In 2025, an outbreak of equine influenza occurred among draft horses at Obihiro Racecourse in Hokkaido, Japan. The number of horses showing clinical signs increased dramatically in April. The proportion affected was significantly higher in 2-year-olds than in older groups. The proportion that developed fever during the outbreak was significantly higher than that observed among Thoroughbreds during the 2007 outbreak. In non-epidemic periods, the geometric mean hemagglutination inhibition antibody titer of 2-year-old draft horses was significantly lower than that of 2-year-old Thoroughbreds. These draft horses had received fewer vaccinations than the Thoroughbreds. Less frequent vaccination likely led to a lower antibody titer and a higher final attack rate in the 2-year-olds and in the overall draft horse population.
FUKUMOTO et al. (Thu,) studied this question.