ANTIBODY-RESPONSES OF JAPANESE HORSES TO INFLUENZA-VIRUSES IN THE PAST FEW YEARS

A total of 305 horse sera collected in the Hidaka district of Hokkaido in the years 1988-90 were tested for the presence of hemagglutination-inhibition (HI) antibodies to A/equine/Newmarket/1/77 (H7N7), A/equine/Tokyo/2/71 (H3N8) and A/equine/Kentucky/1/81 (H3N8, Kentucky) strains of equine influenza (EI) virus. Antibodies to the 3 strains were detected in hardly of the 45 sera from 2-years-old horses which were collected before vaccination. Many of the 51 horses, after vaccination with inactivated EI virus, had HI antibodies to the 3 strains in 37 to 88 per cent. However, the number of positive reactors among the horses aged 3 to 23 years gradually decreased with the advance in age. In particular, no antibody response was found in the 60 horses over 9 years of age, except for 3 cases with HI antibody of low titer, with the Kentucky strain which has recently been prevalent among the horse populations in many countries. In a complement-fixing test, antibodies to the soluble antigen of type A influenza virus were detected in the sera collected from the horses which were exposed to an outbreak of EI virus infection, but not in the sera from the vaccinated horses examined.