Cryptorchid Concerns

Q: I have a 24-month-old Azteca (in this case, Andalusian x APHA) colt that has dropped only one testicle so far. He had two normal-appearing testicles at birth, and both of them withdrew above the inguinal ring before he was a year old.

This past winter, when he was about 20 months old, his left testicle dropped and appears normal, although rather large for his age. My vet palpated his inguinal ring recently and found it to be still open and about 3 inches in diameter, but could not palpate the other testicle.

I would eventually like to use this colt as a breeding stallion. I have not collected him yet to evaluate his sperm quantity and quality. There are no other known cryptorchids from the Andalusian sire. The APHA dam has only one other foal, a colt by the same sire, with two normal testicles at the age of one year.

I have sought advice on the chance of the other testicle dropping, as well as on the heritability of the condition, from a variety of sources. A friend who has been breeding Paints for many years says that it is not unusual for that breed to retain a testicle until the age of 30-36 months.

I have gotten several opinions, from veterinarians and breeders, on the heritability of cryptorchidism--years ago it was considered a taboo condition, but now there seems to be research indicating it may not be hereditary. What is your opinion? And if it is not considered an inheritable risk in a breeding stallion, how do I assuage the fears of potential mare owners, given the stigma the condition seems to carry?

If the trait is reasonably inheritable, then I will seriously consider castrating the colt, although it would be a waste, as he has numerous qualities that would make him very desirable as a sire.

via e-mail

A: Testicular descent is a very complex process, and causes for abnormal descent can be varied and are often difficult to document. According to the literature, and in my experience, testicular descent into the scrotum is generally complete at birth or within the first few days following birth. It is extremely uncommon for retained testes to descend into a scrotal position after a few months of age, but reports exist for testicular descent at 1 to 2 years old in rare instances.

Some newborn colts have open inguinal rings, which permit the testes to ascend into the abdomen and permit intestines to descend into the scrotum.

Treatment of young colts (less than 1-1½ years) with hormones such as GnRH (gonadotropin-releasing hormone) or hCG (human chorionic gonadotropin) has been reported to be relatively successful for testes that are retained in the inguinal canal, but not for those that are retained in the abdomen.

If a retained testis descends into the scrotum when a colt becomes older, it is quite likely that the testis will be smaller than normal. However, stallions with unilateral cryptorchidism (i.e., stallions with only one scrotal testis) often experience compensatory hypertrophy (increase in size) in the scrotal testis, such that it becomes larger than expected as the stallion matures.

It is quite uncommon for an internal inguinal ring to be as large as you have described (3 inches), and this might be a contributing factor to ascent of a testis back into the abdomen following testicular descent; however, this would imply that the structure responsible for "pulling" the testis into the scrotum initially (i.e., the gubernaculum) did not develop and mature properly.

Several reports suggest a genetic basis for cryptorchidism in horses, but this has yet to be confirmed. In one study conducted at Texas A&M University, 56 of 58 colts sired by a cryptorchid Quarter Horse stallion had normally descended testes. The two remaining foals were unavailable for evaluation when the data were compiled.

If cryptorchidism of the above Quarter Horse was heritable, the authors surmised that transmission of the trait by an autosomal dominant gene was highly unlikely, unless the gene had extremely low penetrance (meaning the proportion of individuals with a particular gene variation that expresses an associated phenotypic trait would be quite low). Results of the study also did not support autosomal recessive inheritance, unless the frequency of the recessive gene in the mates of the stallion was near zero.

I suspect that, with the striking advancements being made in the areas of equine genetics and genomics, we might one day soon have an answer to the heritable nature of cryptorchidism. One must keep in mind that many breed organizations currently allow the use of cryptorchid stallions as sires.

About the Author

Dickson Varner, DVM, PhD, Dipl. ACT

Dickson Varner, DVM, PhD, Dipl. ACT, is based at Texas A&M University. He is a member of the AAEP Editorial Advisory Board of The Horse.

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