wnv in california

California animal health officials have confirmed another equine case of West Nile virus (WNV). So far this year, there have been 11 confirmed cases of WNV in California horses.

“On Nov. 2, 2018, a 1-year-old Grade mare in Sacramento County with unknown vaccination history, displaying neurologic signs was confirmed positive for West Nile virus,” the California Department of Food and Agriculture (CDFA) said in a statement on its website. “The mare was euthanized due to severity of clinical signs.”

The CDFA said:

  • The positive horses were located in Amador, Kern, Merced (2), Placer, Sacramento (3), San Joaquin, Shasta, and Stanislaus counties;
  • Five horses were unvaccinated, three had an unknown vaccine history, and three were vaccinated; and
  • Six horses died or were euthanized and five horses are alive.

“(The) CDFA urges horse owners to consult their veterinarian concerning a WNV vaccination program to ensure maximum protection of their horses,” the agency said.

In 2017 there were 21 confirmed cases of WNV in California horses, according to USDA Animal and Plant Health Inspection Service data.

WNV 101

West Nile virus is transmitted to horses via bites from infected mosquitoes. Not all infected horses show clinical signs, but those that do can exhibit:

wnv in california
  • Flulike signs, where the horse seems mildly anorexic and depressed;
  • Fine and coarse muscle and skin fasciculation (twitching);
  • 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 (incoordination).

West Nile has no cure, however some horses can recover with supportive care. Equine mortality rates can reach 30-40%.

Studies have shown that vaccines can be effective WNV prevention tools. Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons. In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. Full immunity takes several weeks to achieve.

In addition to vaccinations, owners should work to reduce mosquito population and breeding areas and limit horses’ mosquito exposure by:

  • Removing stagnant water sources;
  • Dumping, cleaning, and refilling water buckets and troughs regularly;
  • Keeping animals inside during the bugs’ feeding times (typically early in the morning and evening); and
  • Applying mosquito repellents approved for equine use.