Actinobacillus Equuli Infections in Equines

EQUINE | CPD Article Actinobacillus equuli Infections in Equines Dr R D Last – BVSc, M.Med.Vet (Pathology) Specialist Veterinary Pathologist Introduction adult horses and is the isolate most commonly associated with neonatal septicemia (sleepy foal disease), as Actinobacillus equuli are the most common Actino- well as respiratory infections in foals and adult horses. bacillus spp of horses and account for most of the Actinobacillus equuli subsp hemolyticus is a normal equine infections. These are gram-negative, pleomor- inhabitant of the respiratory tract and is commonly phic, commensal bacteria belonging to the family isolated from tracheal wash fluids. However, this sub Pasteurellaceae. This bacterium is the most common species is more commonly associated with non-respi- cause of embolic suppurative nephritis and septicae- ratory infections in adult horses. mia with polyarthritis in foals (sleepy foal disease), with disease in adults including respiratory disease, Actinobacillus equuli subsp hemolyticus produces a cutaneous abscesses, guttural pouch infections, en- repeats-in-toxin (RTX) toxin termed Aqx, similar to the dometritis, pericarditis, endocarditis, peritonitis, en- leukotoxins of Mannheimia hemolytica. Many RTX cephalitis, arthritis, orchitis and abortion, being less toxins are host specific and the pathogenesis of these frequently reported. Other Actinobacillus species bacteria are related to the RTX toxic activity. This Aqx which have been associated with sporadic localized toxin is highly cytolytic to equine erythrocytes and infections in equines include Actinobacillus arthriti- lymphocytes. Although this Aqx toxin is likely a major dis, Actinobacillus ligneresii, Actinobacillus suis and virulence factor of Actinobacillus equuli subsp hemo- Actinobacillus pleuropneumonia. lyticus, it’s absence from Actinobacillus equuli subsp equuli, (the major cause of sleepy foal disease and The Bacteria respiratory infections), indicate that other virulence factors are involved with neonatal septicemia and re- Actinobacillus equuli is composed of 2 sub species spiratory infections associated with this sub species. namely Actinobacillus equuli subsp equuli and Acti- nobacillus equuli subsp hemolyticus, which differ by Endotoxin which is associated with all gram-negative the presence or absence of a repeats-in-toxin (RTX) pathogens, damages endothelium and is therefore, toxin. Actinobacillus equuli subsp equuli is a normal capable of causing vasculitis and thrombosis. Throm- commensal of the oral cavity and alimentary tract of bosis with bacterial emboli is a characteristic and con- • Volume 19 no 4 • December 2017 • 19 EQUINE | CPD Article sistent feature of sleepy foal disease (neonatal septicemia), implicating an important and possible primary role of endotoxin in lesion induction. Pathogenesis Predisposing factors to the development of disease in foals includes failure of passive transfer of maternal antibodies, unsanitary conditions in the foaling area and concurrent immunosuppressive conditions. Infec- tion in foals is acquired in utero, during parturition, or shortly after birth as an umbilical infection or less commonly from gastrointestinal (ingestion) or respira- Figure 2: Foal kidney and adrenal gland - note the early and subtle embolic tory (inhalation) infections. Disease presentation is of- nephritis (arrow) with enlarged and hemorrhagic adrenal gland due to extensive bacterial emboli station of the adrenal with associated vasculitis and hemor- ten peracute with death at birth or within the 1st few rhage. days due to fulminant septicemia. In foals that survive the initial infection and survive for several days, usu- Another target organ besides the kidney is the adre- ally present with the classical microabscessation syn- nal (Figure 3). Death may occur because of fulminat- drome. These neonatal septicemic infections are con- ing septicaemia. In foals that survive for several days, sidered to be primary infections in most instances. micro-abscesses are seen in the kidneys and other organs, and polyarthritis is present, most commonly During bacteraemia, septic emboli lodge in glomeruli involving the hock and stifle joints. and peritubular capillaries producing multifocal glo- merulonephritis and interstitial nephritis. Abscesses are usually cortical but can be seen in the medulla (Figure 1 and Figure 2). Figure 3: Foal adrenal gland – note the multifocal embolic adrenalitis with Figure 1: Foal Kidney - multifocal embolic bacterial nephritis with microabscesses micro-abcessation (arrows) and extensive hemorrhage. The adrenal gland is a (arrows). Image courtesy of the University of Georgia. frequently overlooked target organ of Actinobacillus equuli. 20 • Equine Health Update • EQUINE | CPD Article In adult horses Actinobacillosis is rare and usually acts as a secondary bacterial infection complicating some other primary condition, usually some other concurrent pathogenic bacterial or viral infection, but under the right circumstances Actinobacillus sp can be primary pathogens. Infection in adult horses is gener- ally more localised and include cutaneous abscesses, guttural pouch infections, endometritis, pericarditis, endocarditis, peritonitis, encephalitis, arthritis, orchitis and abortion. The syndromes of acute peritonitis, encephalitis and fatal pulmonary hemorrhage with pneumonia due to endothelial damage caused by bacterial toxins, have been more recently described. In addition, it has been documented that the same mare may abort in successive pregnancies. Rare cases of septicemia associated with both Actinobacillus equuli Figure 4: Foal liver histopathology – note the hepatic sinusoidal bacterial emboli subsp equuli and Actinobacillus equuli subsp hemo- (arrows) with associated micro-abcessation. lyticus, have been documented. Diagnosis Definitive diagnosis of Actinobacillus equuli relies on the isolation of the bacteria by culture. • In live foals the preferred sample is blood for blood culture collected directly into blood culture bottles. • In dead foals at postmortem kidney and lung are the preferred tissues, either fresh biopsies or char- coal swabs. • In adult horses the primary target organ of the disease syndrome is sampled. Figure 5: Foal kidney histopathology – note the plugging of glomerular and Histopathology is also an extremely useful diagnostic periglomerular bloodvessels by large bacterial emboli (arrows). procedure as the presence of large embolic bacterial colonies plugging the capillary’s of various visceral or- Prevention and Control gans, are a common and consistent histological find- ing and easily visible in H&E stained sections (Figure Infected horses may be treated with various antibiotics 4 and Figure 5). including streptomycin, tetracycline, ampicillin and • Volume 19 no 4 • December 2017 • 21 EQUINE | CPD Article trimethoprim – sulfonamide, with antibiotic selection vention measure where infection is suspected, is by being based on the antibiotic sensitivity profile, na- the installation of bovine antibiotic mastitis remedies ture of the infection and the ability of the antibiotic to (containing tetracycline or penicillin), into the mare’s achieve therapeutic concentrations at the site of infec- birth canal shortly prior to foaling. tion. References Neonatal septicemia (sleepy foal disease) is best pre- vented through employing a combination of strict en- 1. Jubb & Kennedy. 2016. Pathology of Domestic vironmental hygiene, practicing of foaling camp rota- Animals 6th edn. tion, ensuring intake of colostrum as soon after birth 2. Layman Q D et al. 2014. A retrospective study as possible, umbilical disinfection and immediate of equine actinobacillus cases:199-2011. Journal removal of the expelled placenta and soiled bedding of Veterinary Diagnostic Investigation. 26:365- post foaling. Remember, the mare’s reproductive tract 375.\ and fetal membranes are the primary source of bac- 3. Merck Veterinary Manual. 2016. Overview of teria for the foal, and therefore another practical pre- Actinobacillosis. MSD, 11th Edition 22 • Equine Health Update •.

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