ISSN 2398-2977      



Synonym(s): Sleepy foal disease, joint ill, neonatal septicemia


  • Cause:Actinobacillus spp, which are Gram-negative common commensal bacteria of the oral cavity, gastrointestinal tract and reproductive tract and can cause disease in both adults and foals.
  • Signs: vary according to the age of the animal and the system affected.
  • Diagnosis: varies according to system affected.
  • Treatment: supportive, IV fluids, antibiotics, treat primary problem if secondary.
  • Prognosis: foals - poor; adults: fair to good.



  • A. equuli  Actinobacillus equuli  .
  • Equine actinobacillosis is caused by severalActinobacillusspp which are Gram-negative, pleomorphic, commensal bacteria belonging to the family Pasteurellaceae.A. equulisubsp.equuliandA. equulisubsphaemolyticusare most commonly isolated from equine cases withA. arthritidis,A. lignieresii,A. suisandA. pleuropneumoniaebeing isolated sporadically and often from localized lesions.
  • A. equulisubspequuliis primarily associated with septicemia of neonatal foals (sleepy foal disease)   Foal: neonatal septicemia syndrome  . Neonatal septicemic actinobacillosis cases have characteristic lesions, including embolic nephritis, embolic pneumonia, lymphoid necrosis, multifocal hepatic necrosis and septic arthritis.
  • A. equulisubsphaemolyticusandA. equulisubspequulican cause respiratory infections   Lung: pneumonia - bacterial  , septicemia, metritis, mastitis   Mammary gland: mastitis  , arthritis, endocarditis, meningitis, mare reproductive loss syndrome   Reproduction: mare reproductive loss syndrome  , cellulitis   Cellulitis  , synovitis, osteomyelitis   Bone: osteitis - septic  , lymphadenitis, cutaneous abscesses and peritonitis in adults   Abdomen: peritonitis  .
  • A. pleuropneumoniaeandA. lignieresiirarely infect horses.A. pleuropneumoniaehas been isolated in cases of metritis and osteomyelitis whilstA. lignieresiihas been isolated from respiratory cases, cuteneous abscesses and cases of metritis and mastitis.
  • A. arthritideshas been isolated from arthritis and septicemia.
  • Contributing factors for systemic actinobacillosis in foals include failure or partial failure of passive transfer (FPT)   Foal: failure of passive transfer (IgG)   and unsanitary conditions in the birthing environment.

Predisposing factors

  • Neonatal septicemia   Foal: neonatal septicemia syndrome   is thought to be associated with failure of passive transfer of antibodies via the colostrum   Foal: failure of passive transfer (IgG)  .
  • Colostrum-deprived foals have been shown to be highly susceptible to septicemia caused byA. equuli.
  • Unhygienic foaling conditions may also contribute to disease in neonates.
  • Infection withStrongylus vulgaris  Strongyle infestation: large  may be linked to the development of peritonitis.
  • Exposure to the eastern tent caterpillar may be associated with the development of pericarditis or abortion (MRLS   Reproduction: mare reproductive loss syndrome  ).
  • In adults, actinobacillosis may be a primary infection or there may be a concurrent viral or bacterial infection or a predisposing lesion such as choke or neoplasia such that theActinobacillusspp is secondary. Alternatively, it may be associated with a stressful event.


  • Generally soft tissue infections.
  • Lymph node involvement   →   systemic spread.
  • Adjacent bony tissues may also be affected.
  • In cases of peritonitis, it has been suggested that infection of the peritoneal cavity may be due to carriage ofA. equuliby strongyle larvae from the intestinal tract.
  • The bacterium has been isolated from verminous aneurysms caused byS. vulgaris  Strongylus spp  larvae in the cranial mesenteric arteries.


  • In foals, infection may occur in utero at parturition or during the first few days of birth. In non-survivors death usually occurs within 18 h to 3 days after birth.
  • Acute peritonitis is associated with rapid onset of clinical signs in <24 h; in some cases signs develop over 14 days; in chronic cases weight loss occurs over several weeks.


