Equis ISSN 2398-2977


Contributor(s): Terry Blanchard, Cody Coyne, Prof Derek Knottenbelt, Graham Munroe, Katrin Schmallenbach, Elaine Watson


  • Testicular neoplasms of the horse occur infrequently, probably because most horses are castrated at an early age.
  • Much higher incidence in retained testicles.
  • Cause: comprised of cells derived from all three germ layers (endoderm, mesoderm, ectoderm)   →   mixed tumor.
  • Signs: unilateral testicular enlargement, pendulous scrotum.
  • Diagnosis: inguinal palpation, rectal palpation, ultrasonography. Confirmed by gross examination of testis.
  • Treatment: surgical removal.
  • Prognosis: good.



  • The origin of tumors is unknown.
  • It is postulated that the higher ambient temperature to which inguinal and abdominal testicles are exposed is a factor in the development of teratomas in these structures.
  • It has been postulated that teratomatous testicles may be congenital.

Predisposing factors

  • In contrast to humans and dogs, for horses, a relationship between cryptorchidism   Testis: cryptorchidism  and formation of testicular neoplasia has not been established.
  • It is thought that teratoma development might prevent testicular descent, rather than cryptorchidism predisposing the animal to the development of teratomas.


  • Most frequently located in the gonads in horses.
  • Benign.
  • Classified as germinal tumor, as they derive from pluripotential germ cells.
  • Complex tumor, composed of a disorderly arrangement of recognizable tissues not native to the testis (bone, cartilage, skin).
  • Only contain mature elements.
  • Neoplastic tissue may be present in late fetal life, impeding normal testicular descent into the scrotum.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Arensburg L, Olivier S, Boussawu B & De Cock H (2012) An abdominal teratoma in a yearling Irish Cob with a strangulating obstruction of the small intestine. Equine Vet Educ 24 (9), 433-436 VetMedResource.
  • Valentine B A (2009) Equine testicular tumours. Equine Vet Educ 21 (4), 177-178 VetMedResource.
  • Pollock P J, Prendergast M, Callanan J J & Skelly C (2002) Testicular teratoma in a three-year-old thoroughbred foal. Vet Rec 150 (11), 348-350 PubMed.
  • May K A, Moll D, Duncan R S, Pleasant R S & Purswell B J (1999) Unilateral Leydig cell tumour resulting in acute colic and scrotal swelling in a stallion with descended testes. Equine Vet J 31 (4), 343-345 PubMed.
  • Schumacher J (1999) Testicular neoplasia of horses - an underreported condition. Equine Vet J 31 (4), 270-272 PubMed.
  • Arighi M et al (1987) Histology of the normal and retained equine testis. Acta Anat (Basal) 129 (2), 127-130 PubMed.
  • Stick J A (1980) Teratoma and cyst formation of the equine cryptorchid testicle. JAVMA 176 (3), 211-214 PubMed.
  • Smyth G B (1979) Testicular teratoma in an equine cryptorchid. Equine Vet J 11 (1), 21-23 PubMed.