Actinobacillus species isolated from Japanese Thoroughbred racehorses in the last two decades

Area of Science:

• Veterinary Microbiology
• Equine Infectious Diseases

Background:

• Actinobacillus species are recognized pathogens in horses, causing actinobacillosis.
• Accurate identification of etiological agents is crucial for effective disease management in equine populations.

Purpose of the Study:

• To identify the specific Actinobacillus species responsible for actinobacillosis in Japanese adult horses.
• To differentiate between subspecies of Actinobacillus equuli and their clinical relevance.

Main Methods:

• Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) for initial identification.
• Polymerase chain reaction (PCR) targeting the A. equuli toxin gene.
• CAMP test and 16S rRNA sequencing for precise species and subspecies determination.

Main Results:

• Actinobacillus equuli subsp. haemolyticus was the most prevalent isolate (16/27), frequently associated with respiratory infections.
• Actinobacillus equuli subsp. equuli (4/27) was identified in chronic or secondary infections.
• Other identified species included A. pleuropneumoniae, unclassified Actinobacillus, and Pasteurella caballi.

Conclusions:

• Actinobacillus equuli, encompassing both subsp. haemolyticus and subsp. equuli, is the predominant bacterial cause of actinobacillosis in Japanese horses.
• Subspecies identification provides insights into clinical presentations and potential treatment strategies for equine actinobacillosis.