On September 25, a 5-year-old Quarter Horse mare in Merced County, California, was confirmed positive for West Nile virus (WNV). The horse, which was not vaccinated, developed clinical signs on September 20, including facial twitching and ataxia. He is now recovering.
EDCC Health Watch is an Equine Network marketing program that uses information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. the EDCC is an independent nonprofit organization supported by industry donations to provide open access to infectious disease information.
West Nile virus is transmitted to horses through bites from infected mosquitoes. Not all infected horses show clinical signs, but infected ones may show:
- Flu-like signs, where the horse seems somewhat anorexic and depressed;
- Fine and coarse muscle and skin fasciculation (involuntary twitching);
- Hyperesthesia (hypersensitivity to touch and sound);
- Cognitive changes (mental activity), when horses seem to be dreaming or “just not”;
- Occasional drowsiness;
- Impulsive walking (driving or pushing forward, often without control); and
- Spinal symptoms, including asymmetrical weakness; and
- Asymmetrical or symmetrical ataxia.
There is no cure for West Nile virus. However, some horses can recover with supportive care. The mortality rate of horses can reach 30-40%.
Studies have shown that vaccines can be effective tools to prevent WNV. Horses vaccinated in previous years need an annual booster shot, but veterinarians may recommend two boosters each year—one in the spring and one in the fall—in areas with a long mosquito season. In contrast, previously unvaccinated horses require a two-shot vaccination series over a three- to six-week period. It takes several weeks for horses to develop protection against the disease after complete vaccination or booster administration.
In addition to vaccinations, owners should work to reduce mosquito populations and breeding sites and limit mosquito exposure to horses by:
- Removal of stagnant water sources;
- Regular disposal, cleaning and refilling of water buckets and troughs;
- Keeping animals indoors during insect feeding (usually early morning and evening); and
- Apply mosquito repellents approved for use on horses.