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

In April 2025, equine influenza re-emerged in Japan for the first time in 17 years, resulting in an outbreak at a horse racecourse for Ban'ei racing, a unique form of horse racing in which draft horses pull heavy sleds. This study aimed to describe the epidemiological characteristics of the outbreak and identify individual- and management-level factors associated with equine influenza cases in order to develop improved control programs. A retrospective analysis was conducted using clinical records reported to the racecourse authority between April 21 and June 18, 2025. Cases were defined based on one or more of the following clinical signs: fever, coughing, and nasal discharge. Generalized linear mixed models were used to assess associations between case status and age, sex, and trainer group. As individual-level spatial information was incomplete, multiple imputation was performed using trainer group-level spatial information to incorporate the effect of each horse's spatial location. A total of 438 out of 856 horses housed in the racecourse met the case definition (attack rate: 51.2%). Two-year-old horses showed significantly higher odds of being classified as a case than older horses, and females had higher odds than males. Evidence of clustering was observed at the trainer-group level (intraclass correlation coefficient = 0.136; 95% confidence interval: 0.044-0.152). Spatial effects were limited, although certain high-traffic areas suggested an elevated risk of being classified as a case. The outbreak demonstrated marked heterogeneity by age, sex, and management unit. The higher frequency of cases among two-year-old horses may reflect differences in vaccination history, although age-related susceptibility cannot be excluded. These findings highlight the importance of understanding age-related risk factors and optimizing vaccination strategies to mitigate future EI outbreaks in this high-density racing setting. Targeted biosecurity measures may also be important for more effective disease control.