wnv in california horses

California public health officials are encouraging owners to vaccinated horses against West Nile virus (WNV), a mosquito-borne illness transmitted to animals and people via a bite from an infected mosquito. In 2018, there were 11 confirmed cases of WNV in California horses. Six of the affected horses (54.5%) died or were euthanized.

Butte County Public Health and the Butte County Mosquito and Vector Control District announced May 29 that they’re concerned about an increase in the number of mosquitoes capable of transmitting WNV in Butte County due to late-season rainstorms and more mosquito breeding sites in the Camp Fire burn zone.

In Butte County, WNV season runs June through October, the groups said.

“Vaccination is a safe, inexpensive, and very effective way to prevent WNV, a disease that results in death or euthanasia in one-third of affected horses,” said Dr. Linda Lewis, Veterinarian and Epidemiologist at Butte County Public Health. “With the potential for more mosquitoes this year, horse owners are urged to vaccinate.”

wnv in california horses

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:

  • Flulike signs, where the horse seems mildly anorexic and depressed;
  • Fine and coarse muscle and skin fasciculation (involuntary 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. It takes several weeks for horses to develop protection against the disease following complete vaccination or booster administration.

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.