Equis ISSN 2398-2977

Semen: bacteriospermia

Contributor(s): Sarah Binns, Jean Pierre Held, Rob Lofstedt, Graham Munroe, Elaine Watson, Madeleine Campbell


  • Bacteria found in semen are often contaminants, particularly since the penis and prepuce harbor a normal bacterial flora - these must be differentiated from pathogens.
  • Cause: many potential agents, including Taylorella equigenitalis, Klebsiella pneumoniae, Pseudomas aeruginosa (for these three bacteria, see the HBLB Codes of Practice), Streptococcus zooepidemicus, E. coli, Actinobacter spp. Not all of these organisms affect fertility; those that do, do so through causing post-breeding bacterial endometritis in mares.
  • Signs: subclinical infection unless specific infection of reproductive tract.
  • Diagnosis: microbiologic examination of swabs and ejaculate; identification of site of infection.
  • Treatment: rest from mating, antimicrobials, specific treatments for reproductive tract infections.
  • Prognosis: fair, depending on organism and site of infection.
  • Management: use of semen extender containing antimicrobials.



Predisposing factors


  • Normal commensals on penis.
  • Poor hygiene in environment.
  • Poor management, eg failure to clean sheath and penis or overzealous washing with antiseptic solutions which removes commensal bacteria and facilitates overgrowth of pathogenic bacteria.
  • Ascending infection from urethritis.


  • Bacterial infection elsewhere in urogenital tract.


  • Bacteriospermia may be caused by contaminating commensals or infection by potential pathogens or infections of the urogenital and reproductive tract.
  • Many bacteria in semen are commensals, which grow in penile and preputial smegma → normal flora discourages growth of pathogens Penis: bacterial colonization.
  • Bacteriospermia is rarely associated with urethritis and/or cystitis Bladder: cystitis - bacterial, or in extremely unusual cases, epididymitis, orchitis Testis: orchitis - bacterial, seminal vesiculitis, or ampullitis.
  • Bacteriospermia is rarely associated with clinical signs or lesions in the male, except that urethral lesions caused by K. pneumoniae may lead to hemospermia Semen: hemospermia.
  • Most mares do not show pathology; they cope with most potentially pathogenic bacteria (non-susceptible).
  • T. equigenitalis, Klebsiella pneumoniae and Pseudomonas aeruginosa infections can lead to endometritis in susceptible and non-susceptible mares. See HBLB Codes of Practice in the UK Reproduction: venereal disease examination.
  • Susceptible mares can become infected with bacteria shed in the sperm of covering stallions.


  • Often chronic or recurrent contamination.


  • Bacteriospermia is usually detected when mares mated to the stallion repeatedly return to estrus with acute bacterial endometritis Uterus: endometritis - bacterial.
  • Semen is almost inevitably contaminated with bacteria.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Crabtree J (2010) Prebreeding examination of the stallion. In Pract 32, 22-28 InPractice.
  • Samper J C & Tibary A (2006) Disease transmission in horses. Theriogenology 66 (3), 551-559 PubMed.
  • Kristula M A & Smith B I (2004) Diagnosis and treatment of four stallions, carriers of the contagious metritis organism--case report. Theriogenology 61 (2-3), 595-601 PubMed.
  • Watson E D (1997) Swabbing protocols in screening for contagious equine metritis. Vet Rec 140 (11), 268-271 PubMed.
  • Watson E D (1997) Fertility problems in stallions. In Pract 19, 260-269 VetMedResource.
  • Madsen M & Christensen P (1995) Bacterial flora of semen collected from Danish Warmblood stallions by artificial vagina. Acta Vet Scan 36 (1), 1-7 PubMed.
  • Brinsko S P, Varner D D, Blanchard T L et al (1992) Bilateral infectious epididymitis in a stallion. Equine Vet J 24 (4), 325-328 PubMed.