The Equine Disease Communication Center (EDCC) reported Aug. 27 that two horses in Maricopa County, Arizona, have tested positive for West Nile virus (WNV). These are the first confirmed cases of WNV in Arizona horses in 2018.
The Arizona Department of Agriculture (ADA) confirmed the first case on Aug. 23.
“On Aug. 17, the 18-month-old filly had acute onset of neurologic signs,” the EDCC said. “The horse had moved with other horses from Wyoming on Aug. 14. No other horses on the property are showing symptoms and this horse is responding to treatment.”
The ADA confirmed the second case the next day.
“The 18-month old (Quarter Horse) gelding had acute onset of neurologic signs,” the EDCC said. “No other horses on the property are showing symptoms and this horse is responding to treatment.”
West Nile virus is 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 (twitching);
- Hyperesthesia (hypersensitivity to touch and sound);
- Changes in mentation (mentality), when horses look like they’re daydreaming or “just not with it”;
- Occasional drowsiness;
- Propulsive walking (driving or pushing forward, often without control); and
- Spinal signs, including asymmetrical weakness; and
- Asymmetrical or symmetrical ataxia (incoordination).
West Nile has no cure, however some horses can recover with supportive care. Equine mortality rates can reach 30-40%.
Studies have shown that vaccines can be effective WNV prevention tools. Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons. In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. Full immunity takes several weeks to achieve.
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 the bugs’ feeding times (typically early in the morning and evening); and
- Applying mosquito repellents approved for equine use.