Cryptorchidism in horses is a condition in which one or both testicles fail to descend into the scrotum, the external sac where the testicles are normally visible in stallions.

During fetal development, the testicles form near the kidneys and gradually descend into the scrotum through a passage called the inguinal canal. In some cases, this descent is incomplete, resulting in one or both testicles being retained within the abdomen or inguinal canal. [1]

Affected horses, often called “rigs,” may face fertility problems and behavioral issues. Cryptorchidism affects approximately 5-8% of male foals. [1][2][3]

Cryptorchidism is often concerning for horse owners as it can impact breeding and athletic prospects. Early detection and ongoing management are essential for ensuring the health and well-being of affected horses.

Testicular Descent in Stallions

Horse owners may find it helpful to familiarize themselves with how the testes descend during normal fetal development to help understand cryptorchidism.

In a healthy horse, testicular development and descent progresses as follows: [1][4]

  1. During early fetal development (5.5 weeks), the testicles begin to form near the kidneys
  2. A strand of tissue called the gubernaculum develops and attaches to the testicle and epididymis, which is the tube where sperm mature and are stored
  3. The testicles grow rapidly in size and by 8 months of gestation, are similar in size to an adult horse’s testicle
  4. Between 270 and 300 days of gestation, the testicles begin to descend, guided by the gubernaculum
  5. The testicles begin to shrink as they continue their descent. The epididymis enters the inguinal canal in the groin
  6. The gubernaculum widens the canal and pressure inside the fetal abdomen helps push the testicles through
  7. The gubernaculum regresses inside the scrotum, allowing the testicles to fully descend

Approximately 50% of foals have descended testicles at birth. In other newborn foals, the testicles continue descending until approximately 2 weeks after birth. At this point, the canal tightens, preventing the testicles from retracting back into the abdomen. [1][4]

If the testicles have not fully descended by the time to canal has fully constricted, they become trapped above the scrotum, and the affected foal becomes a rig. [1][4]

Symptoms

Recognizing the clinical signs associated with cryptorchidism is important for early detection, management, and treatment.

The following signs may indicate cryptorchidism in a horse: [4]

  • Absence of one or both testicles in the scrotum
  • Asymmetry of the scrotum (one side may appear larger or more developed)
  • Stallion behavior in a ‘gelded’ horse (mounting, aggression, increased libido, etc.)
  • Infertility

Causes

Cryptorchidism in horses is complex, with no single known cause. It is thought to result from a mix of genetic, hormonal, and mechanical factors. [4][5]

One suggested cause is that the gubernaculum may fail to regress properly within the scrotum. This in turn may disrupt the normal descent of the testicle from the inguinal canal, trapping it before it is fully descended. [6]

If the testicle does not shrink enough before entering the canal or if the gubernaculum fails to widen the canal sufficiently, it can become stuck. Consequently, the testicle remains in an abnormal position, such as within the abdomen or along the inguinal canal. [6]

Types of Cryptorchidism

Cryptorchidism can be broadly categorized into inguinal and abdominal types based on the location of the retained testicle(s).

  • Inguinal cryptorchidism: the testicle is located within the inguinal canal, which is the passage between the abdomen and the scrotum through which the testicles normally descend.
  • Abdominal cryptorchidism: the undescended testicle is entirely within the abdomen. [1][7]

Inguinal cryptorchidism is more common in horses and may be detected on external palpation, whereas abdominal cryptorchidism often requires more invasive procedures for detection and correction.

Unilateral vs. Bilateral Cryptorchidism

In unilateral cryptorchidism, only one testicle fails to descend, and the other settles as expected in the scrotum. Unilateral cryptorchidism is the most common form, accounting for 80-95% of cases. [4][5]

Inguinal retention of the right testicle is more common than in the left testicle, and abdominal retention of the left testicle (75% of cases) is more common than the right testicle.

