Equis ISSN 2398-2977


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


  • Testicular neoplasms of the horse occur infrequently, probably because most horses are castrated at an early age. Most common neoplasm of young, entire male horses (1-5 years old), and one of the most commonly reported neoplasms of young horses overall in 1-2 year old horses.
  • Much higher incidence in retained testicles; unclear if the tumor is the reason for the lack of normal testicular descent, or the tumor is caused by being within an undescended testicle. May occur in both descended and undescended testicles.
  • Cause: benign lesion arising from gonadal or extragonadal germ cells, comprised of cells derived from all three germ layers (endoderm, mesoderm, ectoderm) → mixed tumor.
  • Signs: unilateral testicular enlargement, pendulous scrotum. Generally slow-growing.
  • 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, as they are (rarely) seen in foals, including neonates.

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; the relationship between cryptorchidism and teratomas remains somewhat unclear.


  • 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.


  • Generally a slow-growing tumor.


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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.