A Serologic Survey on Equine Influenza for the Past Ten Years

Abstract

Over a period of 1966 to 1976, a serologic survey was performed on the serum samples of native and imported horses raised in Hokkaido and Tokyo. No hemagglutination-inhibition (HI) antibodies against the subtype 1 (Prague strain) and the subtype 2 (Miami and Tokyo strains) of equine influenza (EI) virus were detected in any of the sera from 587 native draft horses throughout the observation period. When examined by the HI test over a period of 1966 to 1970, 28 native racing horses had no such antibodies. Twelve and 16 (55 and 73%) of 22 native racing horses tested in 1972 and 1973, respectively, and 67 (72%) of the 93 native racing and riding horses tested in 1976, had HI titers of 1:8 to 1:512 against the strains. Of 80 imported racing horses tested over a period of 1966 to 1970, 48 (60%) had positive titers for the Prague strain and 23 (29%) for the Miami strain. Almost the same percentage was recognized against each of the strains in 28 imported horses tested in 1972 and 1973. In 1976, 12 to 14 (67 to 78%) of 18 imported horses were reactive in positive titers to the strains. These results indicated that there had been a marked difference in the antibody response of racing and riding horses between the time before and the time after the first outbreak of EI infection in 1971.