WNV Confirmed in Seven California Counties

Horses in Stanislaus, Kern, Sacramento, Santa Clara, Butte, Napa and Yuba counties are positive for West Nile virus.

Horses in Stanislaus, Kern, Sacramento, Santa Clara, Butte, Napa and Yuba counties, California, are positive for WNV.
Horses in Stanislaus, Kern, Sacramento, Santa Clara, Butte, Napa and Yuba counties, California, are positive for WNV. | Wikimedia Commons

Seven West Nile virus (WNV) cases have been confirmed in California. The cases are located in Stanislaus, Kern, Sacramento, Santa Clara, Butte, Napa and Yuba counties.

In Stanislaus County, a 10-year-old, unvaccinated Quarter Horse gelding was confirmed on August 15 after developing clinical signs on August 6, including ataxia in all four limbs. He became progressively recumbent and unable to rise. He has been euthanized.

In Kern County, a 3-year-old, under-vaccinated Quarter Horse mare was confirmed positive on August 15 after developing clinical signs on August 7, including acute onset of neurological behavior, pelvic limb weakness, falling when backed and neurologic on tail pull. She has been euthanized.

In Sacramento County, an unvaccinated, 2-year-old Quarter Horse colt was confirmed positive on August 15 after developing clinical signs on August 7, including ataxia in all four limbs and weakness in thoracic limbs. He was recumbent and unable to rise without assistance. The horse is currently affected and alive.

In Santa Clara County, a 5-year-old Lusitano mare was confirmed positive on August 17 after developing clinical sings on July 29, including recumbency, weak pelvic limbs, muscle twitching, ataxia in all four limbs, weakness in thoracic limbs and recumbency. The horse is vaccinated and is now recovering.

In Butte County, a 2-year-old Haflinger gelding was confirmed positive on August 17 after developing clinical signs on August 3, including hyperesthesia and facial and muzzle twitching. The gelding is vaccinated and is now recovering.

In Napa County, an under-vaccinated, 4-year-old Quarter Horse mare was confirmed positive on August 17 after developing clinical signs on August 7, including acute recumbency. She has been euthanized.

In Yuba County, an unvaccinated, 2-year-old Paint filly was confirmed positive on August 16 after developing clinical signs on August 13, including lethargy and ataxia in all four limbs. The unvaccinated filly is now recovering.

There have been nine confirmed cases of WNV in California in 2023. 

EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. The EDCC is an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.

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 (mental activity), 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%.

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 insect feeding times (typically early in the morning and evening); and
  • Applying mosquito repellents approved for equine use.
Brought to you by Boehringer Ingelheim, The Art of the Horse
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