West Nile Virus Confirmed in California Horse
On Sept. 2, California Department of Food and Agriculture (CDFA) officials confirmed a Glenn County horse with West Nile virus (WNV). The 2-year-old Quarter Horse filly began showing neurologic signs of hind-limb dragging, drooping eyelids, a head drop, and stiff hindquarters on Aug. 29. The filly, which had no vaccination history, is affected and alive.

This is California’s seventh confirmed equine case of WNV. Other confirmed cases have resided in the following counties: Amador (1), Merced (1), Riverside (2), San Joaquin (1), and Stanislaus (1). Five were unvaccinated, and two had unknown vaccination histories. Five have survived, one died, and one 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.