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West Nile Virus in Horses

West Nile Virus

West Nile Virus is a mosquito-borne virus that can cause inflammation or swelling of the brain and spinal cord in horses, birds and humans. The virus first appeared in 1937 in Uganda. In 1999 it made its way to the United States, and has become established across North America. The first equine case of West Nile Virus in Saskatchewan occurred in 2002.

There is no evidence that West Nile Virus can be spread from birds to humans, birds to horses, or from horse to horse or horse to human. Only certain species of mosquitoes spread the disease. These mosquitoes become infected as they feed on infected wild birds such as crows, blue jays, magpies and ravens. The prevalence of West Nile Virus varies from year to year. The risk of humans or horses becoming infected tends to peak in Saskatchewan during July, August and early September, when standing water and other mosquito larva habitats are present, and following extended periods of warm weather.

West Nile Virus in Horses

In horses, West Nile Virus causes an encephalomyelitis, or brain and spinal cord infection. Most horses bitten by a mosquito infected with West Nile Virus will not develop clinical disease. They develop an asymptomatic infection, eliminate the virus and are none the worse for it. For those that do become sick, clinical signs may include, but are not limited to:

  • Listlessness
  • Weakness of limbs
  • Inability to swallow
  • Loss of appetite
  • Depression
  • Muscle twitching
  • Inability to stand
  • Stumbling
  • Lack of coordination
  • Fever
  • Death

These signs may be confused with other nervous system disorders in horses such as rabies, sleeping sickness, equine herpes virus and tetanus. There is no specific treatment for horses affected with West Nile Virus. Just over one third of horses showing clinical disease may die or have to be euthanized because of complications. Sometimes horses that do recover may still exhibit permanent neurological symptoms.

Since West Nile Virus is transmitted by mosquitoes, insect control is important. The species of mosquito responsible for West Nile Virus infection breeds in small, warm, still puddles of water. These puddles of water include those found in poorly drained eaves troughs, bird baths, discarded rubber tires and even hoof prints formed in mud. Removal of stagnant water and tall vegetation as well as the use of insect sprays and repellents are some preventative measures that can be taken. In addition, there are approved vaccines that can protect horses against West Nile Virus infection. Horse owners should contact their veterinarian for information about the vaccines available and to receive recommendations on a disease prevention program.

West Nile Virus in horses is an immediately notifiable disease in Saskatchewan. All laboratory-confirmed cases of must be reported within 24 hours to the office of the provincial Chief Veterinary Officer: Phone 306-787-5547 or 306-798-0253, Fax 306-798-0096 or email

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