On Aug. 20, officials at the Oklahoma State Veterinary Office (OSVO) confirmed a 2-year-old Thoroughbred filly at an Oklahoma County racetrack with West Nile virus (WNV). The horse presented with clinical signs of WNV, which included fever, hind-end weakness, loss of appetite, and recumbency (down and unable to rise) on Aug. 12. The mare, who was not vaccinated for WNV, was euthanized.

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.