Testis: cryptorchidism in Horses (Equis) | Vetlexicon
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Testis: cryptorchidism

ISSN 2398-2977


Synonym(s): Rig

Introduction

Presenting signs

  • Failure of one or both testes to descend into normal position in scrotum.
  • Location of retained testis may be in the abdomen, inguinal canal or subcutaneous.
  • Type 1: temporary inguinal retention.
  • Type 2: permanent inguinal retention.
  • Type 3: complete abdominal retention.
  • Type 4: incomplete abdominal retention.

Geographic incidence

  • Worldwide.

Age predisposition

  • 1-3 years (at diagnosis).
  • 4-9 years (at diagnosis).
  • <1 year.
  • >10 years.

Gender predisposition

  • Male.

Breed/Species predisposition

Cost considerations

  • Surgical castration.
  • Investigation.

Pathogenesis

Etiology

  • 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

General

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

Pathophysiology

  • 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 Behavior: stallion.
  • 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 from the inguinal canal back 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.

Timecourse

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

Epidemiology

  • May be a genetic predisposition.

Diagnosis

Presenting problems

Client history

Clinical signs

  • Right and left testicular retention - equal frequency.
  • Unilateral retention: 9 times more than bilateral.
  • Bilateral abdominal: 2.5 times bilateral inguinal retention.
  • Incidence in ponies is different with equal right and left abdominal testicle incidence; temporary inguinal retention on the right side in young horses; and more permanent on the left than right in older horses.

Type 1 - Temporary inguinal retention

  • More common in ponies.
  • Usually unilateral; often right-sided.
  • Testis small; close to superficial inguinal ring.
  • Usually descends spontaneously by 1 year of age.

Type 2 - Permanent inguinal retention

  • All types of horses.
  • Testis located within inguinal canal.
  • More difficult to palpate.
  • Testes weigh generally more than 40 g and may be misshapen.

Type 3 - Complete abdominal retention

  • Testis and epididymis lie completely within abdomen.
  • Vaginal process usually relatively small.

Type 4 - Incomplete abdominal retention

  • Tail of epididymis in inguinal canal.
  • Vaginal process usually well-developed.
  • May be palpated standing in some cases.
  • More often right-sided.
  • One or no testicles in normal position in scrotum.
  • Palpation of retained testicles (under sedation if necessary).
  • Inspect carefully for scrotal scar of castration.

Diagnostic investigation

Serology

Ultrasonography

Other

  • Rectal palpation Urogenital: rectal palpation: difficult to palpate abdominally retained testicle.
  • Identify vaginal ring by palpation. Usually much easier if testicle has descended.
Danger of rectal tears Rectum: tear is increased in male young horses - take care.

Confirmation of diagnosis

Discriminatory diagnostic features

Definitive diagnostic features

  • Hormone assay Endocrine: testosterone assay: very useful in horse with no history to confirm presence of one or two retained testes, or that horse has been previously castrated.

Gross autopsy findings

  • Bilateral or unilateral retention of testicles in abdomen or inguinal canal.
  • Combined with other developmental abnormalities.

Differential diagnosis

  • "False rig": gelding (true castrate) showing stallion-like behavior Behavior: stallion.
  • Monorchid: only one testis has developed.
  • Anorchid: neither testis has developed.

Treatment

Standard treatment

Surgical relocation of the retained testicle into the scrotum is unethical and risks perpetuating a heritable condition.
  • Medical therapy with repeated injection of human chorionic gonadotropin is not effective.
  • Immunocastration has been reported, involving immunization against luteinizing hormone releasing hormone to decrease testosterone levels.
Medical treatment for cryptorchidism is not advocated.

Monitoring

Subsequent management

Treatment

Prevention

Control

  • Do not breed from cryptorchid horses.
  • Do not unilaterally castrate horses with one testicle absent.

Outcomes

Prognosis

  • Good for general health after surgery.
  • Horse will be infertile shortly after both testes are removed.

Expected response to treatment

  • Behavior modification after castration.

Reasons for treatment failure

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Straticò P, Varasano V, Guerri G et al (2020) A retrospective study of cryptorchidectomy in horses: diagnosis, treatment, outcome and complications in 70 cases. Animals (Basel) 10 (12), 2446 PubMed.  
  • Hartman R, Hawkins J F, Adams S B et al (2015) Cryptorchidectomy in equids: 604 cases (1977-2010). JAVMA 246 (7), 777-784 PubMed. 
  • 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.
  • Raś A, Rapacz A, Raś-Noryńska M & Janowski T E (2010) Clinical, hormonal and ultrasonograph approaches to diagnosing cryptorchidism in horses. Pol J Vet Sci 13 (3), 473-477 PubMed. 
  • 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 WileyOnline.
  • 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.