Fertility of the young stallion, or the supposed lack of it, due to the location of his testicles has, at times, sparked heated controversy in the miniature horse industry. It pits buyers against sellers, veterinarians against breeders, friends against friends, and dashed the show hopes of exhibitors. The issue at the root of this controversy is whether or not the developing stallion is a CRYPTORCHID.
For those who may be unclear as to the definition of this term, we provide the following simple explanation. Cryptorchidism is the condition where one or both of a stallion’s testicles do not descend from the abdominal cavity into the scrotum. One of the keys to optimum fertility of the stallion is that both testicles reside in the scrotum. Development of sperm is affected by temperature. Normal sperm development takes place below body temperature in the scrotum. If testicles are retained in the abdomen, sperm do not develop normally because they are kept at body temperature. If you read our article on Stallion Fertility, you will remember that fertility is measured by the total number of motile sperm in the stallion’s ejaculate. Since a retained testicle does not produce normal sperm, the total in ejaculate is low therefore the fertility of the stallion comes into question. It is also thought by some that cryptorchidism may be a genetic trait and passed on from a “crypt” stallion to his male offspring causing them to be “crypt” as well.
In order to understand the actual mechanisms involved, a short discussion of stallion anatomy is in order. The testicles develop in the abdomen of the foal while it is developing in the mare prior to birth. The testicles must migrate downward through the abdomen, and a channel called the “inguinal ring” before they can enter the scrotum.
Conventional wisdom says that testicles complete their migration into the scrotum any time from just prior to birth to a short time after foaling. Conventional wisdom also states that if both testicles have not descended into the scrotum within the first year of life, the horse is a cryptorchid. This principle has been well founded in the full-sized horse industry. Due to the rapid growth of young colts it has been well documented that a full sized horse’s testicles become too large to pass through the inguinal ring as they begin to develop. Since there is almost no written research related to miniature horses on this issue, practitioners generally apply these same norms to miniature horses. Frequently after veterinary examination, some yearling and two-year-old miniature horse colts are determined to be cryptorchids based simply on the knowledge resources of the veterinarian by the norms derived from the large horses.
Experienced breeders of miniature horses have frequently taken exception to these statements due to many years of observing the development of their horses. Miniature horses present a number of unique situations that are not commonly found in large horses. For example, foals routinely born within an unbroken amniotic sac is fairly unique to miniatures. It also turns out that miniature horse testicles do not necessarily grow and descend into the scrotum in the same manner as they do in large horses. Long time breeders have been aware of this for a years. There have been many discussions about colts who have not “dropped” and are two-year-olds. “Old-Timers” have said, “Just be patient. They will come down, usually by their third year.”
Do these “Old timers” really know what they are talking about? Are they trying to pull the wool over our eyes to preserve a sale? Isn’t my veterinarian the appropriate person to diagnose the fertility of this colt? The stress level can elevate when there is a sale involved and the buyers have discovered that their future breeding stallion has been diagnosed as a “cryptorchid” by the veterinarian while the seller is saying, “be patient”. This scenario or a variation has been played out many times over the years. Certainly, sales hinge on this issue, not to mention credibility of sellers, breeding stallions, friendships, and miniature horses in general. The problem lies in the fact very few written studies have been made related to the descent of testicles in miniature horse stallions. Since it is difficult to find documentation, there continues to be this gulf between the general opinions of many veterinarians and those of long-time breeders of miniature horses.
It would be helpful if there was research that would give all of us some guidance as to what the actual situation is. Well, after a lot of reading, we did find ONE document that has been printed in two highly regarded equine veterinary texts. The same article is quoted in both publications. The article is written by J.E. Cox, Ph.D., M.R.C.V.S. Senior Lecturer, Division of Equine Studies, University of Liverpool, England and was published in the texts, Current Trends in Equine Medicine 2 and Equine Reproduction. In his paper Dr. Cox refers to the situation that occurs in miniatures as Temporary Inguinal Retention. He states, “This occurs predominantly in ponies and is characterized by small testes, which are usually readily palpable in the anesthetized horse in dorsal recumbency (anesthetized and lying on his back). If these testes are not removed, they grow and descend into the scrotum usually by the time the animal becomes a three-year-old.” Dr. Cox substantiates much of what breeders have known. There are a numerous cases where colts have been sold and subsequently returned due to apparent cryptorchidism, only to develop into fully functioning stallions with normal fertility when they became three and four-year-olds.
There was one very interesting case we were vicariously involved with in 1997. The owner of a 1 1/2 year old colt wanted to have it gelded but one of the testicles had not descended. Arrangements were made at the Oregon State University Veterinary School to have the retained testicle surgically removed. Upon admission, the surgeon stated that the colt was a cryptorchid and he would have to go into the abdomen of the horse to remove the large testicle that could not pass through the inguinal canal. He was applying the conventional wisdom mentioned above that any colt over a year old is a cryptorchid. The plan was to use a laparoscope to do the surgery. This would minimize the size of the incisions and recuperation time. During the discussion we mentioned that miniature horses are sometimes slow to descend and that frequently miniatures do not drop until they are three. The vet was pretty firm that this colt was a cryptorchid and there was little chance that the horse would ever get it’s second testicle through the inguinal ring.
The surgery went well. After a day of recuperation the horse was ready to take home. When the horse was being discharged, a very interesting discussion occurred with the surgeon. He related his “surprising” findings from the surgery to remove the “cryptorchid” testicle. Upon entering the abdomen of the horse, the surgeon could find no evidence of a large, retained testicle as he was expecting. As he turned the laparoscope toward the inguinal ring he was intrigued to find a small testicle just inside the ring, precisely as described in the Cox article. He mentioned that this small testicle would definitely have been able to pass through the inguinal ring, and, given time, the retained testicle would, no doubt, pass into the scrotum. The surgeon also went on to say that he has definitely revised his opinion as far as cryptorchidism and miniature horse stallions. He now agrees that they can mature slower than full sized horses. Testicles in some miniature horse colts are slow to descend, taking three or perhaps more years to occur. Once the testicles have descended, they mature normally and the fertility of the stallion does not appear to be compromised.
So where does all of this bring us. Until such time as more formal research is done on miniature horses, this issue will probably remain somewhat controversial. The difficulty with this issue is the element of doubt created with those colts whose testicles have not descended. The primary concern involves the relationship between a buyer and seller in a situation such as this. There is the concern about breeding as a two-year-old. There is the issue of showing a horse as a three-year-old and providing veterinary verification of being intact. There are also verified cases of cryptorchidism in miniature horse stallions where testicles have not descended even as adults and eventual gelding occurred. Certainly these topics deserve discussion at the time of purchase. It always seems that the sales relationship is a lot healthier if the issue is covered in advance. Our best suggestion is that potential buyers and sellers come to some mutually agreeable arrangement related to the possibility of cryptorchidism prior to finalizing a sale. Make an agreement in writing. Live by the signed agreement.
By this discussion, hopefully, the reader will gain a greater understanding of this issue as it applies to miniature horses.