On Sept. 23, officials at the California Department of Food and Agriculture (CDFA) confirmed a 14-year-old Quarter Horse mare from Yuba County with West Nile virus (WNV). The mare, who was unvaccinated for WNV, had presented on Sept. 20 with muzzle and ear fasciculations (twitching), recumbency (down and unable to rise), and phases of hyperexcitability. She 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.