The Massachusetts Department of Agricultural Resources (MDAR) is advising horse owners to plan “spring shots” with their veterinarians to ensure proper protection from mosquito-borne diseases like West Nile virus (WNV) and Eastern equine encephalitis virus (EEE).
“Getting these vaccines at the right time of year can provide horses protection for the whole mosquito season,” said MDAR Commissioner John Lebeaux. “As West Nile virus and Eastern equine encephalitis virus pose serious risks to horses, I encourage all owners to get their animals vaccinated promptly.”
Annual vaccinations should be administered during this time of year to ensure their animals are protected prior to the peak arboviral season beginning in late July, and to remain protective through the first hard frost. Owners are urged not to wait until positive cases are reported in their area, since it can take several weeks for an animal to be fully protected by a vaccine.
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.
West Nile is also 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. There are no specific treatments for WNV, however supportive care can help horses recover in some cases. Equine mortality rate can be as high as 30-40%.
Vaccines have proven to be a very effective prevention tool for both EEE and WNV. 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 vaccination, owners should reduce potential mosquito breeding sites on their property by eliminating standing water from containers such as buckets, tires, and wading pools, especially after heavy rains. Water troughs might also provide mosquito breeding habitat and should be cleaned periodically during the summer months to reduce mosquitoes in paddocks. Horse owners should consider keeping horses in indoor stalls during times of peak mosquito activity between dusk and dawn to reduce their risk of exposure to mosquitoes. Use of approved repellents should also be considered.