Equine encephalomyelitis, also commonly known as sleeping sickness or sleeping disease is caused by one of several different strains of the encephalomyelitis virus. It can be extremely dangerous and horses, birds and even humans can develop sleeping sickness if infected. The disease is actually transmitted by mosquitoes that bite an infected bird and pick up the virus, then bite a human, horse or other bird. Once a horse or human is infected, he or she cannot pass sleeping sickness on to another horse or person, so there is no need to be concerned about direct transmission to others. The encephalomyelitis lives in the bird species with the most commonly infected birds being ducks, chickens, pheasants, turkeys and quail with pigeons also being carriers of the disease. In birds the virus is not usually fatal, but it can be when transferred to a human or equine host.
The virus can affect the horse in different severities that are often directly related to the horse's immunity and overall health when they are infected. In mild cases the horse may recover in two to three months but in severe cases death is usually seen in a few weeks. The Western variety of encephalomyelitis has between 24- 45% death rate in horses whereas the much more severe Eastern encephalomyelitis has up to a 90% fatality rate.
The symptoms of encephalomyelitis can range in severity from mild lethargy, depressed appetite and sleeping for long periods of time in the standing position with the lower lip hanging open to lying stretched out on the ground and continually moving the legs in a bicycling motion. The coordination of the horse is usually affected as the virus attacks the central nervous system and the horse may stagger, walk into objects, have poor reflexes, grind his or her teeth as well as run a mild to severe fever. As the virus progresses the horse will become unable to swallow and may not be able to move from the prone position. He or she will usually become paralyzed and death will soon follow.
There is no treatment for equine encephalomyelitis once the horse is infected but good nutrition and care can help the horse's immune system produce antibodies to fight the virus. Additional high nutrient feeds, electrolytes, feed supplements and lots of fresh water are essential in management of the infected horse. Keeping the horse free from any and all other health conditions as much possible is also critical while the body is fighting the virus. The horse should not be stressed, worked or trailered for long periods during the treatment. The good news is that yearly spring vaccinations followed by a booster shot in one to two weeks will provide the protection needed to keep the horse safe from the viral infection for the mosquito season. Horses will need to be vaccinated each spring, there is no retained immunity from season to season.