectors found in Fairfax County, click here.

What is Mosquito Surveillance?

The goal of the Fairfax County mosquito surveillance program is to identify, treat and monitor Culex mosquito breeding sites, and to monitor adult vector mosquito populations and their West Nile virus infection rates in order to better predict risk to human populations, and to try to determine other factors associated that may influence WNV transmission to humans.

The mosquito surveillance program is established throughout the County, and includes trapping stations throughout the county, where traps are set on a weekly basis to collect adult mosquitoes. These traps are a critical element in the County's mosquito management program and provide important data for analysis. If you see this equipment please do not disturb it. To learn more about the trapping methods used by the County, click here.

The County's WNV Surveillance and Mosquito Management team associate mosquito trap data with risk factors in order to assess and predict human risk and refine "triggers" for mosquito control activities. Based on these data, as well as WNV activity in birds, other animals or humans, adult mosquito trapping is expanded or enhanced in areas where WNV is detected. This helps determine zones of potential local transmission and the extent of viral activity thus guiding interventions.

The County uses a molecular diagnostic test to detect WNV in mosquitoes. To learn more about how the County tests for WNV, click here.

Mosquito Facts
To learn more about mosquitoes, click here.

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Technical Questions: Web Administrator

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