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.