EEE is a rare disease that is spread to horses and humans by mosquitos. Eastern equine encephalitis is caused by the eastern equine encephalitis virus, an arbovirus.
Arbovirus is short for arthropod-borne virus.
In North America, arboviruses are spread mainly by infected mosquitoes. Birds are the source of infection for mosquitoes, which can sometimes transmit the infection to horses, cattle, pigs, dogs, emus, ostriches, and, in rare cases, people. The eastern equine encephalitis virus has a complex life cycle involving passerine birds and a specific type of mosquito, Culiseta melanura , that lives in marshes and swamps. These mosquitoes can transmit the virus to animals and people. After infection, the virus invades the central nervous system, including the spinal cord and brain.