A 10-month-old Paint filly in Sacramento County, California, tested positive for West Nile virus (WNV) on Oct. 14. The filly, who was not vaccinated, showed neurologic signs starting on Oct. 7. Fortunately, she is currently alive and recovering.
This case marks the 17th 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 a nonprofit organization supported by industry donations to provide accessible infectious disease information.
WNV 101
West Nile virus is transmitted to horses through bites from infected mosquitoes. While not all infected horses display clinical signs, those that do may show symptoms such as:
- Flulike signs, including mild anorexia and depression;
- Muscle and skin twitching;
- Hypersensitivity to touch and sound;
- Changes in mental activity;
- Drowsiness;
- Propulsive walking;
- Spinal weakness;
- Ataxia.
There is no cure for West Nile virus in horses, but some can recover with supportive care. Mortality rates can range from 30-40%.
Research has shown that vaccines are effective in preventing WNV. Horses previously vaccinated should receive an annual booster shot, while unvaccinated horses may need a two-shot series within a few weeks. It takes time for horses to develop immunity after vaccination.
Apart from vaccinations, owners can reduce mosquito exposure for their horses by eliminating breeding grounds and using approved repellents.