On Aug. 13, Idaho State Department of Agriculture (ISDA) officials confirmed an Idaho horse with West Nile virus (WNV).

The affected horse, a 14-year-old Quarter Horse-cross mare at a private facility in Lemhi County, experienced onset of clinical signs on Aug. 10. Signs included facial paralysis and difficulty standing that progressed to inability to stand. The mare, which was unvaccinated, was transported to a referral hospital. She did not respond to supportive care and was euthanized Aug. 12 at the owner’s request.

About West Nile Virus

WNV transmission occurs when infected mosquitoes feed on animals, as well as humans, after having fed on infected birds.

Clinical signs of WNV in horses include:

• Mild anorexia and depression
• 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);
• Spinal signs, including asymmetrical weakness; and
• Asymmetrical or symmetrical ataxia.

West Nile virus has no cure; however, some horses can recover with supportive care. Equine mortality rates can reach 30-40%. The American Association of Equine Practitioners includes WNV as one of the core diseases all horses should be vaccinated against at least annually.