California Officials Confirm a Horse With WNV

On June 26, California Department of Food and Agriculture (CDFA) officials confirmed the state’s first (index) case of West Nile virus (WNV) in an equine in 2020. The horse, a 20-year-old Quarter Horse gelding from Amador County, began experiencing clinical signs on June 19. Signs included ataxia (incoordination and loss of muscle control) and severe neurologic signs. The horse’s vaccination status is unknown, and he is reported as affected and alive.

About West Nile Virus

West Nile virus 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); and
  • 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.