Equine West Nile Virus

Equine Health Alert

West Nile Virus

West Nile Virus (WNV) is a mosquito-borne virus first detected in the United States (U.S.) in the New York City area in 1999. Since 1999, the virus has spread throughout the U.S. and Canada, infecting birds, humans, horses, and other animals. As of 2015, more than 27,000 horses in the U.S. have been infected since the disease was first identified. The virus is maintained in the wild bird population and is spread between birds by mosquitos. Birds are considered the natural reservoir for WNV since high levels of virus circulate in their bloodstream. Mosquitos acquire WNV in blood meals from infected birds and pass it on to other birds, animals, and people. Mosquitos that feed on an infected horse or human have not demonstrated the ability to ingest enough of the virus to transmit it to other animals or humans; therefore, horses and humans are considered “dead end hosts.”

Clinical Signs

West Nile Virus may cause a wide range of clinical illness ranging from mild” flu-like” signs to encephalitis (inflammation of the brain) that may be fatal to both humans and horses. While horses are susceptible to WNV infection, many infected horses do not develop clinical illness and recover uneventfully.

Consult a veterinarian if your horse exhibits any of the following clinical signs:

  • Fever
  • Incoordination, especially in rear limbs, causing stumbling and falling
  • Generalized weakness, muscle twitching, seizures or coma
  • Drooping lips and lip smacking, head drooping, grinding teeth
  • Hypersensitivity to touch or sound
  • Recumbency (inability to rise)


Any horse displaying abnormal behavior or neurologic signs should be examined by a veterinarian to rule out neurologic diseases, such as WNV, Equine Herpes Virus-1, Equine Protozoal Myeloencephalopathy, Eastern/Western Encephalomyelitis and Rabies.

Blood samples should be collected by a veterinarian and sent for diagnostic testing. The WNV IgM capture ELISA test is a specific test that detects acute WNV infection in animals, is usually positive within six (6) days post infection and can remain positive for up to two (2) months post infection. A positive WNV IgM Capture ELISA test is indicative of recent infection. Unexposed animals vaccinated for WNV will test negative on the WNV IgM capture ELISA test.

Treatment and Prognosis

Currently, there is no specific treatment for WNV. Supportive care includes administration of anti-inflammatory drugs and intravenous fluids. Recumbent WNV positive horses are at a higher risk of dying or requiring euthanasia. For horses exhibiting clinical signs of WNV, the case fatality rate is approximately 33%; however, many infected horses will fully recover following infection.


Vaccination and mosquito control minimize the risk of WNV infection in horses. Although the number of WNV infected horses has declined over the last five years, WNV remains an important disease in unvaccinated horses. The American Association of Equine Practitioners (AAEP) recommends incorporation of a WNV vaccine as an annual core vaccine in equine vaccination protocols. Currently, there are four United States Department of Agriculture (USDA)-licensed WNV vaccines available for horses in the U.S. Horse owners should consult with their veterinary practitioner to ensure current WNV vaccination status of their horses.

Mosquito Control

Minimizing horse exposure to mosquitoes during the peak mosquito feeding periods at dawn and dusk decreases the risk of horse exposure to WNV. Application of mosquito repellant can also effectively reduce the number of mosquito bites on horses. Mosquito control efforts to eliminate mosquito breeding sites are also important. Methods to eliminate mosquito breeding sites include:

  • Draining unnecessary standing water found in wheelbarrows, tires, etc.
  • Cleaning water containers at least weekly (i.e., bird baths, plant saucers)
  • Scheduling pasture irrigation to minimize standing water
  • Keeping swimming pools optimally chlorinated and draining water from pool covers
  • Stocking of water tanks with fish that consume mosquito larvae (Contact local mosquito control for assistance) or use mosquito “dunk” available at hardware stores.

California's WNV Surveillance

WNV surveillance includes testing samples from dead birds, sentinel chickens, mosquito pools, horses and humans. The California Department of Food and Agriculture (CDFA) works with federal, state and local health and agricultural agencies to minimize the impact of WNV on the equine industry. CDFA confirms suspect equine WNV cases, maintains horse surveillance data and educates horse owners on WNV disease control and prevention. Collaborating agencies use the surveillance information to focus disease control efforts.

In October 2003, WNV was first-diagnosed in California in 20-year old unvaccinated Missouri Fox Trotter. In 2004, California became the disease epicenter with 540 confirmed equine clinical cases of WNV. WNV is now endemic in California.

The following chart summarizes the annual confirmed equine WNV clinical cases and the numbers of horses that died or were humanely euthanized in California since 2003.

Confirmed Equine WNV Case Summary Table
Year Confirmed Cases Died or Euthanized
2020 20 5
2019 15 3
2018 11 6
2017 21 8
2016 21 7


19 5
2014 15 2
2013* 8 4
2012 22 8
2011 15 4
2010 19 5
2009 18 7
2008 32 17
2007 28 14
2006 58 24
2005 456 200
2004 540 229
2003 1 0
Totals 1299 543
* Due to budget cuts, during the 2013 calendar year only, equine WNV cases were not confirmed after August 30, 2013.