VEE is a rare disease that is spread to horses and humans by mosquitos. VEE is caused by the venezuela equine encephalitis virus, an arbovirus.
Arbovirus is short for arthropod-borne virus.
In North America, arboviruses are spread mainly by infected mosquitoes. Rodents 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 venezuela equine encephalitis virus has a complex life cycle involving rodents and mosquitos. The endemic virus is rarely able to cause mild disease in humans, and doesn't cause disease in horses. When an epizootic strain emerges, by mutation of the enzootic strain, horses become the main source of the strain to infect mosquitos.