California Confirms Its First 2021 Equine WNV Case

The unvaccinated Fresno County mare is reported as recovering.

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California Confirms First 2021 Equine WNV Case
The unvaccinated Fresno County mare is reported as recovering. | Photo: Wikimedia Commons
On June 28, officials at the California Department of Food and Agriculture (CDFA) confirmed a horse in Fresno County with the state’s first case of West Nile virus (WNV) this year.

On June 18, the unvaccinated 4-year-old Quarter Horse mare started showing clinical signs, including fever, ataxia, (incoordination), lethargy, and proprioceptive deficits in her hindend. 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.

Health Alert: West Nile Virus in Horses
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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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