ISSN 2398-2977      

Actinobacillus equuli





  • Genus:Actinobacillus.
  • Species:equuli.
  • Sub-species:equuliandhaemolyticus, distinguished on the basis of hemolysis and the cyclic AMP reaction; haemolyticusis closely related toA. suis.
  • A newly-described species,A. arthritidis, has been isolated from horses with septicemia and arthritis.
  • Actinobacillusspp are members of the family Pasteurellaceae - close similarities exist among the genera within this family, especially betweenActinobacillusandPasteurella.


  • Gk.actin-, ray.

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Clinical Effects



  • Actinobacilli are commensal parasites on mucous membranes.
  • Actinobacillus equuliis a common inhabitant of the equine oral cavity. 
  • In one study, 28 of 30 adult horses and foals carried the bacterium in the oral cavity.
  • Prevalence in healthy horses in other studies ranges from 1288%.


  • Actinobacillus equuliinfection in foals may be acquired, probably from the normal flora of the dam, via a contaminated umbilicus, or by inhalation or ingestion. Phenotypically and genotypically identical strains have been isolated from mares and their foals.
  • Clinical disease in adult horses is probably caused by exposure to non-commensal strains.

Pathological effects

  • Lesions are suppurative or granulomatous.
  • The Aqx protein ofA. equulis, present in subsp.haemolyticus, is cytotoxic to equine leukocytes and belongs to the RTX family of pore-forming toxins. Non-hemolytic strains are also leukotoxic, but to a lesser degree, indicating a less potent or lower concentration of toxin.
  • The isolation ofActinobacillusspp from multiple cases of fibrinous pericarditis in the horse suggests it is pericardiotrophic.


Control via animal

Control via chemotherapies

  • Infection may be treated with penicillins, chloramphenicol, aminoglycoside, sulfonamides or third-generation cephalosporins.

