Robert G. Scholtens, James H. Steele, Walter R. Dowdle, Wilma B. Yarbrough, Roslyn Q. Robinson, U.S. Epizootic of Equine Influenza, 1963, Public Health Reports (1896-1970), Vol. 79, No. 5 (May, 1964), pp. 393-402
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Scholtens et al. (1964) studied this question.