ISSN 2398-2977      

Penis: bacterial colonization



  • Bacteria are commonly isolated from the penis; harmless commensals must be differentiated from agents of venereal disease.
  • Cause: significant organisms (agents of venereal, ie contagious disease) are Klebsiella pneumoniae, Pseudomonas aeruginosa and Taylorella equigenitalis.
  • Signs: subclinical infection or colonization on external genitalia of stallions. Most commonly, stallion is lesionless carrier. Rarely, a stallion may have internal accessory sex gland infection with Pseudomonas resulting in purulent material being present in ejaculates.
  • Diagnosis: microbiologic culture of swabs from appropriate sites.
  • Treatment: removal of smegma; administration of antimicrobials if necessary.
  • Prognosis: good with appropriate therapy.



  • Normal microflora of the stallion's external genitalia rarely produce reproductive tract infection in immunologically competent mares.
  • Bacteria that may cause individual cases of acute endometritis in in-contact mares, usually prone to repeat uterine infections:
  • Bacteria that may cause outbreaks of contagious acute endometritis in in-contact mares:

Predisposing factors


  • Normal bacterial flora of penis influenced by:
    • Flora in local environment.
    • Fecal contamination.
    • Housing.
    • Breeding management.


  • Breeding to a contaminated mare.
  • Removal of commensal bacteria by overzealous washing with antiseptic solutions predisposes stallion to infection with potentially pathogenic bacteria.
  • Iatrogenic transfer between stallions or stallion/mare via breeding area, personnel or equipment. Poor hygiene.
  • Parasitism of the penis and prepuce predispose to secondary bacterial infection.


  • Organisms can be transmitted to the mare's reproductive tract at time of breeding.
  • Most bacteria colonizing the penis and prepuce are skin commensals, eg coagulase-negative staphylococci, alpha-hemolytic streptococci and coryneforms.
  • There is unavoidable contamination of the mare's reproductive tract with such organisms.
  • Commenal organisms do not normally cause disease in mares, but some pathogenic organisms, eg Streptococcus zooepidemicus and E. coli, can cause occasional cases of endometritis in susceptible mares bred to the stallion.
  • K. pneumoniae, P. aeruginosa and T. equigenitalis are true venereal pathogens, ie are contagious between stallion and mare and vice versa, and can cause venereal disease in mares.
  • K. pneumoniae, P. aeruginosa, T. equigenitalis → no lesions or systemic disease in stallions (K. pneumoniaemay occasionally cause urethral lesions → hemospermia) → mechanical transmission by smegma → bacterial endometritis Uterus: endometritis - bacterial or contagious equine metritis Uterus: contagious equine metritis in mares. Pseudomonas aeruginosa may occasionally cause infection of accessory sex glands of stallion → resultant purulent material in ejaculates. These organisms persist in the smegma of carrier stallions.
  • Commensals and normal flora in smegma discourage growth of pathogenic bacteria.
  • Disruption of normal flora may result in potentially pathogenic bacteria colonizing penis and prepuce.
  • Bacteria can be spread in fresh, chilled or frozen semen being used for artificial insemination despite the inclusion of antibiotics in semen extenders.


  • Usually persistent colonization or infection.


  • Commensal bacteria are obtained from the environment (especially soil/water) or fecal contamination.
  • Other bacteria may be transmitted, particularly between stallions and mares, especially the agents that can cause contagious endometritis: K. pneumoniae, P. aeruginosa and T. equigenitalis.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Pasing S S et al (2013) Development of the genital microflora in stallions used for artificial insemination throughout the breeding season. Anim Repro Sci 139 (1), 53-61 PubMed.
  • Schulman M L, May C E, Keys B & Guthrie A J (2013) Contagious equine metritis: artificial reproduction changes the epidemiologic paradigm. Vet Microbiol 167 (1-2), 2-8 PubMed.
  • Erdman M M et al (2011) Diagnostic and epidemiologic analysis of the 2008–2010 investigation of a multi-year outbreak of contagious equine metritis in the United States. Prev Vet Med 101 (3-4), 219-228 PubMed.
  • Rota A et al (2011) Presence and distribution of fungi and bacteria in the reproductive tract of healthy stallions. Theriogenol 76 (3), 464-470 PubMed.
  • Timoney P J et al (1996) Contagious equine metritis. Comp Immunol Microbiol Infect Dis 19 (3), 199-204 PubMed.
  • Timoney P J et al (1985) Contagious equine metritis, experimental infection in the donkey. Vet Microbiol 10 (3), 259-268 PubMed.
  • Dolan M et al (1984) Serological and bacteriologic survey of three horse studs for contagious equine metritis. Aust Vet J 61 (1), 17-19 PubMed.
  • Timoney P J et al (1982) Isolation of the the contagious equine metritis organism from colts and fillies in the United Kingdom and Ireland. Vet Rec 111 (21), 478-482 PubMed.
  • Timoney P J et al (1982) CEM (contagious equine metritis) in the republic of Ireland. Vet Rec 111 (17), 400-401 PubMed.
  • O'Brien J J et al (1982) CEM (contagious equine metritis) in Northern Ireland. Vet Rec 111 (17), 400 PubMed.
  • Rommel F A et al (1981) Contagious equine metritis; antibody response of experimentally infected pony mares. Vet Immunol Immunopathol 2 (3), 201-213 PubMed.
  • Sahu S P et al (1980) CEM bacteria. Vet Rec 107 (18), 432 PubMed.
  • Sahu S P et al (1980) Contagious equine metritis; isolation and characterization of the etiologic agent. Am J Vet Res 41 (9), 1372-1382 PubMed.
  • Chandler N et al (1979) Swabbing mares and stallions for contagious equine metritis. Vet Rec 105 (24), 561 VetMedResource.
  • Falconer-Taylor R A et al (1979) CEM sampling. Vet Rec 104 (25), 585 PubMed.
  • Swerczek T W et al (1979) Contagious equine metritis-outbreak of the disease in Kentucky and laboratory methods for diagnosing the disease. J Repro Fertil Suppl 27, 361-365 PubMed.
  • Swaney L M et al (1978) CEM; bacteriologic methods. Vet Rec 102 (2), 43 PubMed.

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