Cryptorchid, ridgling, and even rig are terms used to describe a stallion with at least one undescended testis. The condition is not unique to equids, but the horse is of specific importance as the retained testis fails to produce viable sperm, so fertility rates are affected. However, the testis is still capable of producing testosterone, so the animal will show stallion-like behavior. The cost of castrating a cryptorchid is significantly higher than standard castration, and retained testes are at a higher risk of developing malignant (cancerous) tumors.
In the normal stallion, the testes will have gradually descended from just ventral to (below) the kidneys, down through the inguinal canal, and into the scrotum, either in utero or during the first few weeks of life (see "Cryptorchid Anatomy" on page 43). The mechanisms that drive testis descent are not fully understood, although testosterone is thought to have a role. Occasionally, either one or both testes fail to descend, and such stallions are termed cryptorchids. A cryptorchid stallion might be further classified as inguinal (testis is in the inguinal canal), abdominal (testis is in the abdominal cavity), unilateral (one testis is not descended), or bilateral (both testes are undescended). Classification is important when considering management and treatment.
Inguinal cryptorchids, the most common type, are characterized by testes that have failed to descend beyond the inguinal canal, or occasionally lie just under the skin (called ectopic cryptorchids; see "Cryptorchid Anatomy"). Failure of inguinally retained testes to descend might be temporary or permanent. Temporary inguinal cryptorchidism is reported to be most common in ponies and miniature horses, and to mainly affect the right testis. If the condition is temporary, the testes will descend by three years of age. Failure to descend by this time indicates permanent cryptorchidism, a condition seen in all types of horses.
An abdominal cryptorchid is characterized by testes that are still within the body cavity, often near the kidney (see "Cryptorchid Anatomy"). Abdominal cryptorchids can be further classified as complete or incomplete. Complete refers to the retention of both the epididymis (where sperm are stored and mature) and the testis; in such cases the testis tends to be mobile, surrounded by the intestine, and hard to locate. Incomplete abdominal cryptorchids are characterised by the testis being retained within the abdomen, but the epididymis having descended into the inguinal canal. Abdominal cryptorchidism is invariably permanent (unless surgically corrected).
When both testes fail to descend (bilateral), they are normally found within the abdomen (bilateral abdominal). Very occasionally, a stallion might have only one testis (monorchid). The frequency of retention of left and right testes appears to be the same, although significantly more left retained testes appear in the abdomen and more right testes in the inguinal region.
There is considerable debate as to whether the condition is inherited. Circumstantial evidence and some research work does support this view, as well as suggesting a higher incidence in ponies.
The function of the retained testis is affected by two factors: Increased testicular temperature and reduced testicular size.
Testicular Temperature--The normal position for the testes is outside the body cavity within the scrotum, where it maintains a temperature of 3ºC below body temperature (about 94ºF). Sperm production is most efficient at this lower temperature, and is compromised by the higher temperatures within a retained testis. Spermatogenic function of the retained testis is affected more than hormone production, which is not so temperature-dependent.
Testicular Size--The retention of one or both testes decreases testis weight, even if the testis does subsequently descend. The reduction in weight is due to a reduction in testicular tissue.
There are two main cell types within testicular tissue--seminiferous tubules and their associated Sertoli cells, which are responsible for spermatozoa production, and interstitial tissue made up largely of Leydig cells, which are responsible for hormone production (in particular testosterone). The reduction in size and hence function depends upon the cryptorchid's classification.
Abdominally retained testes might be reduced in size by up to twenty times, although the reduction in inguinally retained testes is not as great--up to seven times. The appearance of retained testes also varies with their position--the further they are from the scrotum, the less normal they appear.
Abdominally retained testes are flabby (although they might become hard over time) with very limited seminiferous tubules and interstitial tissue. Spermatogenic activity is, therefore, absent with rarely any development of germ cells, and the low volume of interstitial tissue results in very limited hormone production.
Inguinally retained testes normally show greater testicular development, with both seminiferous tubules and interstitial tissue present. Sertoli cells are seen, as are primary spermatozoa. Inguinal testes might even produce some viable spermatozoa so fertilization of a mare's egg is possible (although fertilization rate is significantly decreased). Hormone production from interstitial tissue might be near normal, so such animals might exhibit considerable libido.
Identifying the Cryptorchid
If the stallion has one normally functioning testis, his libido and fertility rates can be perfectly acceptable, especially if he has only a limited book of mares. Behavior, and even fertility rates, are not necessarily good indicators of cryptorchidism. The most helpful diagnostic aid is the stallion's full history, which should indicate whether he has had an injury, been gelded correctly, or encountered any problems (such as disease or genital infections).
