Cryptorchidism in Horses

Causes, Treatment, and Prevention

vet introducing stallion to mare
Cryptochidism can make male horses act more like stallions than geldings.

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A horse with one or two undescended testes is called a cryptorchid, or rig, and has a condition called cryptorchidism. A rig's testes are located inside its body, near the inguinal canal or abdominal cavity. Some rigs are fertile while others are not, but all cryptorchids produce testosterone, leading to stallion-like behavior, even without testes. Diagnosing cryptorchidism is straightforward and should be done by a veterinarian. Cryptorchidism is usually inherited, although it has yet to be successfully genetically mapped. Breeds like quarter horses, saddlebreds, Percherons, and ponies are more susceptible to cryptorchidism than others. Treatment is generally highly effective with a favorable prognosis, but a highly specialized veterinary surgeon needs to perform the surgery.

What Is Cryptorchidism?

Cryptorchidism is a condition in horses that causes one or both testes to not descend fully into the scrotum. A cryptorchid horse's testes may be located under the skin near the inguinal canal, within the canal, or outside the abdominal cavity. Horses with one descended testis are usually fertile, while one with neither testis dropped is usually infertile. Even though the testes don't descend, a horse will still have libido and aggressive male behavior because testosterone is being produced. Cryptorchid horses may also be referred to as rigs or riglings. Rigs are usually castrated at age three to remove the retained testicle because it can cause discomfort.

Symptoms of Cryptorchidism in Horses

One or two undescended testes are the only reliable sign of cryptorchidism in a horse.


  • One or both undescended testes
  • Regular stallion behavior
  • Infertility

Even if your horse doesn't appear to have testes or is infertile, testosterone is in its body. Your rig will display aggressive male behavior, regardless of fertility status or outward appearance of testicles. Your horse will likely still be fertile even if only one testicle is descended.

Causes of Cryptorchidism

The cause of cryptorchidism in horses is not entirely understood, but there are thought to be several factors at play.

  • Genetics: Cryptorchidism in horses is inheritable but can sometimes occur without a genetic predisposition. While the condition is considered hereditary, there has been limited research on the gene that causes cryptorchidism.
  • Hormonal imbalance: Insufficient testosterone may stunt the development of a horse's testes, causing them not to descend.
  • Developmental issues: Since the testes form prenatally if there are complications with a mare's pregnancy, cryptorchidism could develop in the horse fetus.
  • Breed: Some breeds appear to be more susceptible to cryptorchidism than others. These breeds include quarter horses, saddlebreds, Percherons, and ponies.

Diagnosing Cryptorchidism in Horses

To diagnose a horse with cryptorchidism, your vet will perform external palpation of the scrotum and internal palpation through the rectum. Your horse may also need an ultrasound to locate a testis within the abdomen or inguinal canal. Occasionally, a ridgling with two undescended testes may be mistaken for an already-castrated horse especially if the testes cannot be located by palpitation. If this is the case, Your vet will do blood work to evaluate testosterone levels. The horse may have to be tranquilized for this very invasive internal examination. Often, cryptorchidism goes unnoticed until it's time to geld a colt.

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Treatment & Prevention

Castration is considered to be the best option for treating cryptorchidism in horses. Castrating a rig can prevent medical complications related to undescended testes as well as eliminate aggressive, stallion-like behavior. This can either be done in a standard, surgical fashion or laparoscopically.

While the surgery is not invasive, a horse's testicular anatomy is very complex and requires highly specialized equipment and training. The higher the testes are in the abdomen, the more complicated the surgery will be. Veterinary clinics offering these surgeries may not be readily available to all horse owners.

There is little that can be done to prevent cryptorchidism. Because it is hereditary in most cases, cryptorchids that are capable of reproduction should be castrated and not be bred.

Prognosis for Horses With Cryptorchidism

As with any castration, postoperative complications may arise. Hemorrhaging and inflammation can occur, in which case contact your vet. Rarely more severe complications can occur such as intestinal prolapse or infection of the spermatic cord. Typically treatment is successful, and the prognosis is good. After surgery, your horse will rest in its stall and be walked by hand with a gradual increase in exercise after 10 days to two weeks.

  • Is my cryporchid horse fertile?

    If your horse has one of two testes descended, it is likely fertile. If neither of its testes has descended then it won't be able to reproduce.

  • Is cryptorchidism dangerous?

    Cryptorchidism itself is not dangerous, but retained testes could lead to medical complications like tumors. It's important that you castrate a horse with cryptorchidism.

  • Is castration surgery invasive?

    While the surgery to castrate a horse's retained testes isn't particularly invasive, it requires a highly trained surgeon specializing in complex abdominal anatomy.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Cryptorchidism (Undescended Testicles) in Horses. ACVS.
  2. CRYPTORCHIDISM. Colorado State University Equine Reproduction Lab.

  3. Cryptorchidism in the Horse. Ontario Ministry of Agriculture, Food, and Rural Affairs.