The Michigan Department of Agriculture and Rural Development received confirmation of the state’s first equine West Nile virus (WNV) case of the year on Oct. 10.

“The affected horse is a 5-month-old filly in Calhoun County,” the department said in a statement on its website. “On Oct. 2, the filly began to develop signs of illness, including stumbling and lethargy. The unvaccinated filly is responding to treatment. The dam was vaccinated against WNV two weeks after foaling, and the filly is not yet weaned.”

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 fasciculations (twitching); 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%.

Horse owners should also consult their private practicing veterinarian to determine an appropriate disease prevention plan for their horses. Vaccines have proven to be a very effective prevention tool. Horses that have been vaccinated in past years will need an annual booster shot. 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