A cryptorchid, also called a ridgling, is a male horse in which one or both testicles do not descend into the scrotum. In the developing fetus, the testicles are formed within the abdomen. As the fetus nears term, the inguinal rings and inguinal canal (passage through which the testicles descend) expand to allow for descent of the testicles into the scrotum. This canal contains blood vessels, nerves, and the cremaster muscle, which raises or lowers the testes in response to temperature change or threat of injury.
Based on fetal dissection studies, descent of the testicles is usually completed during the last 30 days of gestation. Testicles in the fetus are quite large, perhaps due to hormonal influences from the dam. They become smaller as the fetus matures. If they stay too large, they might not be able to come down through the inguinal canal.
If migration down into the scrotum is inhibited (if the canal fails to expand or if the testicle is too large to pass through), it is trapped within the abdomen. In these instances, the horse is a true abdominal cryptorchid; the testicle failed to enter the inguinal canal before closure of the internal inguinal ring (where the canal meets the abdominal wall). If it gets trapped somewhere below this ring but above the scrotum, the horse is an inguinal cryptorchid.
One or both testicles might remain high in the flank, stuck in the inguinal canal, and the horse is called a ridgling or high flanker. If the testicle is in the abdomen or very high in the flank, it makes the horse impossible to geld by routine castration. The undescended testicle must be located and removed via abdominal exploratory surgery. Horses with a retained testicle at the external inguinal ring (where the canal meets the scrotum) are called low flankers, and these are easier to remove.
Tom Yarbrough, DVM, PhD, a veterinary surgeon practicing in northern California, explains that a small structure (a cord called the gubernaculum testes) is an important player in the descent of the testicles. In a normally descended testicle, this little cord becomes a scrotal ligament, anchoring the testicle in the region of the scrotum--although it can still be pulled up closer to the body for warmth when necessary. If this structure is weak or absent (an inherited weakness), one or both testicles might remain above the scrotum or up in the abdomen.
Sometimes a cryptorchid is gelded, removing the one descended testicle and leaving the other. If only one testicle is found, some people assume the horse is a monorchid (a rare instance in which the horse only has one testicle) and geld him, not searching for the retained one. Or, the intention is to do surgery later to find the missing testicle--but the horse gets sold in the meantime and the new owner assumes he's bought a gelding.
Even if the properly descended testicle has been removed, the horse will still act like a stallion. He will try to breed mares, even though he is not fertile. The sex gland retained within the body is small and undeveloped compared to a normal testicle, and it is unable to produce viable sperm because body heat is too high to allow sperm to be created and survive. But it is still producing male hormones. The retained testicle must be removed to rid the horse of those tendencies.
Yarbrough says, "We also worry about testicular torsions, because the testicle is not tethered down. It can become twisted or become the focus of entrapment for a section of small intestine. These are the possible long-term health risks for the abdominal cryptorchid." He said the horse owner needs to be aware of these risks.
Ahmed Tibary, DVM, PhD, associate professor of theriogenology at the Veterinary Teaching Hospital at Washington State University, says colic caused by a retained testicle might not be discovered until the horse goes to surgery. "Most of the time this would be an incidental finding when the horse is opened up for colic surgery," says Tibary. "You would not suspect this, unless you know the horse is a cryptorchid."
Another problem might be a tumor that has developed because of a retained testicle.
"The most common thought as to why tumors develop is that because the testicle is retained, it has a different temperature environment," says Tibary. "Cells that are programmed to function under a certain temperature now have a higher temperature, which may trigger abnormal growths. Another theory is that the testicle may have been retained because it was abnormal to begin with. That can help explain the tumors called teratomas, which are made up of misplaced embryonic tissue."
The testicle's reproductive tissue provides ideal conditions for this misdirected tissue growth. In these instances, a teratoma (usually a grapefruit-size mass of fetal material that can contain hair, teeth, and connective tissue) forms in the abdominal cavity where the testicles originally develop. This type of tumor starts early, in the fetus, and might continue to grow during the life of the horse. The tumor can become life-threatening if it becomes large and puts pressure on other organs, or if it becomes malignant.
