California’s 15th Case of WNV Confirmed

On Oct. 5, California Department of Food and Agriculture (CDFA) officials confirmed a San Joaquin County horse with West Nile virus (WNV). The unvaccinated 6-year-old Quarter Horse mare became ill on Sept. 27, showing neurologic signs consisting of agitation, falling over, muscle fasciculations (twitching), recumbency (down and unable to rise), and grimacing. The horse was subsequently euthanized due to the severity of her clinical signs.

Other California horses that have been confirmed with WNV resided in these counties:

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

Eleven horses survived, one died, and three were euthanized. Nine were unvaccinated, four had unknown vaccination histories, and two were vaccinated.

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 (incoordination).

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.