California Horse Confirmed With WNV

The mare’s vaccination history is unknown.

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San Joaquin County, CA
The 6-year-old Quarter Horse mare in San Joaquin County is reported as affected and alive. | Photo: Wikimedia Commons

On Sept. 7, officials at the California Department of Food and Agriculture (CDFA) confirmed a 6-year-old Quarter Horse mare in San Joaquin County with West Nile virus (WNV). She had presented on Sept. 1 with clinical signs that included teeth grinding, hind limb ataxia, poor appetite, and fasciculations (twitching) in neck and front end.  Her vaccination status is unknown. She is reported as affected and alive.

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.


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