"Normally, testicular cancer in horses is not malignant, but there have been cases where these tumors have metastasized," says Tibary.
If you are uncertain whether a horse is a cryptorchid, he can be checked by a veterinarian. Careful palpation of the scrotum and the tissues above it, and rectal palpation to check the pelvic area, can give clues. But a retained testicle is sometimes hard to palpate in the standing horse. If it's in the abdomen, it is usually suspended by a fold of peritoneum (the membrane that lines the abdominal cavity).
According to Tibary, rectal ultrasound can often help locate it, but not in all cases. The testicle is somewhat mobile within the abdomen and might be hard to spot if it has become mixed up with coils of intestine or lies beside the bladder. In some cases, it adheres to the wall of the abdomen or to an internal organ (such as the spleen) and can be hard to identify. A retained testicle within the abdomen is usually small, flabby, and undeveloped, but it can be very large if it becomes tumorous.
"There is a test that can be used to differentiate between a completely gelded horse and one that has a retained testicle in the abdomen," says Tibary. "What you look for in the test is the response of the horse to an injection of HCG (human chorionic gonadotropin) to know if he still has some testicular tissue. Blood samples are taken before the administration of HCG and one to two hours after. If you see a two- to three-fold increase in testosterone, you are dealing with a horse that has a retained testicle. There are other hormonal tests, but I like this one because it is very simple and works well. If interpretation of the testosterone level is not clear enough with the HCG, then the other tests can be done."
Some fully castrated males still have some stallion-like behavior. If you buy a gelding and wonder if he is a gelding or a cryptorchid, the hormone tests are useful.
Yarbrough says, "Any time we are faced with a case in which there is questionable history on the horse--if we don't know if he was completely gelded or not--we run a hormone profile. If the hormone profile comes back indicating that there's a testicle present, we go in with a laparoscope (used to view within the peritoneal cavity; similar to an endoscope) to explore for it."
If only one testicle was removed in an earlier castration, you don't know which side it was on. Using a laparoscope makes it easier to find the missing one. And if a stallion has only one testicle in the scrotum, you don't know whether this might be a rare case of monorchidism (the horse had only one testicle to begin with). A hormone assay can tell you whether to go looking or not, says Yarbrough.
Most owners of a cryptorchid want the retained testicle removed. A testicle caught in the inguinal canal is easiest to address. The veterinarian reaches into that canal to search for the missing testicle and works it out through the canal. "These can often be palpated, or located by placing the ultrasound on the scrotal area," says Tibary.
Yarbrough says the standard inguinal approach to search for the missing testicle is the oldest method. The horse is on his back, anesthetized. "Most practitioners will cut very close to or immediately over the external inguinal ring and go down the canal looking for the inguinal extension of the gubernaculum," says Yarbrough. "They follow it to the internal ring. If the horse is a high flanker, you would encounter the testicle before you get to the abdomen, but otherwise you'd follow the canal right on into the abdomen, to the internal inguinal ring. Once you have identified that structure, you can nip the internal sheath and tease the testicle out."
A testicle retained in the abdomen is usually very soft and smaller than normal. Therefore the inguinal ring doesn't have to be opened very much to get it out. "If you do have to open it, you must make sure none of the small intestine comes out through that hole," says Yarbrough. "If that happens, you must suture the external ring closed."
In earlier years, surgery to find and remove an abdominal testicle could be risky for the horse. If the veterinarian didn't find the testicle within the flank, the incision was enlarged enough to reach a hand into the abdominal cavity, perhaps as far as into the area where the testicles were originally formed. Many of these retained testicles have moved within the abdomen, however, and it could take a lot of searching to find them. The farther up in the abdomen (and the more difficult to find), the more risky the surgery. It might take the veterinarian several hours to find the retained testicle.
