Equine Encephalomyelitis

Equine Encephalomyelitis, infectious disease observed originally in horses and subsequently in humans, other mammals, and birds. A type of encephalitis occurring epidemically, it is caused by a virus that attacks the central nervous system. The disease is transmitted by mosquitoes and possibly by other bloodsucking insects. Three strains of the causative virus have been identified. Two of the strains are confined to North America; one causes a rare but severe type of the disease common to the east, and the other a milder type common to the west. The third strain is responsible for a type that occurs in Venezuela. Each type is characterized by inflammation of the brain and spinal cord, with high fever, headache, muscle pains, and sometimes convulsions as the earliest symptoms. Within one or two days after onset of the disease, somnolence develops, often progressing to coma.

The North American eastern type of the disease causes death in about 90 percent of infected horses. In humans the disease is generally not fatal, although a number of deaths occurred in epidemics of the disease in 1956 in Massachusetts and in 1959 in New Jersey. Severe cases of the disease are most common among children under 15 and adults over 55.

Equine encephalomyelitis can be controlled by annual vaccination of horses and mules before the advent of summer. An antiserum vaccine administered in the early stages of the disease is sometimes effective in the treatment of infected animals. The disease in humans is known popularly as sleeping sickness. A vaccine made from infected chick embryos gives temporary immunity to the disease.