However, if the horse was purchased, his history might not be known. One of the first signs of a problem is an apparent gelding showing stallion-like behavior. It is essential, therefore, to diagnose whether the gelding is a true gelding or a cryptorchid stallion. If he is the latter, the position of the testes and hence the likelihood of the condition being temporary, and the possible courses of action, must be determined.
There are six main techniques used to diagnose cryptorchids. These are external palpation, rectal palpation, laparotomy, endoscopic examination, ultrasound examination, and hormone analysis.
External Palpation--Palpation of the scrotal sac allows the presence of two testes to be identified. Both should be sized about evenly (the left is occasionally slightly larger) and of relatively smooth consistency. If both are not felt, retention must be suspected. But palpation does have disadvantages--many stallions object to this examination and draw their testes tightly up toward their abdomens--a normal response to cold and fear. The scrotal sacs might then seem empty, so sedation is often required.
Additionally, in the young foal the tail of the epididymis may be as big as the testis itself; therefore, if just the epididymis is within the scrotum, this might be confused with the testis.
External palpation can also be used to identify inguinal or ectopic testes.
Rectal Palpation--Rectal palpation uses a well-lubricated arm placed within the rectum to feel for inguinal retained testes. If the testis is known to be retained and is not identified by rectal palpation, it is very likely within the abdomen.
Laparotomy--If the testes are not identified by palpation, and especially if an abdominal cryptorchid is suspected, laparotomy used to be the only method available to detect and treat the condition. Laparotomy is a well-established method and involves opening the abdomen under general anesthesia. The opening is normally in the inguinal region, but might be to the side of the inguinal region or through the flank. Exploratory laparotomy, however, might require a very large incision and considerable manipulation of the internal organs to locate the testis.
Laparoscopic (Endoscopic) Examination--Laparoscopic examination is a relatively new technique involving the insertion of an endoscope (rigid fiber-optic cable with light source) and surgical instruments into the abdominal cavity through a small incision(s). Laparoscopy can be carried out under general anesthesia with the horse in dorsal recumbency (lying on his back) or under local anesthesia and tranquilization with the horse standing. The laparoscope allows the inside of the abdomen to be viewed and abdominal testes to be identified. It is favored by many as being less traumatic and invasive than laparotomy.
Ultrasound Examination--Ultrasonic examination, similar to pregnancy detection in the mare, gives a visual impression of the stallion's abdomen and with skill allows retained testes to be identified.
Hormone Analysis--Hormone analysis is particularly useful as a first indicator of cryptorchidism in an apparent gelding. Plasma concentrations of testosterone and oestrone sulphate, both of which are produced by the testes, are normally higher in cryptorchids than geldings, especially in animals under three years of age. A simple blood test can therefore be used.
In animals older than three years, testing for plasma testosterone is less accurate. A more accurate diagnosis can be obtained by assessing the animal's response to hCG (human chorionic gonadotropin), which has LH (luteinizing hormone)-like activity and thus drives any testicular tissue to produce testosterone. Even the limited amount of testicular tissue present in the cryptorchid will react to hCG by producing testosterone. Blood samples are taken immediately before and several hours after treatment. An increase in testosterone in response to hCG is indicative of a retained testis.
Alternatively, if blood sampling is difficult, such as in feral or unbroken animals, fecal samples can be tested for oestrone sulphate; elevated levels again indicate cryptorchidism. This test, however, is not as accurate as blood sample analysis.
Managing the Cryptorchid
The vast majority of breed societies require a stallion for use in breeding registered stock to be licensed. Getting a license is normally dependent upon passing a veterinary inspection (among other criteria). In some breed registries, a retained testis will cause a stallion to fail the inspection and not be awarded a license. This is largely due to the strong circumstantial evidence that cryptorchidism is an inherited fault.
As discussed previously, inguinal cryptorchidism might be temporary and many breed societies will allow a stallion to be reassessed at two or three years of age to allow time for both testes to descend. Failure of the retained testis or testes to descend by that age normally deems a stallion ineligible for a license.
Treatment is not commonly practiced, although in theory surgical intervention can bring a retained testis down into the scrotum. More recently, hormone therapy has been tried, but neither surgery nor hormone therapy address the problem of inheriting the fault. Thus, these treatments are strongly discouraged by many breed societies. Invariably there is little option except to geld the stallion.
Breeding Cryptorchid Stallions
Although breeding cryptorchids is not actively encouraged, there are several well-known cryptorchid stallions that have had successful breeding careers. It is perfectly possible for a unilateral cryptorchid to successfully cover a mare. Indeed, there is no reason why his descended testis should not function normally.
