On Oct. 6, Idaho Department of Agriculture (IDA) officials confirmed a 7-year-old Quarter Horse mare at a private facility in Gooding County with West Nile virus (WNV). The mare first showed clinical signs consistent with WNV on Sept. 15. Signs included dragging her feet, stumbling, hyperactivity, and being progressively unstable for two to three weeks. The mare was vaccinated two weeks prior to the onset of her clinical signs but not current on the WNV vaccine prior to that. She is reported as recovering.

Fairfield Bain, DVM, MBA, Dipl. ACVIM, ACVP, ACVECC, of Merck Animal Health, notes that horses cannot contract the disease from commercially available WNV vaccines, which use killed virus. Additionally, he said, initial WNV vaccination requires a two-shot series, with peak antibody response occurring between 14 and 21 days following the booster (second) dose.

“Most (WNV) vaccines on the market are very protective when given according to the manufacturers’ direction—meaning primary and booster dose to allow for the maximal protective immune response,” Bain said.

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.