In bilateral cryptorchidism, both testicles fail to descend into the scrotum. Bilateral retention is 2.5 times more likely to occur abdominally rather than in the inguinal canal. [2][4][5]

Type 1 (Temporary Inguinal Retention)

The testicle temporarily resides in the inguinal canal, the passage between the abdomen and the scrotum. While it typically descends into the scrotum naturally by the age of 1, in some cases, it might require surgical intervention to correct. [4]

This type of cryptorchidism is usually unilateral, more frequently found on the right side, and is more common in ponies. [4]

Type 2 (Permanent Inguinal Retention)

The testicle remains permanently lodged in the inguinal canal, failing to descend into the scrotum. This type of cryptorchidism can occur in all types of horses, particularly those with heavier or misshapen testicles.

Surgical intervention is required to correct this condition. [4]

Type 3 (Complete Abdominal Retention)

The testicle and epididymis (tube that stores and carries sperm) are entirely contained within the abdomen, failing to reach the inguinal canal or scrotum. The testicle is usually small, poorly developed and mobile in the abdomen. [4]

This condition often requires more invasive surgical procedures for correction compared to other types of cryptorchidism. [4]

Type 4 (Incomplete Abdominal Retention)

The testicle remains in the abdomen, but part of the epididymis is located in the inguinal canal. The right testicle is typically affected and in some cases, may be palpated while the horse is standing. [9]

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Risk Factors

Although cryptorchidism can occur in any male horse, some risk factors have been identified: [2][4][8]

  • Genetics: if a stallion is cryptorchid, there is an increased risk that his male offspring may also develop the condition.
  • Breed: certain breeds have a higher predisposition for cryptorchidism, including Percherons (and drafts), Saddlebreds, Quarter Horses, Ponies, Thoroughbreds and Arabians.
  • Hormone fluctuations during gestation: testicular descent is regulated by complex hormonal signals coordinating the movement of the testes. Disruptions in hormone balance during gestation can interrupt the descent pathways, resulting in cryptorchidism.

Complications of Cryptorchidism

Male horses with undescended testicles may have reproductive (breeding) and behavioral complications, even after medical intervention. Early detection and treatment provide the best chance of ensuring affected horses have minimal long term health or performance issues.

Infertility

Horses with cryptorchidism may have reduced fertility or infertility. Bilateral abdominal retention, for instance, generally renders a horse sterile as internal body temperature is much higher than in the scrotum, where the testes are usually housed. When the testicles are trapped in the abdomen’s high-temperature environment, sperm production is impaired. [10][11]

Horses with unilateral cryptorchidism, or one descended testicle, can still be fertile but usually have a reduced sperm count. However, it is recommended that stallions with cryptorchidism are not bred, as the trait is likely genetic and can be inherited by offspring. [4][11]

Testicular Tumors (Teratoma)

A teratoma is a tumor that can develop rarely in the testicles of horses during fetal development. Teratomas are composed of cell types from a different region than where they develop, can vary in size and appearance, and may be benign or, rarely, malignant. [12]

While teratomas can develop in fully descended testicles, they are most commonly found in undescended testicles. This suggests a potential link between teratoma presence and cryptorchidism, as the teratoma might impede normal descent of the testicle. However, further research on this relationship is needed. [5][13]

Teratomas generally do not cause any symptoms and are painless, so they are often discovered by chance during cryptorchidism surgery or autopsy. Fortunately, they can usually be removed surgically and the prognosis for recovery is good.

Behavioral Issues

Cryptorchidism can be associated with behavior issues in horses. Testicles can produce similar amounts of testosterone whether they are retained or not. Testosterone regulates various physiological processes, and influences sperm production and temperament. [2][10]

This means that cryptorchid horses, with retained testicles, may display behavior similar to intact stallions, including:

  • Aggression
  • Mounting
  • Difficulty training or handling

Diagnosis

Undescended testicles are often detected during routine veterinary examination early in life.

Diagnosis of cryptorchidism may include a combination of:

  • Palpation
  • Ultrasound
  • Bloodwork

Palpation

Rectal palpation is a relatively quick, straightforward, and generally accurate diagnostic method for detecting cryptorchidism in horses. [8][14]

To perform this procedure, a veterinarian inserts a gloved hand into the horse’s rectum to manually feel for the structures of the abdomen and pelvis, aiming to locate any retained testicle(s). [2]

However, rectal palpation has its limitations. In some cases, especially when the testicles are retained deep within the abdomen, they may be challenging to locate or palpate accurately. Additionally, determining the specific type of cryptorchidism through palpation alone can be difficult. [15]

The size, conformation, and temperament of the horse can complicate the procedure. Young or small horses may face a higher risk of rectal tears. Additionally, anxious or aggressive horses may need sedation to ensure safety for both the horse and the veterinarian. [4][6]

Ultrasonography

Even if a veterinarian confirms undescended testicles on palpation, they often require diagnostic imaging to guide treatment and confirm there are no further congenital defects in the reproductive tract.

