Equis ISSN 2398-2977

Testis: cryptorchidism

Synonym(s): Rig

Contributor(s): Prof G Barrie Edwards, Jean Pierre Held, Rob Lofstedt, Graham Munroe




  • Uncertain.
  • Suggestions include:
    • Improper function of the gubernaculum particularly regression.
    • Genetic predisposition - complex and involves multiple genes.
    • Abnormal fetal and/or maternal hormones.
    • Combined with other congenital abnormalities such as intersexuality   Reproduction: gonadal dysgenesis  .
    • Persistence of suspensory ligament of testis preventing migration.
  • Failure of testicle to regress to a sufficiently small size to transverse the vaginal ring including testicular teratoma   Teratoma  .

Predisposing factors

  • Cryptorchidism in sire.
  • History of cryptorchidism in lineage of either parent.
  • Other congenital or developmental defects.
  • Cross-bred horses.
  • Ponies.


  • Failure of one or both testes to descend to scrotal position.
  • Cause uncertain, may be genetic, hormonal or multifactorial.
  • Note that the epididymis does not produce testosterone, and retention of epididymal tissue cannot account for persistence of stallion-like behavior.
  • Usually unilateral (5-20 (15)% bilateral).

Normal descent of the testis

  • Testes develop at the caudal pole of the kidney at approximately 5.5 weeks of gestation.
  • Suspended by mesorchium (fold of peritoneum) from the dorsal abdominal wall.
  • Attached to body wall by mesenchyme which becomes gubernaculum, which guides descent through the inguinal canal and then becomes the epididymal ligament.
  • From 6 weeks of gestation the testicle increases rapidly in size and by 8 months is as large as an adult testicle.
  • Descent begins at 270-300 days gestation.
  • Epididymis enters the canal first as the gubernaculum grows and dilates the vaginal ring and canal.
  • Increased fetal abdominal pressure helps push the testicle through the vaginal ring.
  • The gubernaculum is within the scrotum and can be mistaken at birth for a testicle. This gradually reduces allowing complete descent of the testicle into the scrotum.
  • Left testicle slightly larger than right on average with possibilities of increased left sided abdominal retention (75%) and right sided inguinal retention (60%).
  • At birth about 50% of foals have descended testes.
  • By 2 weeks after birth, the vaginal ring contracts preventing retraction of testicle into abdomen.
  • But descent into the scrotum may not be complete until 1 month (up to 4 years).
  • Testicular descent after 6 months of age is rare.


  • Some resolve spontaneously by the age of 3 years; others permanent.


  • May be a genetic predisposition.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Brommer H, Grinwis G C M, Van Loon V & Ensink J M (2011) Laparoscopic-assisted diagnosis of anomalous unilateral abdominal cryptorchidism. Equine Vet Educ 23 (8), 391-395 VetMedResource.
  • Barakzai S & Perkins J (2006) Equine cryptorchidism. UK Vet 11 (4), 5-9 VetMedResource.
  • Rakestraw P (2006) The value of laparoscopy in equine cryptorchidism and monorchidism. Equine Vet Educ 18 (2), 88-89 VetMedResource.
  • Mariën T et al (2001) Laparoscopic testis-sparing herniorrhaphy: A new approach for congenital inguinal hernia repair in the foal. Equine Vet Educ 13 (1), 32-35 VetMedResource.
  • Parks A H, Scott E A, Cox J E & Stick J A (1989) Monorchidism in the horse. Equine Vet J 21 (3) 215-217 PubMed.
  • Trotter G W (1988) Normal and cryptorchid castration. Vet Clin North Am Equine Pract (3) 493-513 PubMed.
  • Cox J E, Redhead P H & Dawson F E (1986) Comparison of the measurement of plasma testosterone and plasma estrogens for the diagnosis of cryptorchidism in the horse. Equine Vet J 18 (3) 179-182 PubMed.
  • Hayes H M (1986) Epidemiological features of 5009 cases of equine cryptorchidism.Equine Vet J 18 (6) 467-471 Wiley Online Library.
  • Wilson D G & Nixon A J (1986) Case of equine cryptorchidism resulting from persistence of the suspensory ligament of the gonad. Equine Vet J 18 (5) 412-413 PubMed.
  • Cox J E, Edwards G B & Nea P A (1979) An analysis of 500 cases of equine cryptorchidism. Equine Vet J 11, 113 PubMed.