West Nile Virus Confirmed in California Horse

On Sept. 30, officials at the California Department of Food and Agriculture (CDFA) confirmed an unvaccinated 2-year-old pony colt in Nevada County as positive with West Nile virus (WNV). The colt, which is reported as affected and alive, experienced onset of neurologic signs that included ataxia (loss of muscle control) and recumbency (down and unable to rise) on Sept. 25.

According to a CDFA statement, the colt marks the state’s 14th confirmed equine case this year. The positive horses were located in the following counties:

  • Amador (1)
  • Butte (1)
  • Glenn (1)
  • Kings (1)
  • Merced (1)
  • Nevada (1)
  • Riverside (2)
  • San Bernardino (1)
  • San Joaquin (2)
  • Stanislaus (3)

Of those horses affected, eight were unvaccinated, four had unknown vaccination histories, and two were vaccinated. Eleven horses remain alive. One horse died and two were 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 (twitching);
  • 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.