West Nile Virus

This fact sheet was updated 11/8/2020. 


West Nile Virus (WNV), of the genus or group, called Flaviviruses is a mosquito-borne zoonotic disease that primarily aff­ects birds by attacking their central nervous system. The virus was first isolated from a human patient in the West Nile region of Uganda in 1937.


West Nile Virus infects over 250 species of birds, but crows, blue jays, raptors and ravens are the most susceptible. Mammals, including humans, can also become infected with WNV from the bite of an infected mosquito. About 80% of infected humans will show no symptoms, while as many as 20% will exhibit flu-like symptoms. Less than 1% of people infected with the virus will become seriously ill with potentially fatal neurological disease. Individuals over the age of 50 or under 15 are at 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 many species, including blue jays, American robins, eastern bluebirds, tufted titmice, house wrens, chickadee. Fortunately, by 2005, house wren and blue jay populations had already rebounded and returned to their pre-WNV levels.

Most recently in Pennsylvania, WNV has been shown to have contributed to significant grouse population decline especially in areas of poor quality habitat, and it is thought that WNV may be playing a role in a wider decline in this species as it has in sage grouse on the West.

Species Affected

All North American bird species that have been experimentally challenged are susceptible to infection with WNV. As previously mentioned, crows, jays, and ravens tend to be most susceptible and have the highest mortality rates. House sparrows, common grackles, house finches, Cooper’s hawks, red-tailed hawks, grouse and raptors in general are also commonly infected species.

Horses and humans appear more likely to develop clinical illness than other mammalian species. The virus has been detected in the absence of clinical disease in many wild mammals including white-tailed deer, bears, foxes, mice, rats, opossums, raccoons, caribou, chipmunks, squirrels, skunks, and wolves. 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. Migrating birds play a major role in carrying the virus.

In 1999, WNV was imported into New York City most likely either through the transport of an infected mosquito on a ship or airplane, through import of an infected domesticated bird, or during a migration of a wild bird. Since then, WNV has spread throughout the United States (excluding Hawaii and Alaska), Canada and Mexico.


West Nile Virus is transmitted through the bite of an infected mosquito. Mosquitoes acquire the virus by feeding on the blood of infected birds and then transmit the virus to uninfected birds during a subsequent feeding. In some birds, the virus can persist for more than three months in the blood. Mosquitoes in the Culex group primarily attack birds and are the most common carriers of WNV, though the virus has been detected in over 60 mosquito species in the US. While mosquitoes are considered the primary mode of transmission, the virus may be spread directly from bird to bird via food or water that is contaminated with infected bodily secretions. Raptors can acquire the virus by consuming infected birds.

Humans and other mammals can become infected by Coquillettidia mosquitoes that feed on both birds and mammals. However, mammals do not produce enough viral particles in their blood to transmit WNV to mosquitoes, so they are considered dead-end hosts. Squirrels, chipmunks and racoons have been reported to shed the virus in oral secretions, feces and urine and 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 direct contact.

Clinical Signs

Clinical signs can be very variable, from unapparent to death, and are highly dependent on the type of species affected and the amount of virus available from the infected host.

Birds with West Nile Virus often show neurological signs including loss of coordination, head tilt, tremors, weakness, apparent blindness and lethargy. Most infected crows and jays will die within 3 weeks. On necropsy, hemorrhages on or in the brain, an enlarged spleen and areas of necrosis of the heart muscle are commonly reported.

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 (fever, headache, tiredness, body aches, nausea and vomiting) 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, preferably of the heart, brain or kidney, 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 especially at dawn and dusk when mosquitoes are most active. People should drain standing water around their houses in order to minimize mosquito breeding grounds. Pools and pond water can also be treated with mosquito dunks that kill the larval stage.

There are vaccines available for birds and horses, but not for humans.

Since WNV is a zoonotic disease, birds that may have died from WNV should not be handled without proper personal protective equipment. There have been no reports of transmission of WNV to hunters from handling game birds, but appropriate precautions should be taken with any wild animal exhibiting abnormal behaviour or found dead.

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