  • A. 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 12-88%.
  • A. equuliinfection in foals may be acquired via a contaminated umbilicus, or by inhalation or ingestion. The source of the organism is thought to be the oral cavity, or the gastrointestinal or respiratory tract, of the mare.
  • A. lignieresiis a component of the normal gastrointestinal flora and causes disease when it enters via a puncture wound.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Layman Q D et al (2014) A retrospective study of equine actinobacillosis cases: 1999-2011. J Vet Diag Invest 26 (3), 365-375 PubMed.
  • Watts A E et al (2011) Recurrent Actinobacillus peritonitis in an otherwise healthy thoroughbred horse. Aust Vet J 89 (4), 143-146 PubMed.
  • Sebastian M M et al (2008) Review paper: mare reporoductive loss syndrome. Vet Pathol 45 (5), 710-722 PubMed.
  • Aalbaek B et al (2007) Actinobacillus equuli subsp. equuli associated with equine valvular endocarditis. APMIS 115 (12), 1437-1442 PubMed.
  • Corley K T, Pearce G, Magdesian K G & Wilson W D (2007) Bacteremia in neonatal foals: clinicopathological differences between gram-positive and gram-negative infections and single organism and mixed infections. Equine Vet J 39 (1), 84-89 PubMed.
  • Holyoak G R, Smith C M, Boyette R et al (2007) Serum antibodies in mares and foals to Actinobacillus equuliwhole cells, outer membrane proteins, and Aqx toxin. Vet Immunol Immunopathol 118 (3-4), 310-316 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), 1426-1432 PubMed.
  • Mogg T D & Dykgraaf S (2006) Actinobacillus peritonitis 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 M, Vickers M L et al (2005) Microbiologic and pathologic findings in an epidemic of equine pericarditis. J Vet Diagn Invest 17 (1), 38-44 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), 464-471 PubMed.
  • Berthoud H, Frey J, Sternberg S, Straub R & Kuhnert P (2004) Antibodies to Aqx toxin of Actinobacillus equuli in horses and foals. Vet Rec 155 (8), 231-233 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, 353-359 VetMedResource.
  • Berthoud H, Frey J & Kuhnert P (2002) Characterization of Aqx and its operon: the hemolytic RTX detereminant of Actinobacillus equuliVet Microbiol 87 (2), 159-174 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 subsp. Equuli subsp. nov. and A. equuli subsp. haemolyticus subsp. nov. Int J Syst Evol Microbiol 52 (5), 1569-1576 PubMed.
  • Smith M A & Ross M W (2002) Postoperative infection with Actinobacillus spp in horses: 10 cases (1995-2000). JAVMA 221 (9), 1306-1310 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), 536-539 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), 230-232 PubMed.
  • Sternberg S (2001) Specific immune responses of mares and their newborn foals to Actinobacillus spp present in the oral cavity. Acta Vet Scand 42, 237-242 PubMed.
  • Rycroft A N & Garside L H (2000) Actinobacillus species and their role in animal disease. Vet J 159 (1), 18-36 PubMed.
  • Carmalt J L, Baptiste K E & Chirino-Trejo J M (1999) Actinobacillus lignieresii infection in two horses. JAVMA 215 (6), 826-828 PubMed.
  • Sternberg S & Brandstrom B (1999) Biochemical fingerprinting and ribotyping of isolates of Actinobacillus equuli from healthy and diseased horses. Vet Microbiol 66 (1), 53-65 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), 423-425 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), 254-255 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), 147-156 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), 277-279 PubMed.
  • Raisis A L, Hodgson J L & Hodgson D R (1996) Equine neonatal septicemia: 24 cases. Aust Vet J 73 (4), 137-140 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), 1753-1758 PubMed.
  • Golland L C, Hodgson D R, Hodgson J L et al (1994) Peritonitis associated with Actinobacillus equuli in horses: 15 cases (19821992). JAVMA 205 (2), 340-343 PubMed.
  • Bisgaard M, Piechulla K, Ying Y T, Frederikson W & Mannheim W (1984) Prevalence of organisms described as Actinobacillus suis or hemolytic Actinobacillus equuli in the oral cavity of horses. Acta Pathol Microbiol Immunol Scand Sect B 92 (6), 291-298 PubMed.
  • Gay C C & Lording P M (1980) Peritonitis in horses associated with Actinobacillus equuliAust Vet J 56 (6), 296-300 PubMed.
  • Webb R F, Cockram F A & Pryde L (1976) The isolation of Actinobacillus equuli from equine abortion. Aust Vet J 52 (2), 100-101 PubMed.

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