Abstract
Actinobacillus species are known to be pathogenic to horses. To clarify etiological agents of actinobacillosis in Japanese adult horses, 27 isolates from Japanese Thoroughbred racehorses putatively identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry as Actinobacillus were further identified by PCR of the A. equuli toxin gene, by CAMP test, and by 16S rRNA sequencing analysis. Actinobacillus equuli subsp. haemolyticus was isolated most frequently (16/27) and was related to respiratory infections. Actinobacillus equuli subsp. equuli (4/27) was isolated from chronic cases or concomitant with other bacterial infections. The remainder were A. pleuropneumoniae, unclassified Actinobacillus species and Pasteurella caballi. Actinobacillus equuli including subsp. haemolyticus and subsp. equuli were the species most frequently isolated from equine actinobacillosis in Japan.