The virus reproduces in the body of the mosquito and migrates to the salivary glands. The mosquito can transmit the disease when it bites a horse or human. People and horses are considered dead end hosts because they neither transmit the virus nor serve as reservoirs. About five days post-infection, horses begin to show fever and neurologic signs. These symptoms include extreme depression or drowsiness, paralysis, circling, impaired vision, irregular gait, dysphagia and photosensitivity.
Horses often hang their heads or grind their teeth.
Mildly affected horses my recover over a few weeks but may be left with permanent brain damage. Differential diagnoses for EEE include hepatoencephalopathy, rabies, protozoal myeloencephalitis, verminous encephalitis, botulism, and leukoencephalomalacia. There is no treatment for EEE other than good nursing care and supportive therapy. Human Eastern Equine Encephalitis is a rare but severe neurologic disease. About five human cases are confirmed annually in the United States.