On August 28, a 2-year-old Quarter Horse filly in Contra Costa County, California, was diagnosed with West Nile virus (WNV). The filly, who had not been vaccinated, displayed symptoms such as ataxia, muscle fasciculations, and hypersensitivity. Fortunately, she is currently on the path to recovery.
This case marks the eighth confirmed equine WNV case in California for the year 2026.
EDCC Health Watch is a program by Equine Network that utilizes information from the Equine Disease Communication Center (EDCC) to provide verified equine disease reports. The EDCC is a nonprofit organization supported by industry donations to offer accessible infectious disease information.
WNV 101
Horses can contract West Nile virus through mosquito bites. While not all infected horses show symptoms, those that do may exhibit signs such as flu-like symptoms, muscle twitching, hypersensitivity, changes in behavior, and weakness.
- 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.
There is no cure for West Nile virus in horses, but supportive care can help some recover. Mortality rates for equines can be as high as 30-40%.
Vaccines have been shown to be effective in preventing WNV. Horses need an annual booster shot if previously vaccinated, while unvaccinated horses require a two-shot series within a three- to six-week period. It takes time for horses to build immunity after vaccination.
In addition to vaccinations, horse owners should take steps to reduce mosquito populations and exposure by eliminating stagnant water sources, regularly cleaning water containers, keeping animals indoors during peak mosquito activity times, and using approved mosquito repellents for horses.