Factors associated with equine influenza during the 2025 outbreak in a draft horse population at the Obihiro Racecourse, Japan.

Overview

• This study analyzed the 2025 equine influenza outbreak among draft horses at Japan’s Obihiro Racecourse, identifying factors like age, sex, and trainer group associated with infection risk.
• The findings aim to inform better vaccination and biosecurity measures to control future outbreaks in high-density horse racing environments.

Background and Purpose

• Equine influenza re-emerged in Japan in April 2025 after 17 years without cases, causing an outbreak at the Obihiro Racecourse.
• The racecourse hosts Ban’ei racing, where draft horses pull heavy sleds—a context with unique management practices and horse populations.
• The study aimed to describe the epidemiology of this outbreak and identify individual and management-level factors linked to equine influenza cases.
• Understanding these factors can help improve disease control and prevention strategies in similar settings.

Methods

• The researchers conducted a retrospective analysis using clinical records from April 21 to June 18, 2025.
• Cases were defined as horses showing one or more clinical signs: fever, coughing, or nasal discharge.
• Generalized linear mixed models were utilized to evaluate associations between case status and factors: age, sex, and trainer group.
• Since detailed individual spatial data was incomplete, multiple imputation using trainer group-level location was performed to factor in spatial effects.
• The study population included 856 horses housed at the racecourse during the outbreak timeframe.

Key Findings

• A total of 438 horses (51.2% attack rate) met the case definition, indicating a widespread outbreak.
• Two-year-old horses had significantly higher odds of infection compared to older horses.
• Female horses were more likely to be cases than males.
• There was clustering of cases at the trainer-group level, meaning horses managed by certain trainers had similar infection risks.
• Spatial effects were limited overall, but some high-traffic areas within the racecourse corresponded with elevated infection risk.

Interpretation and Implications

• The age disparity in infection rates may be related to vaccination history differences (younger horses possibly less or differently vaccinated) or inherent age-related susceptibility to equine influenza.
• Sex differences in infection risk suggest that additional factors related to male/female horses’ physiology or management could play a role.
• Clustering by trainer groups highlights the importance of management practices, potentially including hygiene, biosecurity measures, and horse interactions within groups.
• Limited spatial effects suggest that while location within the racecourse matters, other factors like age and trainer influence risk more strongly.
• The findings support prioritizing vaccination strategies focused on younger horses and adapting protocols based on identified risk factors.
• Implementing targeted biosecurity measures at trainer group and high-traffic locations can enhance outbreak control.

Conclusions

• The 2025 equine influenza outbreak at Obihiro Racecourse was characterized by substantial heterogeneity in infection risk across age, sex, and trainer groups.
• Optimizing vaccination protocols and biosecurity tailored to these risk factors can help mitigate future outbreaks.
• This research provides valuable epidemiological insight into managing equine influenza in dense, specialized horse racing environments such as Ban’ei racing.