On August 21, a yearling Quarter Horse filly in Stanislaus County, California, was confirmed to have tested positive for West Nile virus (WNV). The filly displayed clinical signs on August 10, including mild ataxia, muscle fasciculations, and a dull mentation. She is currently in recovery.
This case marks the seventh confirmed equine WNV case in California for the year 2026.
EDCC Health Watch is a marketing program by Equine Network that uses information from the Equine Disease Communication Center (EDCC) to create and distribute verified equine disease reports. The EDCC is an independent nonprofit organization supported by industry donations to provide open access to infectious disease information.
WNV 101
West Nile virus is transmitted to horses through bites from infected mosquitoes. While not all infected horses show clinical signs, those that do may exhibit:
- Flu-like symptoms, with mild anorexia and depression;
- Fine and coarse muscle and skin twitching;
- Hypersensitivity to touch and sound;
- Changes in mental activity, appearing as if daydreaming or disoriented;
- Intermittent drowsiness;
- Propulsive walking without control;
- Spinal signs like asymmetrical weakness; and
- Asymmetrical or symmetrical ataxia.
There is no cure for West Nile virus in horses. However, some horses may recover with supportive care, although equine mortality rates can be as high as 30-40%.
Studies have shown that vaccines are effective in preventing WNV. Horses previously vaccinated should receive an annual booster shot, while in areas with prolonged mosquito seasons, veterinarians may recommend two boosters annually. Unvaccinated horses require a two-shot series within three to six weeks. It takes several weeks for horses to build immunity following vaccination.
In addition to vaccinations, owners should reduce mosquito populations and breeding areas and minimize horses’ exposure to mosquitoes by:
- Eliminating stagnant water sources;
- Regularly cleaning and refilling water buckets and troughs;
- Keeping animals indoors during peak insect feeding times;
- Using approved mosquito repellents for horses.