California Confirms Its 11th Equine WNV Case

California Department of Food and Agriculture (CDFA) officials have confirmed four horses with West Nile virus (WNV) as of Sept. 14.

On Aug. 23, a yearling Quarter Horse gelding in Stanislaus County began showing clinical signs that included mild fever, ataxia (loss of muscle control), and falling over. He had an unknown vaccination status and was confirmed positive on Sept. 3. He is affected and alive.

That day, a 3-year-old Buckskin gelding in San Bernardino County began experiencing ataxia. He was confirmed positive on Sept. 10 and is affected and alive. His vaccination status is unknown.

On Sept. 7, an unvaccinated 13-year-old Mustang stallion from Butte County began exhibiting recumbency (down and unable to rise), weakness, and muscle fasciculations (twitching) in his shoulder. He was confirmed positive on Sept. 10 and was euthanized.

The most recent case began showing signs on Sept. 6. The unvaccinated 5-year-old Friesian mare from San Joaquin County exhibited lethargy, decreased appetite, mild ataxia, difficulty rising, and delayed menace reflex (blink response). She was confirmed positive on Sept. 14 and is reported as affected and alive.

These cases bring the state’s total of confirmed WNV cases in 2020 to 11. Affected counties include:

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

According to a CDFA statement, seven of the 11 horses were unvaccinated, and four had unknown vaccination histories. Eight survived, one died, and two were euthanized.

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.