By Larri Jo Starkey and Ben Espy, DVM, DACT

Which of these commonly accepted truths on cryptorchid horses are true and which are myths? Take our test and learn the answers. Here are the facts about cryptorchidism: It’s a condition in which one or both testicles are retained in the abdomen instead of descending into the scrotum. It’s seen in all domestic animals, and is common in stallions, boars and dogs. You probably know that already. But it’s the other things you “know” about cryptorchidism that might or might not be true. 

The AQHA Journal asked equine reproduction specialist Benjamin Espy, DVM, DACT to clear up any misconceptions. Test your knowledge against the facts. 

True or False? A colt’s testicles can drop and then disappear after birth. 

TRUE. A colt’s testicles should be descended at birth, Espy said. “It’s very common to see them descended at birth, and then when the horse is weeks to months old, have them not be visible any more. The reason is because as their body matures, the testicle may not be palpable in the scrotum. The testicle is not necessarily in the abdomen of the horse, but it maybe so high up inside the groin that it’s not palpable.” Those colts are not true cryptorchids, just horses whose testicles have not descended into the scrotum yet. Different breeds take different amounts of time for the testicles to descend, but in American Quarter Horses and Thoroughbreds, veterinarians and horse owners begin to be concerned at 18 months. “I personally will palpate horses at 18 months,” Espy said. “If the testicles are not descended, then I will check them again at 24 months, and at that point if they are not descended, I’ll send the horse to surgery, because he is not a candidate to castrate on the farm.” 

True or False? The retained testicle can develop cancer.

TRUE. Cancerous growth in the retained testicle is rare but not unknown. “There’s such a small population of cryptorchid horses that there really haven’t been any long-term studies regarding how to assign risk to cancer in a retained testicle. In other species, it has been well-studied, and the increased temperature is blamed for numerous tumor formations, so in general, cryptorchid horses are assumed to have a higher risk of cancer even though in general testicular cancer is really rare in horses.” 

True or False? The retained testicle can produce sperm.

FALSE. “The reason the scrotum is on any male, whether it’s a horse or a human or a bull, is to regulate the temperature inside the testicle,” Espy said. “If the testicle is retained in the abdomen, then that male of whatever species is going to be sterile (in that testicle). The temperature inside the abdomen is too high for the stallion to produce sperm from the retained testicle.” 

True or False? Cryptorchid horses have lowered fertility. 

TRUE. A stallion that only has one descended testicle only has one testicle to produce sperm, which automatically lowers the semen count for that stallion. “If you have a small book of mares, then you can compensate with that one testicle,” Espy said. “As your book of mares gets bigger, the stallion becomes more active, and semen production is required from both testicles, then you’re going to find it more difficult to keep up with the increased reproductive pressure.” 

True or False? The undescended testicle is painful for the horse. 

FALSE. Not necessarily. The most common form of cryptorchidism is the unilateral cryptorchidism, in which one testicle is retained and one testicle is normal. The retained testicle can become predisposed to torsion, or twisting on its axis, because it doesn’t have a scrotum to sit in. “The (testicle with torsion) can swell and become very painful for the horse,” Espy said. Without that torsion, though, the retained testicle doesn’t hurt the horse or his ability to perform. “I’ve seen a reasonably large population of athletic Quarter Horses and athletic Thoroughbreds in Texas and Kentucky, and I haven’t seen any sort of decrease in performance due to cryptorchidism or any sort of colic or abdominal pain due to cryptorchidism,” Espy said. 

True or False? Cryptorchidism is genetic. 

TRUE. The Merck Veterinary Manual lists cryptorchidism under “congenital and inherited anomalies of the reproductive system. “There hasn’t really been any good gene mapping studies on the heritability of cryptorchidism,” Espy said, “but it’s generally accepted that cryptorchidism is congenital or inherited. If you breed your mare to a cryptorchid stallion, the stallion owner should advise you of his horse’s condition and you need to be aware of the risk.” 

