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West Nile Virus


West Nile Virus (WNV) is primarily a disease of birds that affects the central nervous system and is transmitted by the bite of mosquitoes carrying the virus. Humans and other mammals can become infected, but they are considered dead end hosts. The virus was first isolated from a human patient in the West Nile region of Uganda in 1937.


Humans and wildlife can become infected with West Nile Virus from the bite of an infected mosquito. About 80% of people infected with WNV will show no symptoms at all, while as many as 20% will exhibit flu-like symptoms. Approximately one in 150 people infected with the virus will become seriously ill with potentially fatal neurological disease. Adults over the age of 50 are at the greatest risk of developing the severe form of the disease because of age-related deficiencies in immune function.

The United States crow population declined by 30% as a result of the introduction of WNV. There have also been significant declines in blue jays, American robins, eastern bluebirds, tufted titmice, house wrens, and chickadees. However by 2005, house wren and blue jay populations had already rebounded and returned to their pre-WNV levels.

Species Affected

West Nile Virus has been found in more than 250 species of birds, but crows, jays, and ravens are most susceptible to the disease. House sparrows, common grackles, house finches, Cooper's hawks, and red-tailed hawks are also more commonly infected species. Mammals can become infected, but often do not show clinical signs and are considered accidental hosts. Horses and humans appear more likely to develop clinical illness than other mammalian species. WNV has been reported in many wild mammals including caribou, chipmunks, squirrels, skunks, and wolves. Domestic mammals including cats, dogs, cattle, and sheep can become infected. The virus has also been isolated in reptiles and amphibians.


West Nile Virus has been found in Africa, the Middle East, Russia, and southern Europe and Asia. An Israeli strain of the virus was first isolated in North America in August 1999 in New York City. The virus was probably introduced to America via the transport of an infected mosquito by ship or airplane. It may also have been introduced via the import of an infected domestic bird or migration of an infected wild bird. Migrating birds played a major role in carrying the virus throughout the United States. Since its introduction, WNV has spread across the entire continental United States, and into Canada and Mexico. The virus was first found in Pennsylvania in 2000 in mosquitoes, birds, and a horse. Pennsylvania conducts continuous surveillance for the presence of WNV and publishes the data regularly on the United States Geological Survey website as well as the Pennsylvania West Nile Virus Control Program website.


West Nile Virus is most often transmitted in the bite of infected mosquitoes. Mosquitoes acquire the virus by blood feeding on infected birds. The mosquito can then transmit the virus to uninfected birds. Humans and other mammals can become infected by species of mosquitoes that feed on both birds and mammals. Mosquitoes in the Culex group primarily bite birds and are the most common carries of WNV, while Coquillettidia species bite both birds and mammals. Mammals are not known to produce enough viral particles in their blood to transmit WNV to mosquitoes, so they are considered dead-end hosts.

Mosquitoes are considered the primary mode of transmission. However, the virus may be able to spread from bird to bird in food or water that is contaminated with infected bodily secretions, and raptors may be able to acquire the virus by consuming infected birds. West Nile Virus can be transmitted from person to person via organ transplants, blood transfusions, and across the placenta, but it is not thought to be transmitted by direct contact.

Clinical Signs

Birds with West Nile Virus often show neurological signs including loss of coordination, head tilt, tremors, weakness, and lethargy. Most infected crows and jays will die within 3 weeks. Most mammals do not show clinical signs of this disease. Horses and humans are more likely than other mammals to develop a flu-like illness or signs of neurologic disease.


Oral swabs or tissue samples taken from suspect dead birds are used to detect the presence of West Nile Virus antigen.


As with most viruses, there is no specific treatment for West Nile Virus. Some animals may require supportive care to survive. Animals with milder symptoms are more likely to recover than those showing severe neurological signs.


Pennsylvania has a West Nile Virus control program that includes testing mosquitoes and dead birds for the disease. The state also monitors for human and horse cases of WNV to determine the distribution of the disease throughout PA. Disease transmission can be prevented with mosquito population control. People should use insect repellant and wear long sleeves and long pants to avoid mosquito bites when spending time outside. People should also drain standing water around their houses in order to minimize mosquito breeding grounds. There is currently a vaccine available for horses.

Suggested Reading

Bureau of Animal Health and Diagnostic Service. West Nile Virus brochure. Pennsylvania Department of Agriculture, Harrisburg, Pennsylvania, USA.

Centers for Disease Control and Prevention (CDC). 2011. West Nile Virus.

McLean, R. G., and S. R. Ubico. 2007. Arboviruses in birds. Pages 17-62 in N. J. Thomas, D. B. Hunter, and C. T. Atkinson, editors. Infectious diseases of wild birds. Blackwell Publishing, Ames, Iowa, USA.

Michigan Department of Natural Resources. Wildlife Disease. West Nile Virus (WNV). <,1607,7-153-10370_12150_12220-99070--,00.html>. National Wildlife Health Center. 2011. West Nile Virus (WNV). United States Geological Survery.

Pennsylvania's West Nile Virus Control Program.

Pollock, C. G. 2008. West Nile Virus in the Americas. Journal of Avian Medicine and Surgery 22: 151-157.