The California Department of Food and Agriculture has confirmed that a 2-year-old unvaccinated Friesian in Amador County has tested positive for West Nile virus. The stallion displayed neurologic signs of ataxia and muscle twitching on Sept. 6 and was euthanized due to the severity of the condition. This marks the 11th case of West Nile Virus in a horse in California in 2026.
In Cache County, Utah, officials have confirmed that a 2-year-old vaccinated Mustang used for pleasure riding has also tested positive for WNV. The gelding showed signs of mild muscle twitching on Aug. 31 and began to exhibit increasing muscle fasciculations over the next few days. Treatment with dexamethasone started on Sept. 3, and the horse’s condition has been improving.
West Nile virus is transmitted to horses through infected mosquito bites and can cause a range of clinical signs, including flulike symptoms, muscle twitching, hypersensitivity, changes in mental activity, and weakness. While there is no cure for WNV, some horses can recover with supportive care, although mortality rates can be as high as 30-40%.
Vaccination is an effective way to prevent WNV in horses, with annual booster shots recommended for previously vaccinated horses and a two-shot series for unvaccinated horses. It is also important for owners to reduce mosquito populations and limit their horse’s exposure to mosquitoes by eliminating stagnant water sources, regularly cleaning water buckets and troughs, keeping animals indoors during peak insect feeding times, and using approved mosquito repellents for horses.