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Uterus: endometritis - bacterial


Synonym(s): Bacterial endometritis


  • Bacterial endometritis is probably the most significant cause of reduced reproductive performance in mares.
  • Cause: bacterial contamination combined with reduced uterine defenses or clearance mechanisms.
  • Signs: subfertility or infertility; vulval discharge.
  • Diagnosis: clinical signs, vaginoscopy, endometrial swabs, smears and/or biopsy.
  • Treatment: remove predisposing factors; antimicrobial therapy.
  • Prognosis: fair; some cases intractable.



Predisposing factors




  • Endometritis results from contamination of the reproductive tract, combined with a failure of uterine defense or clearance mechanisms.
  • Bacteria generally considered as either venereal pathogen, or non-pathogenic opportunistic (see Etiology above).
  • Mating, artificial insemination, parturition or veterinary procedures all causes acute endometritis characterized by polymorphonuclear infiltration of the endothelium. This is transient in genitally healthy mares, but in mares with compromised uterine defense or clearance mechanisms it can cause persistent endometritis → subfertility or infertility.
  • Mating (whether by 'natural service' or AI) → unavoidably deposits pathogenic or commensal bacteria directly into uterus from stallion's penis Penis: bacterial colonization → bacterial contamination of uterus via cervical canal.
  • In young mares → uterine defenses → rapid inflammatory response → clears the contamination caused by mating within 4-5 days allowing the embryo to survive when it descends into the uterus at c6 days post-ovulation.
  • In older mares, or those with history of endometritis or predisposing factors, less intense and more persistent inflammatory response to breeding occurs, so that the uterus is persistently inflamed/contaminated when the embryo descends into it Reproduction: persistent post-breeding endometritis.
  • Generally, mares gradually lose ability to combat bacterial contamination with age and multiparity, due to compromise in uterine clearance, and repeated insults to the reproductive tract at mating and parturition.
  • Recent work suggests that in some mares, chronic bacterial endometritis is attributable to the development of 'biofilm', ie an extracellular matrix produced by a community of bacteria. The biofilm increases resistance to antimicrobial treatment by physically reducing penetration of the drug, and by reducing the metabolism of bacteria. The presence of a biofilm ultimately allows a subpopulation of bacteria which are resistant to antimicrobial drugs to persist in the uterus.


  • Acute onset, may become chronic or recurrent.


  • Bacterial contamination occurs at mating, during parturition, or due to fecal contamination of vagina associated with poor perineal conformation.
  • Streptococcus zooepidemicus Streptococcus spp, is a common skin saprophyte on external genitalia →  introduced in pneumovagina, breeding, foaling, and veterinary procedures.
  • E. coli Escherichia coli, endometritis is usually associated with anatomical defects predisposing to pneumovagina and fecal contamination.
  • Pseudomonas aeruginosa Pseudomonas aeruginosa may be present in large numbers in the ejaculate of healthy stallions.
  • Klebsiella pneumoniae Klebsiella pneumoniae may be a true venereal infection, as it can cause lesions in stallions. The organism also persists in the clitoral fossa and caudal vagina of mares.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Ferris R A (2014) Bacterial endometritis: a focus on biofilms. Clinical Theriogenology 6 (3), 315-319 VetMedResource.
  • MacPherson M L (2013) What to do with the problem mare: new approaches for diagnosis and management. In: Proc Annual Symposium of the AAEP. pp 105-118.
  • LeBlanc M M (2010) Advances in the diagnosis and treatment of chronic infectious and post-mating-induced endometritis in the mare. Repro Domest Anim 45 Suppl 2, 21-27 PubMed.
  • Ghasemzadeh-nava H et al (2004) A review of mare endometritis in Iran. J Equine Vet Sci 24 (5), 188-192 VetMedResource.
  • Nikolakopoulos E & Watson E D (1999) Uterine contractility is necessary for the clearance of intrauterine fluid but not bacteria after bacterial infusion in the mare. Theriogenology 52 (3), 413-423 PubMed.
  • Troedsson M H (1999) Uterine clearance and resistance to persistent endometritis in the mare. Theriogenology 52 (3), 461-471 PubMed.
  • Katila T (1996) Uterine defence mechanisms in the mare. Anim Repro Sci 42, 197-204 VetMedResource.
  • Pycock J F & Newcombe J R (1996) The relationship between intraluminal uterine fluid, endometritis and pregnancy rate in the mare. Eq Pract 18 (6), 19-22.
  • Ley W B (1994) Treating endometritis in mares. Vet Med 89 (8), 778-788 VetMedResource.
  • Ricketts S W & Alonso S (1991) The effect of age and parity on the development of equine chronic endometrial disease. Equine Vet J 23 (3), 189-192 PubMed.
  • Ley W B, Ochs D L, Metclaf E S et al (1990) Dimethly sulfoxide intrauterine therapy in the mare - effect upon uterine-derived neutofil function. Theriogenology 33 (6), 1177-1189 ScienceDirect.
  • Ley W B, Bowen D P, Sponenberg et al (1989) Dimethly suloxide intrauterine therapy in the mare - effects upon endometrial histologic features and biopsy classification. Theriogenology 32(2), 263-276 PubMed.
  • Roszel J F & Freeman K P (1988) Equine endometrial cytology. Vet Clin North Am Equine Pract 4 (2), 247-262 VetMedResource.

Other sources of information

  • Horserace Betting Levy Board (2017) Codes of Practice. 5th Floor, 21 Bloomsbury Street, London WC1B 3HF, UK. Tel: +44 (0)207 333 0043; Fax: +44 (0)207 333 0041; Email:; Website:
  • Troedsson M H T (2011) Endometritis. In: Equine Reproduction. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. Wiley-Blackwell, UK. pp 2608-2619.
  • Brinsko S P, Rigby S L, Varner D D & Blanchard T L (2003) A Practical Method for Recognizing Mares Susceptible to Post-breeding Endometritis. In: Proc 49th AAEP Convention. pp 363-365.

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