West Nile Virus Facts


West Nile virus was first detected in Fairfax County in 2000 when a single crow was found to be infected. Fairfax County has since implemented an intensive West Nile Virus Surveillance and Mosquito Management Program that includes surveillance of mosquitoe and human cases, as well as laboratory testing for West Nile virus, source reduction (elimination of standing water), larviciding, public education and community outreach.

Human Cases of West Nile Virus

West Nile virus was first detected in humans in Fairfax County in 2002, in which there were 13 cases with one fatality. There were three human cases of West Nile virus with no fatalities in 2003, and a single case, which was fatal, in 2004.

For more information about WNV activity in humans in Fairfax County, click here.

West Nile Virus Activity in Mosquitoes

The goal of the Fairfax County mosquito surveillance program is 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.

More information about WNV activity in mosquitoes.

West Nile Virus Activity in Birds

The Health Department has observed a drop in the number of dead birds reported in the County over the past three years. Much of this is due to the fact that those birds adversely affected by West Nile virus infection have died. Those not adversely affected by the infection have survived, developed immunity and continue to contribute to the gene pool. The ones that have died are no longer in the gene pool, while surviving birds have developed immunity to the virus. Since West Nile virus has been in the region for several years, there are a large number of immune birds in the population.

Mosquito Control Efforts

The Health Department treats catch basins throughout the County with a larvicide, an insecticide used to kill mosquito larvae, in programmed cycles aimed at reducing mosquito populations that breed in this environment. The first treatment cycle typically begins in May each year, with the number and magnitude of each cycle dependent on climatic factors and mosquito surveillance results.


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