Annual vaccination can help protect horses against mosquito-borne diseases, including EEE, WEE, and WNV. | Photo: Kevin Thompson/The Horse

Horses face high mortality rates from core diseases (those that can infect any horse of any age), including Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE) and West Nile virus (WNV). And, because all three diseases are transmitted via bites from infected mosquitoes, horses are at the greatest risk when these insects are most prevalent—late spring through fall in the United States. Risk management is critical for horse owners during this time.

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.

Clinical signs for WNV, also transmitted by bites from infected mosquitoes, include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; hyperesthesia; 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%