EEE, WNV Confirmed in Massachusetts Horses

EEE, WNV Confirmed in Massachusetts Horses

Both EEE and WNV are viral diseases transmitted to horses by infected mosquitoes.


On Sept. 20 the Massachusetts Department of Public Health (DPH) confirmed the commonwealth's fourth case of Eastern equine encephalitis (EEE) in a horse, stabled in Middleborough in Plymouth County. The DPH also confirmed the first case of West Nile virus (WNV) in a horse this year, in an animal stabled in Dartmouth.

“Infected mosquitoes continue to be present in our environment and will be around until the first hard frost. Everyone needs to continue to take precautions to avoid getting bitten,” said DPH State Public Health Veterinarian Catherine Brown, DVM, MSc. “With cooler nighttime temperatures, mosquitoes tend to be less active. But the next few days in particular are still warm enough for them to be out looking for food during the dusk to dawn hours, and humans are a source of food for them.”

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. Its fatality rate 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.

Similarly, WNV is also a viral disease transmitted to horses by infected mosquitoes. Clinical signs for WNV 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. Equine mortality rate can be as high as 30-40%.

If an animal is diagnosed with WNV or EEE in Massachusetts, owners are required to report to Department of Agriculture Division of Animal Health by calling 617/626-1795 and to the Department of Public Health by calling 617/983-6800.

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