With laparoscopy, this is easier and safer, and very helpful in cases where the testicle is difficult to find. A laparoscope can be inserted through a small incision in the abdomen, into the peritoneal cavity, and the testicle can be brought out through this small incision after it is located. "It's not as invasive, and it's visual," says Tibary. "You don't have to open the abdominal cavity, as was done in the past.
"Our surgeon here at WSU, Dr. Claude Ragle, is very good at laparoscopy and does quite a few of these surgeries," continues Tibary. "He gets a lot of referral horses, particularly show horses and performance horses where the owner does not want to leave much scar. In some cases, the procedure can be done with the horse standing, just sedated--which makes for less stress on the horse."
Yarbrough says laparoscopic surgery is a bit more time-consuming (45 minutes to an hour) than a traditional surgery (15-20 minutes) if the horse is on his back and anesthetized. The table is tilted so the abdominal contents fall forward, away from the area to be explored, and the abdomen is inflated like a balloon for the laparoscopic examination.
If the laparoscopy is done standing, the time and cost might be more comparable to regular surgery because you won't have the added cost of anesthesia. As a rule, however, a laparoscopic procedure will cost a bit more due to the cost of the equipment being used. But in a complicated case, the laparoscope can save time and stress on the horse. "Some veterinarians do all their cryptorchid surgeries with a laparoscope, but most of us use it simply for optimal case management--in cases where traditional surgery would be more difficult or risky," says Yarbrough.
"If a veterinarian had already explored the horse inguinally and could not find the testicle, or maybe removed one testicle and came back later to do a hormone profile to see if there is another one up inside and has proven that there is, that's usually when we go in with the laparoscope," he says.
If the horse has both testicles retained, or has already had the descended testicle removed, it's easy to do the laparoscopic surgery standing. If there is still a down testicle to deal with, however, it's usually safer to have the horse anesthetized and on his back, because standing castration can be risky, says Yarbrough.
"The laparoscope allows you to do a much greater exploration, and the area where you remove the testicle is at less risk for evisceration (prolapse of the intestine through the incision)," explains Yarbrough. "The incision is smaller, and in a structure you can suture nicely. If the horse is standing, you pull the testicle out through a region low in the flank. If he's on his back, you pull it out through the dense muscle and fascia, which you can suture--making a more solid closure. The inguinal ring and inguinal canal structures (that you go through to find the testicle in a traditional surgery) are more risky to suture."
When to Do It
Sometimes people wait to geld a cryptorchid, trying to give him time for the undescended testicle to come down. A few young horses do have temporarily retained testes that are caught in the inguinal canal below the abdomen. This occurs most often in ponies. The pony colt might have one small testicle that does not descend when the normal one does. The smaller one might eventually grow larger and descend into the scrotum by the time the animal is two or three years old.
If you check a young horse and cannot feel the testicles, how long you should wait before you determine the colt is truly a cryptorchid?
Tibary says the testicles should both be in the scrotum at birth or within a few months. "However, there are reports of some stallions being two years old before the testicles descend," he says.
If a testicle is caught partway up the inguinal canal, there is a chance it will come down when the horse matures. Enlargement brought about by puberty, and increased weight of the testicle, helps pull it down. But body heat in the flank might inhibit proper growth of the testicle and it might not enlarge and descend.
"Once it has been determined that a colt is a cryptorchid, it is generally recommended that the retained testicle be removed by the time he is about 12 months old," recommends Tibary. "If the horse is still a high flanker or abdominal cryptorchid, it's time to do something about it."
Yarbrough says that if the testicle has not descended through the internal inguinal ring by the time the ring starts to contract (at about six months of age or earlier), it will not get out and will be permanently trapped in the abdomen. "It is possible for a high flanker to have a testicle not make it all the way down to the scrotum until two years of age, but that's very unusual," says Yarbrough. "If you know the colt is a cryptorchid, there is no real advantage to waiting. It's actually easier to do an inguinal approach on foals because they have less inguinal fat, and the blood vessels are smaller, so the exploration is easier."
