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University of Minnesota Extension

West Nile Virus in horses

Quick facts

  • Mosquitoes can infect horses with West Nile Virus (WNV).
  • WNV infection causes muscle tremors, incoordination and sometimes death.
  • Vaccinating your horse, turning them in at dawn and dusk, and using spray repellents can help prevent WNV.

What is West Nile virus?

West Nile Virus (WNV) is a disease-causing virus transmitted to people and horses through the bite of an infected mosquito. With abundant mosquito and bird populations, WNV has established itself in Minnesota. Similar to other mosquito-spread diseases such as Western and Eastern Equine Encephalitis, WNV causes periodic outbreaks of illness in horses.

  • Horses with WNV express neurologic signs and muscle trembling.
  • WNV kills about a third of affected horses. 

How does it spread?

  • WNV is not contagious and can’t pass between horses or humans.
  • Mosquitoes pass WNV from infected birds to horses.

Signs of illness

Signs of WNV depend on the severity of the case. If your horse experiences any of these signs, contact your veterinarian immediately. It’s important to rule out any other neurologic disease.

  • Mild cases show muscle tremors, stumbling, incoordination and weak limbs. Prognosis for horses with mild cases is usually good.
  • Severe cases of WNV often result in death.
  • About 30 percent of severely affected horses go down and can’t get up.

All cases of WNV in horses should be reported to the Minnesota Board of Animal Health at 651-296-2942.


Horses with WNV require veterinary care. Treatment for WNV will vary with the severity of the case.

Some horses may need clinical care while others can receive treatment at home. Usually, horses with WNV are given anti-inflammatory and pain medications to help relieve their symptoms.

Aside from supportive therapy, horses must recover on their own.   

Preventing West Nile in your horse

  • Vaccinate your horse for WNV in the spring.
  • Turn in horses at dusk and dawn when mosquitoes are most actively feeding.
  • Attach and maintain screens on windows and doors to keep mosquitoes out of the barn.
  • Keep your farm free of water-holding containers, where mosquitoes can breed. 
    • Remove old tires.
    • Cover or drill holes in barrels, buckets or tubs stored outside.
    • Turn over wheelbarrows when not in use. 
  • Use sprays on your horse to repel mosquitoes.
  • Use fans to help deter mosquitoes. 

Authors: College of Veterinary Medicine and Julie Wilson, DVM

Reviewed in 2021

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