Eastern and Western Equine Encephalomyelitis

EEE, WEE and VEE (Venezuelan) are the most commonly recognized viruses causing encephalitis in horses in the Western Hemisphere.  While WEE is primarily found on the western side of North America and EEE on the eastern side, there is a significant amount of overlap, hence why we vaccinate for both strains.  Natural reservoirs are birds and small mammals which harbor the disease, mosquitoes are the vector that transmits it to horses and people.  While the mosquito vector is the most important for transmission, it can be transmitted from horse to horse as well.

Humans can be infected with EEE and the mortality rate is 50-75% in those infected.  Symptoms include headache, encephalitis, altered state of consciousness and seizures.  Fatality from WEE is less common (5-15% mortality rate).

Clinical signs are more extreme in unvaccinated animals and develop within 1-3 weeks of exposure in experimental studies.  These signs include fever, anorexia, depression and stiffness.  While signs of WEE often do not progress beyond this point, animals affected with EEE will often develop symptoms of neurologic disease ranging from depression and proprioceptive deficits to aggression, hyperesthesia, propulsive walking and excitability.  As the disease progresses, animals may head press, shows signs of blindness, develop a head tilt or begin circling.  Other neurologic signs, such as paralysis of the larynx, pharynx and tongue may become apparent as well.

Treatment is supportive care but once neurologic signs develop, mortality rate is 75-100% for horses affected with EEE and approximately 20% for WEE.  For horses that survive, most have permanent neurologic deficits.


Prevention is key!  Vaccination is very effective and horses develop a significant titer within 3 days of vaccination.  Consider vaccinating twice yearly in the face of an outbreak or in endemic areas.  Use mosquito repellents, fly sheets and masks whenever possible.



Bertone, J., Viral Encephalitis in: Equine Internal Medicine. 3rd Ed. Saunders 2010. Pps.624-627.