In 1931 Meyer, Haring and Howitt1 reported the discovery of a virus as the cause of an epizootic of encephalomyelitis among horses and mules in the San Joaquin Valley of California during the summer of 1930. Since that time the disease has occurred in several Western and Midwestern states. Similar epizootics have occurred during the past seventy years, especially in the West Central states, but have been confused with other equine diseases. In 1933 the disease appeared in Maryland, Delaware, Virginia and New Jersey. In this year Ten Broeck and Merrill2 and Giltner and Shahan3 reported the isolation of an eastern strain of virus of equine encephalomyelitis which, though similar to the western strain, differed serologically. The disease in the horse was more acute and fatal, and the virus appeared to be more virulent for laboratory animals.
The viruses of equine encephalomyelitis differ immunologically from the viruses