On Sept. 16, officials at the California Department of Food and Agriculture (CDFA) confirmed a 2-year-old Andalusian filly in San Joaquin County with West Nile virus (WNV). The filly, which presented with clinical signs that included ataxia (incoordination), hyper-reactivity, muzzle fasciculations (twitching), and falling to her knees, was unvaccinated for WNV. She is reported as alive.

The filly marks the state’s eighth confirmed equine case of WNV in 2021. Positive horses have  resided in Fresno County (2), Kings County (1), Merced County (1), Sacramento County (2), and San Joaquin County (2).

Five of the affected horses were unvaccinated for WNV, two had unknown vaccination histories, and one was vaccinated.

Seven of the horses have survived, and one died. CDFA regularly monitors neurologic cases for the presence of WNV in California.

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.