Wisconsin horse diagnosed with West Nile Virus

Wisconsin State Farmer
Veterinarian Wes Schroeder fills his syringe as he prepares to vaccinate yet another horse for the West Nile virus.


A 4-year-old unvaccinated Quarter Horse gelding from St. Croix County is the first reported Wisconsin horse to have become infected and die from West Nile Virus (WNV) this year.  Horse owners are urged to get horses vaccinated to prevent further spread of WNV and another mosquito-borne disease, Eastern equine encephalitis (EEE).

"Horses require two doses of the vaccinations initially, and then boosters at least annually,” says Dr. Julie McGwin, equine program veterinarian for the Wisconsin Department of Agriculture, Trade and Consumer Protection.

"Work with your veterinarian on your horse’s vaccination program, so you get the best formulation for your horse and advice about additional boosters later in the season," McGwin says.

Both WNV and EEE are caused by viruses transmitted by mosquitoes, and both may cause encephalitis, or inflammation of the brain. Both viruses can be fatal to horses.  Symptoms are similar for both diseases: depression, appetite loss, drooping eyelids and lower lip, fever, weakness, twitching, paralysis or lack of coordination, aimless wandering, circling and blindness.

Neither of the viruses is contagious between horses, but can be carried by mosquitos from an avian, or bird, host to horses and humans. While humans may also be infected by both WNV and EEE, the viruses do not pass directly between people and horses.  Mosquitoes biting warm-blooded animals is the only route of transmission.

Besides vaccination, McGwin recommends taking other steps to limit horses' exposure to mosquitoes during warm weather:

  • Remove items from surrounding property that could collect stagnant water such as old tires, tin cans, plastic containers.
  • Keep rain gutters clean and draining properly.
  • Clean and chlorinate swimming pools, outdoor saunas and hot tubs, and drain water from pool covers.
  • Turn wading pools and wheelbarrows upside down when not in use.
  • Empty and replace water in birdbaths at least once a week.
  • Consider keeping horses in the barn from dusk to dawn, when mosquitoes are most active.
  • Discuss using equine mosquito repellents with your veterinarian.