WNV in Montana Horses

Montana Department of Livestock officials confirmed the state’s first reported cases of equine West Nile virus (WNV) state for 2018. They’ve confirmed WNV in Montana horses residing in Musselshell and Lake Counties.

This follows WNV detection in mosquito surveillance pools in Cascade, Hill, and Lewis and Clark Counties. Montana typically sees WNV cases through late summer and into fall.

WNV 101

Not all infected horses show clinical signs, but those that do can exhibit:

  • Flulike signs, where the horse seems mildly anorexic and depressed;
  • Fine and coarse muscle and skin fasciculation;
  • Hyperesthesia (hypersensitivity to touch and sound);
  • Changes in mentation (mentality), when horses look like they’re daydreaming or “just not with it”;
  • Occasional drowsiness;
  • Propulsive walking (driving or pushing forward, often without control); and
  • Spinal signs, including asymmetrical weakness; and
  • Asymmetrical or symmetrical ataxia (incoordination).

Equine mortality rates can reach 30-40%.

“There is no direct treatment for the virus in horses, but vaccination is highly effective in preventing disease,” said Tahnee Szymanski, DVM, Montana’s assistant state veterinarian. “Horses that are vaccinated rarely die or are euthanized because of the disease.

“Vaccination is typically administered in the spring but may offer some protection even this late in the season,” she added. “Work with your veterinarian to determine if your horse could still benefit from vaccination.”

Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons. Previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period.

In addition to vaccinations, owners should work to reduce mosquito breeding areas and limit horses’ mosquito exposure by removing stagnant water sources; dumping, cleaning, and refilling water buckets regularly; keeping animals inside during the bugs’ feeding times (typically early in the morning and evening); and applying mosquito repellents approved for equine use.