On Sept. 13 the Ohio Department of Agriculture confirmed the first positive cases of West Nile virus (WNV) in Ohio horses for 2017. Multiple cases in different parts of the state have been reported. None of the horses confirmed to have contracted WNV had been vaccinated.

The spread of WNV in horses is preventable with proper vaccination and horse owners are urged to ensure their animal’s vaccinations and boosters are up-to-date.

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West Nile is transmitted to horses via bites from infected mosquitoes. Clinical signs for 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%.

The WNV cases follow the Ohio Department of Agriculture’s report of Eastern equine encephalitis (EEE) in an Ashtabula County horse on Aug. 24. That horse was unvaccinated against EEE.

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A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care; fatality rates reach 75-80% among horses. Horses that survive might have long-lasting impairments and neurologic problems.

“These cases should serve as an alert to all horse owners to vaccinate their animals against West Nile virus and Eastern equine encephalitis,” said State Veterinarian Tony Forshey, DVM. “Vaccines are a proven and effective prevention tool and I encourage all owners to talk to their local vet for options and advice on how to keep their animals healthy.”

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.

In addition to vaccinations, horse owners also need to reduce the mosquito populations and their possible breeding areas. Recommendations include removing stagnant water sources, keeping animals inside during the bugs’ feeding times (typically early in the morning and evening), using fans to disrupt mosquitoes’ flight patterns, and applying mosquito repellents approved for equine use.