To the Editor: The renewed interest in equine encephalitis because of the 16 cases occurring in Massachusetts in 1955 and 1956 serves as the pretext for recording here the circumstances surrounding the discovery of the first human infections with the equine viruses.Early in August, 1938, "horse sleeping sickness" broke out in Massachusetts, and before the season was over nearly 300 horses had succumbed to the disease. As a result, many telephone calls were received by the Division of Communicable Diseases from those who were fearful of catching the disease from horses. These people were assured that no cases in . . .