On Oct. 21, a Standardbred stallion in Green Lake County, Wisconsin, tested positive for West Nile virus (WNV). The unvaccinated horse presented with left-hind stiffness and mild ataxia. He is now recovering.EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. TheEDCCis an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.WNV 101West Nile virusis transmitted to horses via bites from infected mosquitoes. Not all infected horses show clinical signs, but those that do can exhibit: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 theyre daydreaming or just not with it;Occasional drowsiness;Propulsive walking (driving or pushing forward, often without control); andSpinal signs, including asymmetrical weakness; andAsymmetrical or symmetrical ataxia.West Nile virus has no cure. However, some horses can recover with supportive care. Equine mortality rates can reach 30-40%.Studies have shown thatvaccines can be effective WNV prevention tools. Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annuallyone in the spring and another in the fallin areas with prolonged mosquito seasons. In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. It takes several weeks for horses to develop protection against the disease following complete vaccination or booster administration.In addition to vaccinations, owners should work to reduce mosquito population and breeding areas and limit horses mosquito exposure by:Removing stagnant water sources;Dumping, cleaning, and refilling water buckets and troughs regularly;Keeping animals inside during insect feeding times (typically early in the morning and evening); andApplying mosquito repellents approved for equine use.