In South-Central Oregon’s Klamath County veterinarians confirmed a 4-year-old Quarter Horse mare and a yearling Quarter Horse filly with the disease. The filly experienced onset of clinical signs on Aug. 22. She was confirmed positive on Aug. 27. Her vaccination status was not reported; she is recovering. The 4-year-old mare began showing unspecified neurologic signs on Sept. 1 and was confirmed positive on Sept. 8. She was euthanized.

The horse in Malheur County, located in far Southeastern Oregon, was an unvaccinated 15-year-old Quarter Horse gelding at a private facility. He showed neurologic signs and was confirmed positive on Aug. 20. He was euthanized.

The horse in Baker County, located just north of Malheur County, was a 4-year-old Quarter Horse gelding. He experienced onset of clinical signs on Aug. 27 and was confirmed Sept. 3. His vaccination and health status were not reported.

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.