Canis ISSN: 2398-2942

Streptococcus equi

Synonym(s): S. equi

Contributor(s): David Bemis, Kerstin Erles




  • PhylumFirmicutes
  • ClassBacilli
  • Order: Lactobacillales
  • FamilyStreptococcaceae
  • GenusStreptococcus
  • Speciesequi
  • Subspeciesequi (Synonym: S. equizooepidemicus (Synonyms: S. zooepidemicus)


  • L. gen. N. equi, of a horse.
  • Gr. n. zoon, an animal; Gr. adj epidemios, among the people, epidemic; L. masc. suff. icus, suffix used with various meanings; N.L. masc. adj. zooepidemicus, prevalent among animals.

Active Forms

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Clinical Effects



  • S. equi subsp. equi found on upper respiratory mucosa and associated lymphatics of infected horses.
  • S. equi subsp. zooepidemicus common commensal on mucous membranes of the upper respiratory tract and lower genital tract of horses.
  • S. equi subsp. zooepidemicus is found with varying frequencies as a commensal or opportunistic pathogen in many other animal hosts, including ruminants, pigs, poultry, guinea pigs and, rarely, dogs and people.


  • Cells divide by binary fission to produce pairs or chains.


  • Both S. equi subspecies may be transmitted by inhalation or ingestion.
  • Indirect transmission by contaminated fomites also occurs.
  • S. equi subsp.zooepidemicus may also be transmitted by sexual contact or congenitally during passage through the birth canal.
  • Most individual infections arise from endogenous, pre-existing clones; however, outbreaks may result from the horizontal spread of a single clone.
  • Transmission of S. equi subsp. zooepidemicusf rom dog to man reported.

Pathological effects


  • Host defense depends on phagocytosis.
  • Hyaluronic acid capsules antiphagocytic.
  • Antibodies raised against M protein, an antiphagocytic surface protein.
  • Recovered animals have short-term resistance to infection.
  • Immunity is serotype-specific.


  • Pyogenic bacteria produce suppurative host response.
  • Adhesins mediate attachment.
  • Extracellular products include streptolysin O and S, hyaluronidase, protease, streptokinase, streptococcal pyrogenic exotoxins.
  • Virulence mechanisms of highly pathogenic infections are unresolved.

Other Host Effects


  • S. equi subs .equi - associated with submandibular swelling and lymphadenitis.
  • S. equi subsp. zooepidemicus - rare commensal in upper respiratory and lower genital tracts.
  • S. equi subsp .zooepidemicus - associated with neonatal septicemia and severe forms of respiratory disease with hemorrhagic pneumonia Lung: bacterial pneumonia.


  • Co-pathogens found in some (but not all) outbreaks of severe respiratory disease.


Control via animal

  • Isolate and treat clinically affected animals.
  • Isolate and treat exposed animals (to control outbreak).
  • Quarantine until 2 weeks after resolution of signs.

Control via chemotherapies

  • Usually susceptible to penicillins, cephalosporins, potentiated sulfas, macrolides, chloramphenicol Chloramphenicol.
  • Often resistant to aminoglycosides and tetracyclines Tetracycline.
  • Treatment only effective if started early following onset of clinical signs.

Control via environment

  • Clean, disinfect and dry all environmental surfaces that were exposed to infected dogs.
  • Most disinfectants, at recommended use dilutions and contact times, effectively kill pathogenic streptococci.
  • Control via other means.
  • Use of separate protective clothing (glove, mask, footwear, etc) when examining infected animals.


  • No streptococcal vaccines available for use in dogs.

Other countermeasures

  • Isolate newly admitted dogs from clinically affected and exposed dogs.
  • Prophylactic treatment of newly admitted dogs may be indicated in some circumstances.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Abbott Y, Acke E, Khan S et al (2010) Zoonotic transmission of Streptococcus equi subsp. Zooepidemicus from a dog to a handler. J Med Microbiol 59 (Pt 1), 120-123 PubMed.
  • Pesavento P A, Hurley K F, Bannasch M J et al (2008) A clonal outbreak of acute fatal hemorrhagic pneumonia in intensively housed (shelter) dogs caused by Streptococcus equi subsp. zooepidemicus. Vet Pathol 45 (1), 51-53 PubMed.
  • Kim M K, Jee H, Shin S W et al (2007) Outbreak and control of haemorrhagic pneumonia due to Streptococcus equi subspecies zooepidemicus in dogs. Vet Rec 161 (15), 528-530 PubMed.
  • Ladlow J, Scase T & Waller A (2006) Canine strangles case reveals a new host susceptible to infection with Streptococcus equi. J Clin Microbiol 44 (7) 2664-2665 PubMed.
  • Chalker V J, Brooks H W & Brownlie J (2003) The association of Streptococcus equi subsp. zooepidemicus with canine infectious respiratory disease. Vet Microbiol 95 (1-2), 149-156 PubMed.
  • Sundberg J P, Hill D, Wyand D S et al (1981) Streptococcus zooepidemicus as the cause of septicemia in racing Greyhounds. Vet Med Small Anim Clin 76 (6), 839-842 PubMed.

Other sources of information

  • Hirsh D C & Biberstein E L (2004) Streptococcus and Enterococcus. In: Hirsh D C, MacLachlan N J & Walker R L (eds).Veterinary Microbiology. 2nd ed. Blackwell Publishing, Ames, IA,USA pp. 159-167.