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 •
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