The Lemhi County horse, an 8-year-old mare, first showed clinical signs on Aug. 27. Signs included ataxia (incoordination) and muscle twitching in her cheeks. The mare was last vaccinated several years ago. She is reported as recovering.

The Ada County horse, a yearling gelding, experienced onset of clinical signs on Sept. 3 and was confirmed on Sept. 8. His signs consisted of ataxia, difficulty eating, poor balance, and muscle fasciculations (twitching). He is also recovering. His vaccination status is unknown.

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.