West Nile Virus

West Nile Virus Information for Horse Owners

West Nile Virus (WNV) is a virus that can cause a potentially fatal encephalomyelitis (inflammation of the brain and spinal cord) in humans, horses, and members of the corvid family (crows, ravens, magpies, blue and gray jays). The main reservoir for the virus is thought to be wild waterfowl; although these birds do not show symptoms of the disease. The WNV is spread from bird to bird, and to other animals, by the bites of mosquitoes. The horse is a dead-end host for WNV - the virus does not spread from horse to horse, or to other animals, including humans.

WNV first appeared in North America in 1999 and spread steadily across the continent. The virus was detected in birds in Manitoba in early July 2002, and now is found throughout Manitoba. Cases of WNV in horses occur in most years in unvaccinated horses.  

What to Look for in Your Horse

Horses and other members of the equine species become infected following being bitten by a mosquito carrying WNV. The virus enters the bloodstream and quickly spreads to the brain and / or spinal cord causing inflammation or swelling. Symptoms include listlessness, depression, loss of appetite, stumbling and incoordination, weakness of limbs, muscle twitching, partial paralysis, inability to swallow, head pressing, recumbency, convulsions and death. Fever may or may not be present. Clinical signs of disease typically present within three to 15 days of exposure. In severe cases the symptoms will progress from the very early, mild symptoms through to recumbency and death in a period of five to seven days.

These symptoms are easily confused with other nervous system disorders in the horse. Diseases to be considered when making a diagnosis include rabies, sleeping sickness, equine viral rhinopneumonitis, equine protozoal myeloencephalitis, and tetanus.

All ages of horses are susceptible to the disease, although very young or very old horses, or horses already compromised by some other disease condition, are more likely to be affected.


When a horse owner identifies abnormal behavior or any of the symptoms previously outlined, a veterinarian should be consulted. Neurological diseases, including rabies, cannot be differentiated on clinical signs alone.  A variety of tests are available to make a diagnosis of WNV infection or the other aforementioned differential diseases. These tests most typically involve taking a blood sample and then identifying the virus, viral antigens, viral genetic material, or antibodies produced by the horse in response to WNV infection.

Treatment Options 

WNV can be a serious disease, with a case fatality rate approaching 35 - 40 %. Horses that become recumbent have a greater risk of dying or requiring humane euthanasia than infected horses that remain standing during the course of disease. Many infected horses will recover completely; however, some horses (approximately 40%) may experience residual clinical signs. Caution must be used around horses that continue to exhibit neurological deficits after recovering from West Nile virus. Horses that stumble can accidentally injure their caregivers and should not be ridden. Treatment is primarily supportive, and may include fluid therapy, non-steroidal anti inflammatory drugs, diuretics, and excellent nursing care. Approximately two-thirds of affected horses recover from infection and when symptoms are identified early and supportive therapy is instituted, the success rate can be improved.


Since there is no cure for WNV, only supportive therapy, prevention is key to minimizing the chances of horses becoming infected with the virus. Current preventative measures include vaccination, management strategies to reduce exposure to mosquitoes, and ensuring your horse is in optimal health.

Consult with your veterinarian to set up a health program for your horse. WNV is only one of many diseases to be considered in a vaccination program. Strategies to limit exposure to mosquitoes include stabling horses at night in a barn with mosquito netting in place or the use of fans to maintain air movement. Incorporating the use of insect sprays and repellants will reduce the number of mosquito bites. Removal of tall vegetation around corrals and barns and get rid of all sources of standing, stagnant water will reduce mosquito habitat.  

Final Comments

West Nile Virus is a disease that is now endemic to our region. It can be a serious, fatal disease in the horse. The risk of your horse contracting WNV is extremely low if your horse is properly immunized. Consult with your veterinarian on vaccination strategies for your horse and if you see signs of a central nervous system disorder in your horse, call for veterinary assistance to rule out diseases such as rabies, and to institute a treatment regimen.

Additional Resources


For more information, or if you suspect any animal health related concerns, please contact the Chief Veterinary Office or call 204-945-7663 in Winnipeg.