With the mosquito season approaching, Massachusetts Department of Agricultural Resources (DAR) officials are advising horse owners to take steps to protect animals against mosquito-borne diseases like West Nile virus (WNV) and Eastern equine encephalitis (EEE).

The DAR reminds horse owners that annual vaccines should be administered during this time of year to ensure animals are protected prior to the peak arboviral season, beginning  in Massachusetts in late July and August. 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.

"It is important to stay a step ahead of these diseases and administering vaccinations in a timely manner is the best way to protect our equine population," said DAR Commissioner Greg Watson.

Both WNV and EEE pose serious risks to horses and other equids, sometimes causing neurologic symptoms that can lead to death. There were two confirmed equine WNV cases and four equine EEE cases in Massachusetts in 2013.

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.

Also a viral disease transmitted by infected mosquitoes, WNV's clinical signs 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 drowsiness; propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia (incoordination). Equine mortality rate can be as high as 30-40%.

In addition to vaccination, owners are advised to:

  • Redue potential mosquito breeding sites by eliminating standing water from containers such as buckets, tires and wading pools – especially after heavy rains;

  • Clean water troughs, which can provide mosquito breeding habitat, regularly during the summer months to reduce mosquitoes near paddock areas;

  • Keep horses in indoor stalls during times of peak mosquito activity between dusk and dawn to reduce their risk of exposure to mosquitoes; and 

  • Apply mosquito repellents designed for use on horses.

If an animal is suspected of having WNV or EEE, owners are required to report to DAR Division of Animal Health by calling 617/626-1795 and to the Department of Public Health by calling 617/983-6800.