Two recent cases of equine West Nile virus (WNV) have been reported to the Equine Disease Communication Center.
In Shasta County, California, a 2-year-old Quarter Horse colt, who was not vaccinated, tested positive for WNV on Sept. 15. The horse showed hind-end ataxia and recumbency but is currently on the path to recovery. This marks the 13th confirmed equine WNV case in California for the year 2026.
In Minidoka County, Idaho, a horse at a veterinary hospital also tested positive for WNV. Further details about this case are not yet available.
EDCC Health Watch is a marketing program by Equine Network that utilizes information provided by the Equine Disease Communication Center (EDCC) to create and distribute verified reports on equine diseases. The EDCC is an independent nonprofit organization supported by industry donations to provide accessible information on infectious diseases.
WNV 101
West Nile virus is transmitted to horses through bites from infected mosquitoes. While not all infected horses show symptoms, those that do may exhibit:
- Flu-like signs, with mild anorexia and depression;
- Fine and coarse muscle and skin twitching;
- Hypersensitivity to touch and sound;
- Changes in mental activity, such as appearing drowsy or disoriented;
- Propulsive walking without control;
- Spinal signs like weakness or ataxia.
There is no cure for West Nile virus in horses, but supportive care can help some recover. Mortality rates among infected equines can be as high as 30-40%.
Research has shown that vaccines are effective in preventing WNV. Horses previously vaccinated need an annual booster, while unvaccinated horses may require a two-shot series within a few weeks. It takes time for horses to develop immunity after vaccination.
Apart from vaccination, owners can reduce the risk of WNV by managing mosquito populations and exposure to mosquitoes by:
- Eliminating stagnant water sources;
- Cleaning and refilling water containers regularly;
- Keeping horses indoors during peak mosquito hours;
- Using approved mosquito repellents on horses.