West Nile Virus

NWDC is in the process of updating this fact sheet. 


West Nile Virus (WNV) is a virus from the Flavivirus genus. It is a mosquito-borne disease that primarily affects birds by attacking the central nervous system. 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, while as many as 20% will exhibit u-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. Fortunately, 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. Crows, jays, and ravens tend to be most susceptible to the disease. House sparrows, common grackles, house finches, Cooper’s hawks, and red-tailed hawks, and raptors in general are also commonly infected species. While humans and other mammals do become infected and can develop serious disease, most mammals do not develop symptoms. 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 identified in Africa, the Middle East, Russia, southern Europe, and Asia. An Israeli strain of the virus was first isolated from crows in North America in August 1999 in New York City. The virus was likely introduced to America through the transport of an infected mosquito via ship or airplane. It may also have been introduced through the import of an infected domestic bird or during the migration of an infected wild bird. Migrating birds played a major role in carrying the virus throughout the United States. Since its introduction in 1999, WNV has spread across the entire continental United States and into Canada and Mexico. 



West Nile Virus is most often transmitted through the bite of infected mosquitoes. Mosquitoes acquire the virus by feeding on the blood of infected birds and then transmit the virus to uninfected birds during a subsequent feeding. Humans and other mammals can become infected by the 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 do not produce enough viral particles in their blood to transmit WNV to mosquitoes, so they are considered dead-end hosts. While mosquitoes are considered the primary mode of transmission, the virus may be spread from bird to bird in food or water that is contaminated with infected bodily secretions. Raptors can 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 believed to be able to be spread through 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. Severe debilitating disease and death are most likely to occur in the elderly or those with compromised immune function.


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


The CDC has developed a national monitoring and surveillance system called ArboNET to help manage and control the incidence of West Nile Virus. ArboNET is managed by both national and state health departments and maintains surveillance data on human, mosquito, bird, sentinel animal, and veterinarian reported cases of WNV. Disease transmission can be prevented with mosquito population control. People should use insect repellent 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.

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