The Nevada Department of Agriculture (NDA) has confirmed three horses tested positive for West Nile virus (WNV) in the western part of the state in the last two weeks. None of the three horses were vaccinated.

“Vaccination is the best protection horse owners have for their animals,” JJ Goicoechea, DVM, the NDA’s state veterinarian, said. “With the increased numbers of mosquitoes this year, it’s important all horse owners take this precaution to prevent the spread of disease.”

West Nile is transmitted to horses via bites from infected mosquitoes. Clinical signs of WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; hyperesthesia (hypersensitivity to touch and sound); changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.

Horses that have been vaccinated in past years will need an annual booster shot; in areas with a prolonged mosquito season, veterinarians might recommend two boosters annually—one in the spring and another in the fall. However, if an owner did not vaccinate their animal in previous years, the horse will need the two-shot vaccination series within a three- to six-week period