Swarm of mosquitoes

West Nile Virus


  • West Nile Virus (WNV) is a mosquito-borne virus that affects humans and other animals, particularly horses.
  • The effects of WNV infection in horses can range from mild to potentially fatal inflammation of the brain.
  • Although most horses recover from WNV infection, some experience long-term effects.
  • The best way to protect horses from WNV infection is by including a WNV vaccine as an annual core vaccine. It is also important to practice good mosquito control and minimize horses’ exposure to mosquitoes.

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What is West Nile virus?

West Nile Virus (WNV) is a mosquito-borne virus that affects humans and other animals, of which horses represent 96.9% of reported non-human cases. Introduced to the United States in 1999, WNV is now found in all of the contiguous 48 states.

Wild birds are the natural hosts for WNV. Humans and horses are considered dead end hosts. The virus is only transmitted through mosquitoes; humans and horses cannot give it to each other.

Many horses that become infected with WNV do not show signs of illness, but some develop neurologic disease. The virus enters the bloodstream, where it multiplies, through a bite from an infected mosquito. If the virus crosses the blood-brain barrier, it can cause inflammation of the brain and possibly death.

What are the clinical signs of West Nile virus?

The effects of WNV in horses range from mild, “flu-like” signs to inflammation of the brain (West Nile Encephalitis), which can be fatal. Some infected horses show no clinical signs and recover without intervention.

Clinical signs of WNV infection may include fever, incoordination, stumbling, falling, weakness, muscle twitching, seizures, drooping lips, lip smacking, head drooping, grinding teeth, and abnormal sensitivity to touch or sound. Serious cases may be unable to rise (recumbent).

How is West Nile virus diagnosed?

To make a diagnosis of WNV in horses, veterinarians will first rule out other neurologic diseases. An IgM capture enzyme-linked immunosorbent assay (ELISA) test is the most reliable method to detect WNV infection. It is capable of identifying the virus within six days, and up to two months, after infection.

How is West Nile virus treated?

There is no specific treatment for WNV in horses. Supportive care for clinical signs may include anti-inflammatory drugs and intravenous (IV) fluids.

What is the prognosis for West Nile virus?

Many horses recover fully from WNV infection. Some that recover exhibit long-term effects, including gait and behavioral abnormalities. The prognosis is poorer for horses that are severely affected. Older horses are more likely to experience severe signs than younger horses. The case fatality rate is approximately 33% and recumbent horses are at greater risk of death or requiring euthanasia.

How can West Nile virus be prevented?

The American Association of Equine Practitioners (AAEP) recommends including a WNV vaccine as an annual core vaccine to protect horses from infection. Owners and facilities can further protect horses by practicing good mosquito control, including eliminating breeding sites by removing standing water, cleaning water containers and stocking water troughs with fish that feed on mosquito larvae. It is also important to minimize horses’ exposure to mosquitoes through use of repellents and bringing horses in at peak mosquito feeding periods (dawn and dusk).

For more information:

CDFA - Equine West Nile Virus

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu