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The Journal of veterinary medical science2026; doi: 10.1292/jvms.26-0092

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

Abstract: 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.
Publication Date: 2026-03-25 PubMed ID: 41882872DOI: 10.1292/jvms.26-0092Google Scholar: Lookup
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Summary

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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.

Cite This Article

APA
Fukumoto N, Shibata M, Hayashi M, Kawauchi K, Yabuuchi Y, Oue Y, Matsuyama R, Yamamoto T, Mita H, Kawanishi N, Bannai H, Tsujimura K, Yamanaka T, Nemoto M. (2026). The 2025 outbreak of equine influenza among draft horses at Obihiro Racecourse, Hokkaido, Japan. J Vet Med Sci. https://doi.org/10.1292/jvms.26-0092

Publication

ISSN: 1347-7439
NlmUniqueID: 9105360
Country: Japan
Language: English

Researcher Affiliations

Fukumoto, Natsuko
  • Athena Integrative Veterinary Care for Ban'ei Race Horse.
Shibata, Makoto
  • Athena Integrative Veterinary Care for Ban'ei Race Horse.
Hayashi, Mayu
  • Athena Integrative Veterinary Care for Ban'ei Race Horse.
Kawauchi, Kyoko
  • Hokkaido Tokachi Livestock Hygiene Service Center.
Yabuuchi, Yukiko
  • Hokkaido Tokachi Livestock Hygiene Service Center.
Oue, Yasuhiro
  • Hokkaido Tokachi Livestock Hygiene Service Center.
Matsuyama, Ryota
  • Epidemiology and Arbovirus Group, Division of Transboundary Animal Disease Research, National Institute of Animal Health, National Agriculture and Food Research Organization.
Yamamoto, Takehisa
  • Epidemiology and Arbovirus Group, Division of Transboundary Animal Disease Research, National Institute of Animal Health, National Agriculture and Food Research Organization.
Mita, Hiroshi
  • Equine Research Institute, Japan Racing Association.
Kawanishi, Nanako
  • Equine Research Institute, Japan Racing Association.
Bannai, Hiroshi
  • Equine Research Institute, Japan Racing Association.
Tsujimura, Koji
  • Equine Research Institute, Japan Racing Association.
Yamanaka, Takashi
  • Miho Training Center Racehorse Clinic, Japan Racing Association.
Nemoto, Manabu
  • Equine Research Institute, Japan Racing Association.

Citations

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