The 2025 outbreak of equine influenza among draft horses at Obihiro Racecourse, Hokkaido, Japan.

The 2025 outbreak of equine influenza at Obihiro Racecourse in Hokkaido, Japan, particularly affected draft horses, with young horses showing a higher rate of infection and more severe symptoms compared to older horses and previous outbreaks among Thoroughbreds.

Overview of the Outbreak

• The outbreak occurred in 2025 among draft horses at Obihiro Racecourse, located in Hokkaido, Japan.
• There was a dramatic increase in the number of horses exhibiting clinical signs of equine influenza starting in April.
• The disease spread rapidly within the draft horse population, especially among younger horses.

Age-related Susceptibility

• The proportion of horses affected by the influenza was significantly higher in 2-year-old horses compared to older horses.
• This suggests that younger horses were more susceptible to infection or developed more noticeable symptoms during the outbreak.

Clinical Signs and Severity

• The proportion of draft horses that developed a fever during the 2025 outbreak was significantly greater than the rate observed among Thoroughbred horses during a previous outbreak in 2007.
• Fever is a key clinical sign of equine influenza, indicating a more active immune response and potentially more severe infection.

Immunity and Vaccination Status

• During non-epidemic periods, the geometric mean hemagglutination inhibition (HI) antibody titer — a measure of immunity — was significantly lower in 2-year-old draft horses compared to 2-year-old Thoroughbreds.
• Lower antibody titers indicate a weaker immune response or less protection against influenza infection.
• Draft horses received fewer vaccinations than Thoroughbreds, contributing to their lower antibody levels.
• Less frequent vaccination likely resulted in poorer immunity and contributed to the higher attack rate observed in 2-year-old draft horses during the outbreak.

Implications and Conclusions

• The study highlights the importance of regular and adequate vaccination protocols for draft horses to maintain sufficient immunity against equine influenza.
• The higher susceptibility of younger horses stresses the need for targeted preventive measures in this age group.
• Comparisons with previous outbreaks among Thoroughbreds suggest breed-related differences in vaccination practices and possibly immune response.
• Overall, better vaccination coverage could reduce the severity and spread of future equine influenza outbreaks in draft horse populations.