The daily sperm production (DSP) of the average stallion is 10-20 billion if he is used one or two times per day with a rest day every seven to 10 days. A cryptorchid with one descended testis can be expected to produce 50% of the normal DSP (5-10 billion). As it is known that the number of mares a stallion can successfully cover is dependent upon his DSP, a cryptorchid can only be expected to cover half the maximum number of mares of a normal stallion and still give acceptable fertilization rates of 60-70% (a normal stallion should be able to cover twice a day, with a rest day every few days).
Most stallions are not expected to cover anywhere near their maximum number of mares; most cover only 50-60 per season. Thus, their fertility levels aren't challenged, so if the stallion is a cryptorchid, there should be no noticeable problem and no special management should be required.
However, lately a potential problem has arisen in breeds such as the Thoroughbred, as it becomes more evident that some stallions can successfully cover 250-plus mares per season by natural service. These numbers of mares challenge the fertility of a normal stallion and certainly can present the cryptorchid with a problem.
The key to successful breeding with a cryptorchid stallion is not to present him with an excessive book of mares. He should be able to cope with 50-60 mares per season by natural service. If, however, he has a large book of mares, he should be managed like a normal stallion of low fertility. Ideally, he should be presented with only one mare per day, and each mare must be monitored carefully to make sure she is bred at the ideal time to avoid rebreeding. The stallion's semen quality and fertility rates should be monitored closely, and you should be prepared to withdraw him temporarily from service if a problem occurs.
Artificial insemination (AI) is a good option for most breeds (but prohibited in the Thoroughbred). Semen can be collected and treated identically to that of normal stallions, evaluated to determine sperm concentration, then concentrated or diluted to give the required dose for insemination. AI also allows more mares to be covered per ejaculate than natural service, which compensates for the cryptorchid's lower DSP.
Unlike DSP, libido should not be affected too much by cryptorchidism unless he is a bilateral abdominal cryptorchid. As stated earlier, in unilateral cryptorchids, the descended testis will continue to produce testosterone normally. An inguinal retained testis will also produce testosterone, as it's often a reasonable size and hormone production is relatively unaffected by the higher testicular temperature. In an abdominal cryptorchid, the testis is much smaller, so the limited testicular tissue is less able to produce testosterone; hence, libido will be reduced. Disinterest in covering mares is rarely a problem specific to cryptorchids and, as with entire stallions, there is always significant variation between animals.
Gelding a cryptorchid is not a simple castration procedure. In particular, abdominally retained testes are very hard to access. While inguinal testes are easier to assess, they still require more complicated and expensive surgery than normal castration.
There are two methods of removing retained testes--laparotomy and laparoscopy.
Laparotomy allows the retained testis to be located and removed through the incision. This is fairly simple in inguinal cryptorchids, but abdominal testis removal is much tougher, with deeper dissection and considerable manipulation of internal organs, and might not even prove successful.
Laparoscopy is a more recent technique where the inside of the abdomen is observed via endoscopy and the testis located, tied off, and removed through a small incision. It is thus particularly useful for removing abdominal testes, which are very hard to access via laparotomy as stated. This procedure is preferred by many as the incisions are smaller and there is little post-operative pain, allowing the horse to return to training much sooner. However, it's still only offered by a few specialty centers.
Obviously, gelding a cryptorchid can be complicated. To save expense, many cryptorchids, especially abdominal cryptorchids, are not gelded correctly, thus leaving the retained testis in place. This can cause serious problems as the testicular tissue within the retained testis, while not spermatogenically active, still produces testosterone. Such animals outwardly appear to be geldings, but might show libido and prove unpredictable and dangerous.
Hormone therapy is currently experimental. The aim is to encourage the release of natural testosterone, which might encourage testes within the inguinal canal to fully descend. Testosterone secretion is encouraged using either hCG or GnRH (gonadotropin-releasing hormone); both have LH-like activity or encourage the natural release of LH, which in turn drives testosterone production. Such treatment is reportedly successful in some horses.
The cryptorchid stallion presents the owner with a dilemma. Due to the likely heritable nature of the condition, gelding is an option if the horse won't be bred or the breed registry doesn't accept cryptorchids. This in itself is not simple and can prove costly. Thus, it is not uncommon for such animals to be gelded improperly and the horse erroneously sold as a gelding--a particularly dangerous situation for subsequent purchasers. A veterinary examination can identify any retained testicles, and the horse can then be managed accordingly or castrated as the owner wishes.
About the Author
Mina Davies Morel, PhD, is head of the equine group at the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University in the United Kingdom. She has particular interest in equine reproductive physiology and its application to stud management, and she is the author of a number of scientific papers and text books on the subject. She is a leisure rider and owner of Welsh Cob Section Ds.