Oregon Horse Tests Positive for WNV

The affected county first discovered WNV-infected mosquitoes about Aug. 1.
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Malheur County, OR
On Aug. 27, officials confirmed a 15-year-old Paint mare at a private facility in Malheur County with West Nile virus (WNV). | Photo: Wikimedia Commons
On Aug. 27, officials at the Oregon Department of Agriculture (ODA) confirmed a 15-year-old Paint mare at a private facility in Malheur County with West Nile virus (WNV). The mare first experienced clinical signs of WNV on Aug. 24. Signs included facial and skin fasciculations (twitching), hyperesthetic (extremely sensitive to touch) muzzle, tachycardia (rapid heart beat), and tachypnea (rapid breathing). Her vaccination status is unknown. She is reportedly recovering.

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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