Control via environment

  • Maintain hygiene at foaling.
  • Avoid contamination of surgical and other wounds.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Holyoak G R, Smith C M, Boyette R et al (2007) Serum antibodies in mares and foals toActinobacillus equuliwhole cells, outer membrane proteins, and Aqx toxin. Vet Immunol Immunopathol 118 (3-4), 310316 PubMed.
  • Donahue J M, Sells S F & Bolin D C (2006) Classification of Actinobacillus spp isolates from horses involved in mare reproductive loss syndrome. Am J Vet Res 67 (8), 14261432 PubMed.
  • Mogg T D & Dykgraaf S (2006) Actinobacillusperitonitis in a Warmblood gelding. Vet Clin North Am Equine Pract 22 (1), e916 PubMed.
  • Stewart A J (2006) Actinobacillus pleuritis and peritonitis in a quarter horse mare. Vet Clin North Am Equine Pract 22 (1), e77-93 PubMed.
  • Bolin D C, Donahue J Met al (2005) Microbiologic and pathologic findings in an epidemic of equine pericarditis. J Vet Diagn Invest 17 (1), 3844 PubMed.
  • Stewart A J, Hinchcliff K W, Saville W J et al (2002) Actinobacillus sp. bacteremia in foals: clinical signs and prognosis. J Vet Intern Med 16 (4), 464471 PubMed.
  • Berthoud H, Frey J, Sternberg S, Straub R & Kuhnert P (2004) Antibodies to Aqx toxin ofActinobacillus equuliin horses and foals. Vet Rec 155 (8), 231233 PubMed.
  • Kuhnert P, Berthoud H, Christensen H, Bisgaard M & Frey J (2003) Phylogenetic relationship of equine Actinobacillus species and distribution of RTX toxin genes amongst clusters. Vet Res 34 (3), 353359 PubMed.
  • Berthoud H, Frey J & Kuhnert P (2002) Characterization of Aqx and its operon: the hemolytic RTX determinant of Actinobacillus equuliVet Microbiol 87 (2), 159174 PubMed.
  • Christensen H, Bisgaard M & Olsen J E (2002) Reclassification of equine isolates previously reported as Actinobacillus equuli, variants of A. equuliActinobacillus suis or Bisgaard taxon 11 and proposal of A. equuli sub sp. Equuli subsp. nov. and A. equuli subsp. haemolyticus subsp. nov. Int J Syst Evol Microbiol 52 (5), 15691576 PubMed.
  • Smith M A & Ross M W (2002) Postoperative infection with Actinobacillus spp in horses: 10 cases (1995-2000). JAVMA 221 (9), 13061310 PubMed.
  • Matthews S, Dart A J, Dowling B A, Hodgson J L & Hodgson D R (2001) Peritonitis associated with Actinobacillus equuli in horses: 51 cases. Aust Vet J 79 (8), 536539 PubMed.
  • Patterson-Kane J C, Donahue J M & Harrison L R (2001) Septicemia and peritonitis due to Actinobacillus equuli infection in an adult horse. Vet Pathol 38 (2), 230232 PubMed.
  • Sternberg S (2001) Specific immune responses of mares and their newborn foals to Actinobacillus spp present in the oral cavity. Acta Vet Sc and 42, 237242.
  • Rycroft A N & Garside L H (2000) Actinobacillus species and their role in animal disease. Vet J 159 (1), 1836 PubMed.
  • Carmalt J L, Baptiste K E & Chirino-Trejo J M (1999) Actinobacillus lignieresii infection in two horses. JAVMA 215 (6), 826828 PubMed.
  • Sternberg S & Brandstrom B (1999) Biochemical fingerprinting and ribotyping of isolates of Actinobacillus equuli from healthy and diseased horses. Vet Microbiol 66 (1), 5365 PubMed.
  • Blackall P J, Christensen J P & Bisgaard M (1998) Diversity among isolates of Actinobacillus equuli and related organisms as revealed by ribotyping. Aust Vet J 76 (6), 423425 PubMed.
  • Rycroft A N, Woldeselassie A, Gordon P I & Bjornson A (1998) Serum antibody in equine neonatal septicemia due to Actinobacillus equuliVet Rec 143 (9), 254255 PubMed.
  • Sternberg S (1998) Isolation of Actinobacillus equuli from the oral cavity of horses and comparison of isolates by restriction enzyme digestion and pulsed-field gel electrophoresis. Vet Microbiol 59 (2-3), 147156 PubMed.
  • Ward C L, Wood J L, Houghton S B, Mumford J A & Chanter N (1998) Actinobacillus and Pasteurella species isolated from horses with lower airway disease. Vet Rec 143 (10), 277279 PubMed.
  • Raisis A L, Hodgson J L & Hodgson D R (1996) Equine neonatal septicemia: 24 cases. Aust Vet J 73 (4), 137140 PubMed.
  • Collins M B, Hodgson D R & Hutchins D R (1994) Pleural effusions associated with acute and chronic pleuropneumonia in pleuritis secondary to thoracic wounds in horses: 43 cases (19821992). JAVMA 205 (12), 17531758 PubMed.
  • Golland L C, Hodgson D R et al (1994) Peritonitis associated with Actinobacillus equuli in horses: 15 cases (19821992). JAVMA 205 (2), 340343 PubMed.
  • Bisgaard M, Piechulla K, Ying Y T, Frederikson W & Mannheim W (1984) Prevalence of organisms described as Actinobacillus suis or haemolytic Actinobacillus equuli in the oral cavity of horses. Acta Pathol Microbiol Immunol Scand Sect B 92 (6), 291298 PubMed.
  • Gay C C & Lording P M (1980) Peritonitis in horses associated with Actinobacillus equuliAust Vet J 56 (6), 296300 PubMed.
  • Webb R F, Cockram F A & Pryde L (1976) The isolation of Actinobacillus equuli from equine abortion. Aust Vet J 52 (2), 100101 PubMed.

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