Cryptorchidism – The “Hidden” Testicle

By Garrett Metcalf, DVM

Cryptorchidism is not an issue that everyone faces but might have heard about. Horses with at least once undescended testicle have been called “rig” or “ridgling” which is still used in the horse world. This condition can be problematic for horse owners and buyers leading to unwanted expense, stallion behavior and decreased fertility if a breeding stallion is desired plus ethical concerns in breeding animals.  Although cryptorchidism has a fairly low incidence in horses, they are the most commonly effected among the domesticated species.

 Cryptorchidism is when one or both testes are retained in the abdomen of a horse that never descended properly. Testicles are formed in utero behind the kidney within the abdomen and begin descended from the abdomen into the inguinal canal which is the tube like structure that allows the testicles to leave the abdomen and to pass to the scrotum.  Around day 270 to 300 is when this process begins but the testicles are not always in the scrotum at birth. There are many factors that can lead to failure of the testicles to descend but it is thought to be related to hormone, mechanical and hereditary causes. Although the retained testicle(s) are not able to produce viable sperm for breeding it does produced testosterone that will give the horse stallion behaviors and characteristics. From an ethical stand point cryptorchidism is considered a hereditary trait and it is recommended not to continue breeding of sires that contain a retained testicle.

Cryptorchid horses will often display signs of stallion like behavior and of course lack evidence of one or both testicles. They will be sexually mature if they are the correct age and will attempt breeding, but if both testicles are retained they will be infertile because of the effects of body temperature on the sperm. Some geldings will display stallion like behavior even without testicular tissue especially if they were castrated later in life and have bred in the past. This learned behavior and ingrained characteristics are sometimes difficult to eliminate even when testosterone levels are very low like a gelding should be after castration.  Monorchidism is when a stallion has only one developed functioning testicle. This is really rare but has been described in horses and experienced in cases when trying to diagnose the presents of a possible retained testicle.

There are several ways to confirm or diagnose if a horse has a cryptorchid testicle or not. With just a simple history can generally answer that question especially if there is one owner and the horse has never had surgical castration before. It’s more difficult if the horse was purchased or if a surgical attempt has been made and only one testicle was removed as an example for a unilateral cryptorchid stallion. Retained testicles can be diagnosed with hormonal assays such as testosterone blood levels or other hormone tests like estrone sulfate or anti-mullerian hormone. Rectal palpation with or without the use of ultrasound can be used to diagnose the presents of an intra-abdominal testicle. This is often necessary to determine which side to do surgery on if the horse had a previous castration of the one descended testicle. Other methods are surgical exploration or with laparoscopic instruments to visualized the retained testicle using minimally invasive techniques.

Surgical removal of a retained testicle can be performed using several different approaches. The most common way to approach a retained testicle is through an inguinal approach that uses the same tunnel or inguinal canal that the testicle passes through from the abdomen to the scrotum. This approach is best used when the correct side the retained testicle is on can be confirmed by methods listed before or if there is only one retained testicle to confirm which side. If both testicles are retained this approach is still used when accurate history ensures that the horse has not had an attempted castration that failed, has hormone assay that determines cryptorchidism or both testicles can be determined to be in the abdomen. Another approach is making a small incision into the abdomen either with a standing sedated horse using local anesthesia or under general anesthesia. This allows access to the area near the inguinal rings were the testicle passes through. Lastly laparoscopic techniques allow rather small incisions to pass a camera and other instruments to aid in removing the retained testicle. Laparoscopic techniques is the most superior way to explore for a retained testicle when it cannot be found with other means and there is no information to determine if it is present or not.

Read more in the March issue of Oklahoma Farm & Ranch.