For a number of years, in the summer months occasional cases of encephalitis of unknown cause have occurred in human beings in the Yakima valley, Washington. In 1939 the number of these cases first reached epidemic proportions. During the same period deaths occurred sporadically and in small outbreaks among horses, attributed generally to forage poisoning or botulism or occasionally simply diagnosed as blind staggers. In 1939 more than 600 cases of encephalomyelitis in horses were reported for the same region in addition to 31 cases of encephalitis in man. It was presumed at this time that both human beings and horses had been infected with the virus of Western equine encephalomyelitis, and a few samples of blood serum from each group, when tested by Miss Beatrice Howitt, of the Hooper foundation, were found to contain antibody for this virus.
In 1940 encephalitis again appeared in epidemic proportions, and on August
HAMMON WM. ENCEPHALITIS IN THE YAKIMA VALLEY: MIXED ST. LOUIS AND WESTERN EQUINE TYPES. JAMA. 1941;117(3):161–167. doi:10.1001/jama.1941.02820290003002
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