As a result of heavy rains and increased mosquito populations, the Maryland Department of Agriculture is strongly encouraging horse owners to make sure their equids are vaccinated against West Nile virus and Eastern equine encephalomyelitis, two potentially fatal diseases spread by mosquitoes.
“With all of the rain the region has experienced and with mosquito season now in full swing, it is important that horse owners are being proactive and vaccinating their equines against mosquito-borne diseases,” said Maryland State Veterinarian Michael Radebaugh, VMD. “We urge horse owners to speak with their veterinarians about vaccinations, and also remind veterinarians throughout the state of Maryland to report any cases of equine arboviruses to the department.”
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 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%.
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, which are typically early in the morning and evening, and applying mosquito repellents approved for equine use.