Ashley Griffin, University of Kentucky

Equine encephalomyelitis, also called “sleeping sickness,” is an infectious disease that affects the brain of the horse. Three strains have been identified: Eastern, Western, and Venezuelan. The mortality of the three strains runs from moderate to high. The Eastern strain occurs more frequently and has the highest mortality rate.

The cause of encephalomyelitis is a virus. Reservoir hosts include birds, reptiles, and rodents. The mosquito acts as the vector, transmitting the virus from the reservoir host to the horse. Therefore, most infections are apparent from midsummer to frost. Although horses can spread the disease to one another by rubbing noses or sharing feed and water containers, they are considered dead-end hosts because they have such a low viral count. It is highly unlikely that other feeding mosquitoes can pick up the virus from them.

Clinical Signs


1. fever and depression, beginning five days after infection
2. diarrhea
3. overall drowsy appearance and no desire to move
4. self-mutilation, hyperexcitability, irritability, and a refusal of food and water
5. central nervous system (CNS) symptoms of incoordination, head pressing, circling, paralysis, convulsions, and coma
6. death two to three days after the first signs appear.




Treatment is mainly supportive and should be administered in a shaded, well-padded area. If symptoms are severe, food and water may need to be forced into and manually evacuated from the animal. If the horse lies down for long periods of time, it will need to be rotated to prevent sores. Mortality is high, and if the horse survives, it is usually considered a “dummy” because of impaired cerebral function.




Immunization and mosquito control are recommended for prevention. The first vaccine should be given prior to mosquito season in the spring or early summer and be followed by a second vaccination in two to four weeks. Immunization lasts six months and needs to be repeated yearly. See the vaccination table at the end of this unit.