Equis ISSN 2398-2977

Semen: azoospermia / oligospermia

Contributor(s): Annalisa Barrelet, John Dascanio, William Ley, Graham Munroe, Vetstream Ltd, Madeleine L H Campbell


  • Oligospermia is a deficiency of spermatozoa in ejaculated semen.
  • Azoospermia is an absolute absence of spermatozoa in an ejaculate.
  • Etiologies are similar, but the magnitude of the problem varies.
  • Cause: azoospermia = failure to ejaculate/completely blocked epididymal or deferent ducts; oligospermia = incomplete ejaculation/partial obstruction of the epididymal or deferent ducts/advanced testicular degeneration. History of medications, including treatment for fertility, use of androgenic steroids and use of an anti-GnRH vaccine may also be relevant.
  • Signs: azoospermia = absence of sperm in ejaculate; oligospermia = few sperm in ejaculate, often abnormal.
  • Diagnosis: clinical examination, semen collection and analysis +/- reproductive ultrasonography of the stallion, testicular biopsy if indicated.
  • Treatment: depends on cause.
  • Prognosis: depends on cause.


Predisposing factors


  • Reduced spermatogenesis due to testicular hypoplasia   Testis: hypoplasia, degeneration or atrophy.
  • Sexual overuse:
    • Frequency of ejaculation affects sperm numbers in successive ejaculates; however, total sperm numbers are unlikely to fall below critical threshold in a mature normal breeding stallion before its libido is affected.
    • Early in physiologic breeding season when day length increased.
  • Coverings/estrous period and decreased daily sperm production/output: at the end of the breeding season.
  • Immaturity: <3 year old stallions produce ejaculates with low spermatozoal concentrations and total numbers.
  • Senility:
    • Fertility may decline with age after 10 years of age.
    • Older stallions may develop testicular atrophy.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Rode K et al (2016) Effects of repeated testicular biopsies in adult warmblood stallions and their diagnostic potential. J Equine Vet Sci 38, 33-47 VetMedResource.
  • Turner R & McDonnell S (2003) Alkaline phosphatase in stallion semen: characterization and clinical applications. Theriogenology 60 (1), 1-10 PubMed.
  • Gehlen H et al (2001) Azoospermia due to testicular degeneration in a breeding stallion. J Equine Vet Sci 21 (3), 137-139 VetMedResource.
  • Brinsko S P (1996) GnRH therapy for subfertile stallions. Clin North Am Equine Pract 12 (1), 149-160 PubMed.
  • Love C C (1992) Ultrasonographic evaluation of the testis, epididymis, and spermatic cord of the stallion. Vet Clin North Am Equine Pract 8 (1),167-82 PubMed.

Other sources of information

  • Roser J et al (1998) Endocrine Cntrol of Tsticular Function in the Stallion; Endocrine Diagnostics. In: Proc of Stallion Reprod Symposium. American College of Theriogenology, Society for Theriogenology, AAEP. pp 22-26.