Mass. State Officials: Vaccinate Horses Against WNV, EEE

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

Annual vaccinations should be administered during the spring to ensure animals are protected prior to the peak arboviral season beginning in late July and August (in Massachusetts). 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.

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. The fatality rate for EEE in horses is 75-95%. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care. Horses that survive might have long-lasting impairments and neurologic problems. 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.

Clinical signs for WNV include flu-like signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (twitching); hyperesthesia, or 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 (incoordination on one or both sides, respectively). Equine mortality rate can be as high as 30-40%.

Other species such as pheasants, emus, llamas, and alpacas can be affected as well. Owners should consult with their veterinarian regarding other at-risk species. One equine case of WNV and one equine case of EEE were confirmed in Massachuestts in 2011.

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 may provide mosquito breeding habitat and should be cleaned periodically during the summer months to reduce mosquitoes near paddock areas. 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.

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

The State Reclamation and Mosquito Control Board within the Massachusetts Department of Agricultural Resources oversee mosquito control in Massachusetts. For more information call 617/626-1777 or visit

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