The first outbreak of equine coronavirus infection in 13 years among draft horses at Obihiro Racecourse in Japan in 2025.

Equine coronavirus (ECoV) causes fever, anorexia and sometimes enteric signs (e.g. diarrhea) in adult horses. Three ECoV outbreaks among draft horses at the same Japanese racecourse have been reported previously, in 2004, 2009 and 2012. We describe the epidemiological, clinical and genetic features of the fourth ECoV outbreak at the same racecourse in 2025. In January 2025, a sudden increase in the number of draft horses showing various combinations of fever, anorexia and enteric signs was observed at Obihiro Racecourse, in Hokkaido, Japan. Of about 800 horses, 176 showed clinical signs. Specifically, 83.5% (147/176) horses had fever, 98.9% (174/176) were anorexic, and 10.2% (18/176) had enteric signs. Real-time reverse transcription -polymerase chain reaction detected ECoV RNA in 23 of 26 horses. Seroconversion to ECoV was confirmed in 25 of 26 horses by neutralization assay. The results indicate that all 26 horses tested were infected with ECoV. Molecular analyses showed that the spike and nucleocapsid genes were highly conserved compared to other ECoVs. However, phylogenetic analysis showed that these genes of the 2025 virus did not form a group with the strains detected in the previous three outbreaks. These results indicate that ECoV was the cause of the sudden increase in the number of draft horses with fever, anorexia and enteric signs. Although the origin of the 2025 virus is unknown, this outbreak was likely caused by a virus different from the one that caused the previous outbreaks.