On Aug. 30, officials at the California Department of Agriculture (CDFA) confirmed a 2-year-old filly at a Kings County training facility with West Nile virus (WNV). The filly, which was unvaccinated for WNV, experienced onset of clinical signs on Aug. 19. Signs included fever, inappetance, stumbling, ataxia (incoordination), and increased lung sounds. She is reported as alive.

According to a CDFA statement, the filly is California’s fifth confirmed equine case of WNV in 2021. Previous cases resided in Fresno County (2), Merced County (1), and Sacramento County (1). Four of the horses were unvaccinated and one was vaccinated. Four horses survived and one died.

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.