Eastern Equine Encephalitis (EEE), Western Equine Encephalitis (WEE) and Venezuelan Equine Encephalitis (VEE) are all viral diseases in the arbovirus family. All arboviruses are transmitted via a vector (i.e. mosquito), hence they are classified as vector-borne diseases. These viruses cause inflammation of the brain and spinal cord. Horses affected with EEE/WEE are not contagious, as the disease cannot be transmitted from horse to horse. Birds are typically the reservoir host for the virus and act as a source for vector transmission.

Incubation: The incubation period is the time from exposure to the virus to when a horse shows clinical signs of infection. The incubation period for EEE is five to 14 days. The incubation period for WEE ranges from two days to three weeks. The incubation period for VEE is 12 hours to five days.

Transmission: EEE, WEE, and VEE are transmitted via mosquitoes and biting insects that carry the pathogen and then bite a horse.

Clinical Signs: Signs of EEE, WEE, and VEE include depression, fever, lack of appetite, lethargy, and sudden onset of neurologic signs such as ataxia (wobbly gait), proprioception defects, and recumbency.
Diagnostic Testing: EEE, WEE, and VEE are diagnosed with a blood titer testing.

Treatment: There is no cure for EEE, WEE, or VEE. Unvaccinated horses affected with EEE or VEE often die within a few days after initially showing symptoms. Vaccinated horses or horses with WEE may survive with supportive care.

Prevention: Keeping horses up to date on vaccines is the primary method of prevention for EEE, WEE, and VEE. Vector management is also effective and can include the use of insect repellant and eliminating standing water and brush piles.

Prognosis: Most horses are not able to spread EEE amongst themselves, however mortality rates range between 75-90%. Mortality rates for WEE are 20-40%. Mortality rates for VEE are between 50-60%.