UC Davis Center for Equine Health

West Nile virus (WNV) is a mosquito-borne virus that affects humans and other animals, of which horses represent 96.9 percent of reported non-human cases. Introduced to the United States in 1999, WNV is now found in all of the contiguous 48 states. In Canada, 86 human cases and three deaths had been reported to the Public Health Agency of Canada (PHAC) in between January 1 and October 24, 2020 as well as 40 WNV-positive wild birds and six positive horses. In addition, a total of 15,945 mosquito pools had been tested for WNV in four provinces: Saskatchewan (71), Manitoba (1,264), Ontario (13,179) and Québec (1,431). Of these, 198 WNV-positive mosquito pools have been reported to PHAC.

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.

Related: Eastern and Western Equine Encephalitis

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 neurological 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 behavioural 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 for horses exhibiting clinical signs is approximately 33 percent and recumbent horses are at greater risk of death or requiring euthanasia.

How can West Nile virus be prevented?

Including WNV vaccine as an annual core vaccine can protect horses from infection, and local veterinarians can provide guidance on appropriate products and strategies. 

Related: What Are You Vaccinating Against?

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).

Printed with the kind permission of the UC Davis Center for Equine Health. The UC Davis Center for Equine Health is dedicated to advancing the health, welfare, performance and veterinary care of horses through research, education and public service. https://ceh.vetmed.ucdavis.edu/ 

Photo: Shutterstock/Rita Kochmarjova