Equine Health Update EHU Vol 19 Issue 4 - Page 19

EQUINE | CPD Article Actinobacillus equuli Infections in Equines Dr R D Last – BVSc, M.Med.Vet (Pathology) Specialist Veterinary Pathologist Introduction A ctinobacillus equuli are the most common Actinobacillus spp of horses and account for most of the equine infections. These are gram-nega- tive, pleomorphic, commensal bacteria belonging to the family Pasteurellaceae. This bacterium is the most common cause of embolic suppurative nephritis and septicaemia with polyarthritis in foals (sleepy foal dis- ease), with disease in adults including respiratory dis- ease, cutaneous abscesses, guttural pouch infections, endometritis, pericarditis, endocarditis, peritonitis, encephalitis, arthritis, orchitis and abortion, being less frequently reported. Other Actinobacillus species which have been associated with sporadic localized infections in equines include Actinobacillus arthriti- dis, Actinobacillus ligneresii, Actinobacillus suis and Actinobacillus pleuropneumonia. The Bacteria Actinobacillus equuli is composed of 2 sub species namely Actinobacillus equuli subsp equuli and Acti- nobacillus equuli subsp hemolyticus, which differ by the presence or absence of a repeats-in-toxin (RTX) toxin. Actinobacillus equuli subsp equuli is a normal commensal of the oral cavity and alimentary tract of adult horses and is the isolate most commonly associ- ated with neonatal septicemia (sleepy foal disease), as well as respiratory infections in foals and adult horses. Actinobacillus equuli subsp hemolyticus is a normal inhabitant of the respiratory tract and is commonly isolated from tracheal wash fluids. However, this sub species is more commonly associated with non-respi- ratory infections in adult horses. Actinobacillus equuli subsp hemolyticus produces a repeats-in-toxin (RTX) toxin termed Aqx, similar to the leukotoxins of Mannheimia hemolytica. Many RTX toxins are host specific and the pathogenesis of these bacteria are related to the RTX toxic activity. This Aqx toxin is highly cytolytic to equine erythrocytes and lymphocytes. Although this Aqx toxin is likely a major virulence factor of Actinobacillus equuli subsp hemo- lyticus, it’s absence from Actinobacillus equuli subsp equuli, (the major cause of sleepy foal disease and respiratory infections), indicate that other virulence factors are involved with neonatal septicemia and re- spiratory infections associated with this sub species. Endotoxin which is associated with all gram-negative pathogens, damages endothelium and is therefore, capable of causing vasculitis and thrombosis. Throm- bosis with bacterial emboli is a characteristic and con- • Volume 19 no 4 • December 2017 • 19