After nearly two months of drought conditions, mosquitoes have wasted little time hatching after recent rains, which elevates the need for horse owners to vaccinate their animals against Eastern equine encephalitis (EEE) and West Nile virus (WNV), two mosquito-borne diseases that together have stricken hundreds of horses in Wisconsin since 2001.

"The drought we have experienced in the last couple of months may have given us a false sense of security when it comes to mosquitoes," says State Veterinarian Robert Ehlenfeldt, DVM, of the Wisconsin Department of Agriculture, Trade, and Consumer Protection. "Cases have now emerged in Illinois and Michigan, demonstrating that the viruses may be ready to re-emerge in Wisconsin."

Horses require two doses of the vaccination initially, and then boosters at least annually. "We don’t recommend administering the vaccinations yourself," Ehlenfeldt says. "Work with your veterinarian, so you get the best formulation for your horse and advice about additional boosters later in the season."

Clinical signs for WNV include 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 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%