Inguinal and transrectal ultrasounds are used in veterinary medicine to visualize internal structures in horses, particularly when diagnosing conditions like cryptorchidism. [2]

For inguinal ultrasonography, the ultrasound probe is used externally on and around the groin to visualize the inguinal canal. Transrectal ultrasonography involves inserting the probe into the rectum to visualize structures in the pelvic and abdominal areas. [6]

These sensitive diagnostic techniques allow veterinarians to pinpoint the specific location of undescended testicles.

Bloodwork

Blood tests are used to support diagnosis of cryptorchidism by measuring hormone levels associated with testicular function.

The following blood tests may be performed during investigation of a cryptorchid horse: [4][10][16] [17][18]

  • Testosterone: a control blood sample is taken from the horse before administering intravenous human chorionic gonadotropin (HCG), a hormone that stimulates testosterone production in the testicles. A second blood sample is taken 30 minutes to 24 hours after the HCG injection. If the testosterone level in the second sample is higher than in the pre-injection sample, it indicates the presence of functional testicular tissue.
  • Serum estrone sulfate (Ages 3+): elevated levels of estrone sulfate in a male horse can indicate the presence of mature testicular tissue, suggesting cryptorchidism.
  • Anti-Müllerian hormone (AMH):  AMH is a hormone produced by the testicles during fetal development and is responsible for the regression of Müllerian ducts, which would otherwise develop into female reproductive structures. Elevated levels of AMH can indicate the presence of testicular tissue, suggesting cryptorchidism.

Differential Diagnosis

Once cryptorchidism is suspected, the veterinarian must rule out other similar conditions to narrow down treatment options. Conditions that produce similar symptoms include: [4][19][20]

  • Monorchia (rare): the horse only has one testicle from birth, meaning the missing one is not undescended; instead, it does not exist.
  • Anorchia (rare): the horse has no testicles present at all.
  • False rig: a gelding that displays stallion-like behavior, such as aggression, mounting, and dominance, despite being successfully castrated.

Treatment

The most effective and ethical treatment for cryptorchidism in horses is surgical removal of the retained testicle, also known as a cryptorchidectomy. Medical treatments, such as hormone therapy or medications, are not recommended for resolving cryptorchidism in horses. [4]

Laparotomy

Laparotomy is a surgery where the abdomen is opened while the horse is under general anesthesia. Different approaches can be used to open the abdomen, such as cutting near the groin or on the flank. [21]

Surgeons usually access the abdominal cavity via the inguinal canal  by making an incision in the skin near the groin. As they carefully separate the layers of tissue, they locate structures like the vaginal process (a structure involved in fetal development of the reproductive tract), epididymis, and scrotal ligament. In most cases the vaginal process is opened to reveal the location of the retained testicle so it can be excised. [1]

If these structures are hard to find, the surgeon may need to explore deeper, which makes the procedure more invasive in these cases. Once the testicle is found, it is carefully removed. [6]

Standing Laparoscopy

A standing laparoscopy is a minimally invasive surgical technique used to address abdominal cryptorchidism in horses. During this procedure, the horse remains standing under local anesthesia. [6]

Laparoscopy involves making a small incision in the flank area through which a laparoscope, a thin tube with a camera and light source, is inserted into the abdomen. This allows the surgeon to explore the abdominal cavity to locate the undescended testicle(s).