True or False? Cryptorchids are harder to castrate. 

TRUE and FALSE. While removing an undescended testicle is always a surgical procedure, it’s not as hard as it used to be. Until recently, removing the testicle involved anesthetizing the horse, flipping him onto his back and probing past the inguinal ring into the abdomen, searching for the testicle. “Horses can become paralyzed from lying on certain muscle groups too long,” Espy said, “plus you have the inherent risks of abdominal surgery on horses and the risk of peritonitis.” “The newest, safest, most advanced way to retrieve cryptorchid testicles is through a laparoscope,” Espy said. “It’s by far the preferred method now, and with a laparoscope, you can cauterize blood vessels with lasers. It’s just as advanced as any surgery you would see in any human hospital.” In the laparoscopic procedure, a laparoscope – a rigid arthroscope, like a camera – goes in through a standing horse’s flank. Another hole is pierced for the instruments. The veterinarian can see what the camera sees through a monitor, and the horse has a much smaller incision on the flank to heal instead of a wound in his belly. “It’s very atraumatic to the horse because he is standing the whole time,” Espy said. TRUE – It’s harder than castrating a horse that isn’t cryptorchid -- and FALSE – because it’s easier now than it used to be. 

True or False? Cryptorchids are more aggressive. 

FALSE. Whether retained or descended, testicles produce testosterone. Normal stallions and cryptorchid stallions produce the same amount of testosterone. Cryptorchid horses that haven’t been completely castrated will continue to show the same aggressiveness as a full stallion, Espy said, because they have the same hormone production. “On more than one occasion, I have found horses that have been castrated on one side and then sold as geldings,” Espy said. “Then three or four owners down the line, someone will check and find the horse is cryptorchid.” 

True or False? Cryptorchid testicles can only be identified by palpation. 

FALSE. Three ways to check whether a horse you thought was a gelding is really a cryptorchid are rectal palpation, transrectal ultrasound and hormone challenge testing. Espy prefers the hormone challenge test. “You give the male horse a certain regimen of hormones, and then draw his blood a certain number of hours afterward,” Espy said. “The only tissue in a male horse that makes certain testosterones and estrogens is the testicle, so if you see a rise in those androgens or hormones, the testicle is still in the horse. If the horse doesn’t have any testosterone production, then he doesn’t have any testicular tissue. If a horse has been castrated but still has aggressive, stallion-like behavior, then the horse has learned those behaviors,” Espy said. 

True or False? The best time to talk about cryptorchids is in the spring. 

FALSE. "Asking questions about cryptorchidism in the middle of breeding season is a little late," Espy said. “You need the information before you make the decision on whether or not to breed to a cryptorchid stallion.” 

If you have questions or concerns about cryptorchidism, talk to your equine veterinarian. You can locate an equine veterinarian in your area through AAEP's Get-A-DVM at www.aaep.org in the horse owner section. 

AQHA Rules: AQHA’s founders considered cryptorchidism a failing in a horse. According to Rule 448 (e)(2), stallions older than 2 shown in halter classes must have two visible testicles. Cryptorchids should be excused from the ring. When AQHA was still doing physical inspections of appendix horses before advancing them to full registry, inspectors didn’t rule a horse out because of parrot mouth or cryptorchidism, but the horse had to be exceptional in every other aspect of conformation, said Gary Griffith, AQHA’s executive director of registration. 

When AQHA stopped performing physical inspections, the rule was changed so that cryptorchid stallions could no longer advance to full registry. AQHA’s founders considered halter classes to be the classes where the breed standard was preserved. “The thought was that if you’re going to be showing in a breeding class, then your horse should be free of genetic defects,” Griffith said. There is no prohibition against cryptorchids being shown in performance classes.

Reprinted courtesy of the AQHA and the Quarter Horse Journal.

Reviewed by original authors in 2016.