veterinarians castrating a cryptorchid horse

The World of Cryptorchids

I have a theory that states the size of a stallion’s testicles is inversely proportional to their “undesirable” behavior. Stated another way, the smaller the testicles, the more the attitude; the larger the testicles, the calmer-minded the stallion. Over 20 years of equine veterinary medicine, I have found this to be true of horses and men alike. Work your way through a crowded barn of yearling stud colts at a sale or hang out behind the bucking chutes at a rodeo, and you’ll see the basis of my thought process.

Now, anytime I start an article off about horses and testicles, the next paragraph is bound to be about removing them. (Testicles, that is.) While nature has provided each male with a pair, in some, only one is external. This is called cryptorchid because one or both of the colt’s testicles is retained within the body and did not descend into the scrotum like normal. There have been a good number of colts I swore had three or four testicles, as rank as they acted when I went to geld them. But, in the truly normal equine foal, both testicles descend into the scrotum somewhere between 30 days before birth and 10 days after birth.

I do not want to overlook the great research that was done to identify and document the anatomy and physiology that occurs in equine embryology, which explains testicular descent, but this article has to fit one page. So, suffice it to say that at five weeks of gestation, the male testis has developed in close association with the fetal kidney, high up in the abdomen. Fast forward to the 10th month of gestation and the testis has followed, under hormonal control, a thin band of tissue through the abdomen that connects the bottom end of the testis to the inside of the scrotum. Crazy stuff, I know.

The retained, or cryptorchid, testicle is located in one of three places — either just inside the body adjacent to the abdominal wall, within the inguinal canal or just outside the body and inguinal canal, but not in the scrotum. The inguinal canal, which allows the testes to exit the body during development, is the anatomic space created by the two rings formed from naturally occurring slits in the abdominal oblique muscles. For the sake of reference, this is where inguinal hernias occur when intestines exit these slits.

Between you and I, just remember that normal testicular descent depends on various hormones and structures playing well together for things to go right. However, in 5 to 8 percent of colts, they do not, and you get a cryptorchid. Our beloved Quarter Horse has the highest incidence of cryptorchidism of all breeds, with only about 14 percent of those being bilateral, where both testicles are retained. With retained testicles, the inability to regulate temperature causes sterility in the affected testicle. Testicles being outside the body at a lower temperature in the scrotum allow for normal semen production. Spermatogenesis only occurs at certain temperatures, and internal body temps (99.5-100.5 degrees Fahrenheit) are too high for that to occur; however, you still get all the attitude testosterone can provide, because the Leydig cells of the testes, which produce testosterone, are much more heat tolerant than the seminiferous tubules that produce sperm cells.

The diagnosis of a cryptorchid can be as simple as visual inspection and careful palpation to determine if and which testicle is retained. Sedation greatly facilitates this task, even in compliant colts, as they may relax enough to confirm the presence of both testicles. You’d be surprised how far up they can pull their testicles when the need arises. In colts with questionable or unknown histories, simple blood tests can be diagnostic. Baseline testosterone levels in a gelding average about 0.12 ng/mL, whereas horses with testicular tissue average about 0.80 ng/mL. I prefer to run human chorionic gonadotropin (hCG) stimulation tests on horses I’m not sure about. A simple baseline blood sample is taken, and then a large dose of hCG is given intravenously. This hormone stimulates the Leydig cells to produce testosterone. Blood samples are taken at one- and two-hour intervals after the injection. These samples are evaluated for increases in testosterone levels from the baseline sample. A flat-line testosterone level proves the absence of testicular tissue. There was no testicular tissue to stimulate. A positive test will increase testosterone levels four to 20 times the baseline because of the external stimulation by the hCG, meaning that there is testicular tissue present.

Further diagnostics can be done by rectal palpation with or without ultrasound to identify abdominally located testicles, but this carries a greater risk of inadvertent rectal tears in the colt. Some argument can be made for testing estrone sulfate levels, as well, but only in a certain few instances. Either way, castration is and should be the treatment of choice.

There are multiple reasons for cryptorchidism based on anatomy and physiology, but genetics and heredity play as big a role as the others combined. Cryptorchid stallions left intact have a high incidence of tumor formation in the retained testicle, all while spreading a genetically preventable defect of the horse. So, if someone tells you that what you can’t see won’t hurt you, ask them if they can count to two.


This article was originally published in the February 1, 2019, issue of Quarter Horse News.