On Aug. 26, officials at the California Department of Food and Agriculture confirmed a yearling Quarter Horse colt at a private facility in Merced County with West Nile virus (WNV). The colt, which succumbed to the disease, had not been vaccinated for WNV. He presented on Aug. 16 with clinical signs that included twitching, nervousness, and acute recumbency (down and unable to rise).

This is California’s fourth confirmed equine case of WNV this year. Additional cases were confirmed in Fresno County (2) and Sacramento County (1). Three were unvaccinated for WNV, one had unknown vaccine history, and one was vaccinated. The three horses in other counties survived.

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.