Equis ISSN 2398-2977

Penis: erection failure/abnormality

Contributor(s): Terry Blanchard, Rob Lofstedt, Graham Munroe, Elaine Watson, Madeleine L H Campbell


  • Cause: can be a consequence of a lack of libido Male: lack of libido Male: libido/mating ability evaluation or arise from physical injury, vasculogenic or neurogenic problems.
  • Signs: failure to achieve normal erection, or loss of erection before ejaculation.
  • Diagnosis: history, observation of mating behavior.
  • Treatment: depends on cause.
  • Prognosis: depends on cause.



  • Psychogenic:
    • Lack of libido.
  • Non-psychogenic:
    • Vascular disease, eg aortoiliac disease which compromised perfusion of the pelvic organs, penis and hindlimbs. May result in partial or deviated erection.
    • Neurological compromise, eg following a breeding accident or other genital injury which affects the mechanisms controlling erection.

Predisposing factors


  • Lack of libido Male: lack of libido.
  • In horses with good libido the most common cause of erection failure is injury at breeding Penis: trauma or misuse of stallion aids to prevent masturbation, eg stallion rings, brush attached to penis and occlusion of vascular supply. Tail hairs from the mare can have a 'cheese wire' effect and cut the penis. An artificial vagina which is too hot, too tight or used inappropriately can damage the penis.
  • Phenothiazine-derived penile prolapse or priapism Penis: paralysis/priapism caused by phenothiazine tranquilizers Etorphine hydrochloride which can cause blocked (thrombosed) corpus cavernosum penis (CCP). Stallions may subsequently regain some ability to retract penis, but not be able to achieve full engorgement of CCP with sexual stimulation.
  • Paraphimosis Penis: paraphimosis due to preputial trauma will inevitably lead to edema, vascular compromise (which can result in absence of/abnormal erection) and may ultimately cause ischemia.
  • Ruptured penile suspensory apparatus.


  • Excessive teasing, overuse and poor management can all cause psychogenic failure.
  • Fear or pain resulting from encounters with difficult violent mares or handlers.
  • Low circulating luteinizing hormone, testosterone and oestradiol 17B have been implicated by some authors but have not been adequately demonstrated.
  • Premature tumescence may be associated with generalized neurological problems in older stallions, and accompanied by signs of neurological dysfunction elsewhere in the pelvic area, eg reduced anal and bladder tone.


  • Erection is the lengthening and stiffening of the penis which, in the stallion, results from engorgement with blood of the corpus cavernosum penis and, subsequently, the corpus spongiosum penis. Normal, physiological erection occurs as follows:
    • Psychogenic stimulation initiates responses in the cerebrum, eg air passing over the vomeronasal organ during teasing Nose: Flehmen response; visualization of the mare/dummy mare.
    • Sensory stimulation of the glans penis also initiates erection directly.
    • Parasympathetic impulses pass from the second, third and fourth sacral segments of the spinal cord to the splanchnic nerves and hence to the penis.
    • This parasympathetic stimulation allows dilation of the penile arterioles by overriding the sympathetic stimulus which, in a resting state, keeps the arterioles in the penis partly constricted.
    • Blood flow into the corpus cavernosum and corpus spongiosum increases.
    • Concurrently, the extrinsic muscles of the penis contract, pushing the deep and dorsal veins of the penis against the ischial arch, and inhibiting venous return from the corpus cavernosum and corpus spongiosum.
    • Consequently, the corpus cavernosum and corpus spongiosum are filled due to the ‘shunting’ effect of simultaneous increased blood flow to those structures and decreased venous return from them.
    • Cardiac output may also increase.
    • Erection subsides when sympathetic impulses cause the arterioles to constrict, and pressure on the veins if relieved by the extrinsic muscles of the penis relaxing.
  • Disruption of any part of this normal process, eg due to psychogenic factors; injury; vascular of neurological abnormalities, causes failure or abnormality of erection.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Pozor M (2005) Diagnostic applications of ultrasound to stallion's reproductive tract. Theriogenology 64 (3), 505-509 PubMed.
  • Watson E (1997) Fertility problems in stallions. In Pract 19 (5), 260-269 VetMedResource.

Other sources of information

  • McDonnell S M (2011) Specific Erection and Ejaculatory Dysfunctions. Equine Repro 1 (2), 1419-1422.
  • McKinnon A O, Squires E L, Vaala W E & Varner D V (2011) Specific Erection and Ejaculatory Dysfunctions. In: Equine Reproduction. 2nd edn. Blackwell Publishing Ltd. ISBN: 0813819717. pp 1419-1422.
  • McDonnell S M, Blanchard T L & Varner D D (1992) Normal and Abnormal Sexual Behavior. Vet Clin North Am Equine Pract 8 (1), 71-89.
  • Colahan P T et al (1991) Equine Medicine and Surgery. 4th edn. American Veterinary Publications, Inc. ISBN: 0939674270. pp 884 (concise summary of main points).