Once the undescended testicle is identified, specialized instruments are inserted through additional small incisions to assist in its removal. [6]

While this approach effectively treats abdominal cryptorchidism, there is debate surrounding the removal of an inguinal testicle this way, as it may involve cutting the inguinal ring, potentially leading to hernias. [6]

Prognosis

After surgical removal of both testes, the prognosis for cryptorchid horses’ overall health is generally good. Horses that undergo a bilateral cryptorchidectomy become sterile shortly after the procedure. If only one testicle is removed, the horse may retain some fertility, but is likely to have reduced reproductive potential. [11]

Most vets recommend removing both testicles from cryptorchid horses, as breeding these individuals is not advised. [11]

Summary

Cryptorchidism occurs in male horses when one or both testicles fail to descend into the scrotum during fetal development or shortly after birth.

  • Cryptorchidism affects approximately 5-8% of male foals and is more common in certain breeds like Percherons, Saddlebreds, and Quarter Horses
  • The condition is classified into inguinal and abdominal types based on the location of the retained testicle(s)
  • Complications linked to undescended testicles include infertility, testicular tumors and behavioral issues
  • Diagnosis can be made through rectal palpation, ultrasonography and blood tests
  • Surgical removal (cryptorchidectomy) is the recommended treatment method
  • Horses become sterile following surgery, but the prognosis for health and quality of life is good

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References

  1. Trotter G.W., Normal and Cryptorchid Castration. Veterinary Clinics of North America: Equine Practice. 1988. View Summary
  2. Straticò P. et al., A Retrospective Study of Cryptorchidectomy in Horses: Diagnosis, Treatment, Outcome and Complications in 70 Cases. Animals (Basel). 2020. View Summary
  3. Murase H. et al., A Case of Equine Cryptorchidism with Undetectable Serum Anti-Müllerian Hormone. J Vet Med Sci. 2020. View Summary
  4. Lofstedt R. et al., Testis: cryptorchidism in Horses (Equis). Vet Lexicon.
  5. Hayes H.M., Epidemiological Features of 5009 Cases of Equine Cryptorchidism. Equine Vet J. 1986. View Summary
  6. Schambourg M.A. et al., Use of Transabdominal Ultrasonography to Determine the Location of Cryptorchid Testes in the Horse. Equine Veterinary Journal. 2010. View Summary
  7. Amann R.P. and Veeramachaneni D.N.R., Cryptorchidism and Associated Problems in Animals. International Symposium on Animal Biology of Reproduction. 2006.
  8. Hartman R. et al., Cryptorchidectomy in Equids: 604 Cases (1977–2010). Journal of the American Veterinary Medical Association. 2015.
  9. Cryptorchid (Rig) Fact Sheet. The University of Edinburgh.
  10. Starkley L.J. and Espy B., Tales from the Crypt (Cryptorchid Horses). AAEP.
  11. Tibary A., Congenital and Inherited Disorders of the Reproductive System in Horses – Horse Owners. Merck. 2019
  12. Leonardi L. et al., Dentigerous Equine Teratoma in a Stallion: Surgical Management and Clinicopathology. Vet Sci. 2021. View Summary
  13. Blanchard T. et al., Teratoma in Horses (Equis). Vet Lexicon.
  14. Pollock P., Approach to the Cryptorchid Horse. In Practice. 2017.
  15. Searle D. et al., Equine Castration: Review of Anatomy, Approaches, Techniques and Complications in Normal, Cryptorchid and Monorchid Horses. Aust Veterinary J. 1999.
  16. Barrelet A. and Taylor F., Blood: biochemistry – glucose in Horses (Equis). Vet Lexicon.
  17. Illera J.C. et al., Amplified Androstenedione Enzymeimmunoassay for the Diagnosis of Cryptorchidism in the Male Horse: Comparison with Testosterone and Estrone Sulphate Methods. J Steroid Biochem Mol Biol. 2003. View Summary
  18. Murase H. et al., Anti-Müllerian Hormone as an Indicator of Hemi-Castrated Unilateral Cryptorchid Horses. J Equine Sci. 2015. View Summary
  19. Turner R.M.O., CHAPTER 30 – Testicular Abnormalities. Current Therapy in Equine Reproduction. W.B. Saunders. 2007.
  20. Cox J.E., Behaviour of the False Rig: Causes and Treatments. Vet Rec. 1986. View Summary
  21. Cryptorchidism in the horse. Government of Ontario: Ministry of Agriculture, Food, and Affairs. 2022.