A third Massachusetts horse has died after being infected with Eastern equine encephalitis (EEE). On Aug. 13 a 3-year-old horse from Lancaster, located in Worcester County, died after developing clinical signs of the illness the previous day.

On Aug. 1 another Worcester County horse died after being diagnosed with the disease, and a Middleborough colt was euthanized on July 21 after developing severe clinical signs of EEE the day prior.

"The most common symptoms of infection with Eastern equine encephalitis are fever, depression, lack of appetite, and neurological signs," said J. Fred Nostrant, DVM, MS, an equine veterinarian practicing in Worcester County. "These neurological signs can range from mild ataxia (incoordination) and depression to circling, head tilting, and aggressive behavior."

EEE is a mosquito-borne illness that affects both horses and humans. It is transmitted by the bite of an infected mosquito. For horses, the disease is often fatal once contracted.

"Unfortunately complete recovery from EEE once the neurological signs are seen is very rare," Nostrant said. "Mortality rates for horses showing neurological signs due to EEE infection are 75-100%, and horses that do survive often have permanent neurological problems."

Elsewhere, two horses in Brooks Country, Ga., tested positive for EEE earlier this year, and more than 69 horses have tested positive for the disease in Florida. Both Virginia and Michigan are also seeing high numbers of horses with positive EEE tests as well.

"EEE is most ofte