Cryptorchids are intact males with hidden testes. While the condition isn’t unique to horses, it is more prevalent in them than in any other domesticated species. Approximately 3-4% of colts are cryptorchids (also called ridglings or “rigs”), with higher incidences seen in breeds such as Quarter Horses, Percherons, and American Saddlebreds. This condition is thought to be hereditary but can have any combination of inherited, hormonal, or mechanical causes. It’s important because affected stallions can have decreased fertility, and some breed associations will not register cryptorchid stallions.

Classification

Horses can be either unilateral (one testicle not descended) or bilateral (both testicles not descended) cryptorchids. Unilateral cryptorchids are more common and still fertile, whereas the less-common bilateral cryptorchids are sterile. The hidden testicles can be located inguinally (within the inguinal ­canal/groin area) or abdominally. Either the right or the left testicle can be retained; however, right testicles are generally retained within the inguinal canal, and left testicles are more likely to be retained within the abdomen. Very rarely, horses can have monorchidism, which is the complete absence of one testicle. Veterinarians diagnose this condition only after performing a physical exam, laboratory testing, and possible surgical exploration. 

Diagnosis

Normally, male horses are born with both testicles within the scrotum. Sometimes they are not palpable for the first few weeks or months. Owners shouldn’t become concerned that a colt is a true cryptorchid until he’s reached 18 months of age. Once older than 2 years, a horse without palpable testes is considered a true cryptorchid because at this point the testicles will not drop.

Your primary care veterinarian can perform external palpation to diagnose cryptorchidism. If he or she only is able to palpate one testicle, deeper external palpation might reveal the testicle in the inguinal canal. Sometimes veterinarians use sedation to relax the horse and allow for better palpation or allow the testicle to relax into the scrotum. If the veterinarian is unable to palpate externally, he or she might use rectal palpation with or without an ultrasound probe to locate the retained testicle. If this approach is unsuccessful or you are unsure of the horse’s surgical history, the practitioner might also draw blood to perform testosterone, estrone sulfate, or hCG-response testing. For these tests to be accurate, the horse must be at least 12 months of age. Veterinarians might run testosterone tests in conjunction with estrone sulfate tests to increase their chances of making an accurate diagnosis; these hormone levels are normally elevated in cryptorchids. If results are still ambiguous, they can follow up with the hCG-response test or with the newer, more diagnostic AMH (anti-Müllerian hormone) test.

Consequences

The retained testicle in a unilateral cryptorchid is often small. In fact, an abdominally retained testicle can be up to 20 times smaller than normal. Retained testicles, however, can still produce testosterone, and affected horses can still display stallionlike behaviors. Unilateral cryptorchids have reduced fertility rates because the retained testicle is less viable than the descended one, and they aren’t able to service as many mares. If the retained testicle is within the abdomen, it is considered sterile because the increased temperature of the abdomen does not allow sperm to develop. Retained testicles can also be more prone to cancer, even though testicular cancer in horses is rare.

Treatment

Once your veterinarian has diagnosed cryptorchidism, he or she will likely advise castration due to the condition’s heritability. Inguinal retained testicles are easier than abdominally located testicles to remove, but either procedure can be complicated. If the retained testicles are in the abdomen, an ACVS-board-certified surgeon should perform the surgery. Veterinarians can perform both surgeries via laparotomy (abdominal incision) or open abdominal surgery. Laparoscopy in the dorsally recumbent or standing horse is much less invasive and allows for a faster recovery time.

Many owners ask, “Can’t you just remove the one?” Removing only the retained testicle, however, would be unethical and can set future owners up for unnecessary costs and headaches, such as testing, possible surgeries, and a stallionlike supposed gelding. If an owner proceeds with this method, however, it’s imperative they pass along that information to subsequent owners.

Take-Home Message

A cryptorchid can present an owner with increased costs and dilemmas. A thorough exam by your veterinarian can help determine the extent of this condition and allow for the best plan of action.