CRYPTORCHIDS AS STALLIONS
Keeping a cryptorchid as a breeding stallion is very controversial, and most horse owners and veterinarians advise against it. There is a genetic component to this problem, and offspring might have a greater chance of having the same problem. Some breeds allow cryptorchids to be stallions; others don't. Cryptorchids are more difficult (and expensive) to geld, and a cryptorchid stallion can be at a disadvantage for breeding. If anything happens to his one functional testicle, he is out of business as a sire. He will still be fertile with only one testicle, but will not produce as many total sperm as he would with two testicles, and he won't be able to breed as many mares.
"For a testicle to develop and function normally, it must be able to thermoregulate," says Tom Yarbrough, DVM, PhD, a veterinary surgeon practicing in northern California. "It needs to be at a temperature a little below normal body temperature. That's why the cremaster muscle will draw the testicle up or let it down, depending upon whether the air temperature is warm or cold. So a testicle retained within the abdomen will not be producing viable sperm, and cannot function normally."
This genetic defect is not as harmful to the horse as some other inherited faults, since the affected animal can live and function normally (but he might have aggressive behavior that could put handlers at risk). "We castrate a high percent of these animals, and as long as horse owners don't mind the added expense to geld the progeny of a cryptorchid stallion, cryptorchidism may be an acceptable inconvenience if the horse has other qualities that are desired," says Yarbrough. "It's a matter of whether the horse owner wants to put up with the inconvenience, and whether we want to take on a bigger financial burden when gelding the offspring."
Ahmed Tibary, DVM, PhD, associate professor of theriogenology at the Veterinary Teaching Hospital at Washington State University, says leaving a horse a cryptorchid can be dangerous for people handling him. "Some people will tell you that cryptorchid stallions can become a lot more dangerous than normal stallions, and I have seen this in Thoroughbreds," states Tibary. "The crankiest Thoroughbreds I have seen were cryptorchids."
If a tumor develops in that testicle, it can also change the horse's behavior--by altering the hormone balance, triggering a change in personality.--Heather Smith Thomas
INCIDENCE AND HERITABILITY
Cryptorchidism occurs in most mammals, perhaps more often in domestic animals than in wild ones. We have bred horses selectively for traits that suit our purposes, and sometimes in our efforts to choose breeding stock with characteristics we desire, we overlook others. Thus we might perpetuate or increase the incidence of certain problems.
Cryptorchidism in horses is thought to be inherited, although the actual mechanism of inheritance is not yet fully understood. Ahmed Tibary, DVM, PhD, associate professor of theriogenology at the Veterinary Teaching Hospital at Washington State University says, "It has been confirmed in sheep and swine to be hereditary, and we assume it is in horses. Some authors think it's inherited as a dominant autosome (in a pair of chromosomes), and others think it's a recessive gene (that must be inherited from both parents to show up in the offspring). In my experience, I've seen a lot of cryptorchids in Akhal-Teke horses, a breed with a limited gene pool."
Within all breeds, there are some bloodlines that produce a greater number of cryptorchids than the average for that breed, and some breeds have a higher incidence. A study that looked at more than 5,000 cryptorchid horses (documenting breed incidence, among other things) found the highest number in Percherons, Quarter Horses, Saddlebreds, and ponies, says Tibary.
"From that study, it was also surmised that the breeds that don't have many cryptorchids are the ones that have tried to eliminate this trait by not allowing them to be used for breeding," he adds. "This lends credence to the idea that it's inherited, but at this point we're just using the experiments from other species to assume it is hereditary in horses," he says.
"In that study, it was also found that the left testicle is more commonly retained than the right in horses that are abdominal cryptorchids," notes Tibary. "One speculation as to why this occurs is because of the way the kidneys are placed. One kidney is higher than the other. So it has to do with position of the kidneys and testicles while the fetus is developing."
In ponies with complete abdominal retention, the left testicle is retained as often as the right, but in horses, left side abdominal retentions outnumber the right side cases two to one. Sometimes both testicles are retained in the abdomen (bilateral cryptorchidism), but this is rare. More often just one is retained.--Heather Smith Thomas
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.
POLL: Rehabbing the Injured Horse