Page 114 - January 2021

CRYPTORCHID STALLIONS
by Heather Smith Thomas
Acryptorchid is a male horse in which one or both testicles are retained in the abdomen or inguinal canal and do not
descend into the scrotum. In the developing fetus, the testicles are formed within the abdomen, but as the fetus nears term, the inguinal rings and inguinal canal (the passage through which the testicles descend) expand to allow for descent of the testicles to the scrotum. This canal contains blood vessels and nerves, and the cremaster mus- cle, which raises or lowers the testes in response to temperature change or threat of injury.
Based on fetal dissection studies, descent of the testicles usually occurs during the last 30 days of gestation. Testicles in the developing fetus are quite large, perhaps due to hormonal influence from the dam. They become smaller as the fetus matures. If they stay too large, however, they may not be able to come down through the inguinal canal.
If migration down to the scrotum is inhibited such as when 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 be- low this ring, the horse is an inguinal cryptorchid.
In some instances, the testicle comes down from the abdomen but does not make it all the way into the scrotum. One or both testicles may 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. 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.
Dr. Tom Yarbrough explains that a small 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 scro- tal ligament, anchoring the testicle in the region of the scrotum, though 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 may remain above the scrotum or up in the abdomen.
PROBLEMS WITH INCOMPLETELY GELDED CRYPTORCHIDS
Sometimes a cryptorchid is gelded, remov- ing the one descended testicle and leaving the other. If only one testicle is found, some people assume the horse is a monorchid (a rare in- stance in which the horse only has one testicle) and they go ahead and geld him, not searching for the retained one. Or the intention may be to do surgery later to find the missing testicle, but the horse gets sold in the meantime and the new owner assumes it’s a gelding.
Even if the one descended testicle has been removed, the horse will still act like a stallion. He will try to breed mares, even though he is not fer- tile. The testicle retained within the body is small
and undeveloped compared to a normal testicle, and 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.
There is also some risk for a retained testicle in the abdomen to become tumorous as the horse gets older. Ahmed Tibary, DVM, PhD (Profes- sor of Theriogenology at the Veterinary Teaching Hospital at Washington State University) says retained testicles may develop abnormal growths. There are various kinds of tumors and these can change a horse’s behavior by altering the hormone balance. Since the testicle produces both male and female hormones (the latter in very small amounts, under normal conditions), a tumor may cause more pronounced masculine or feminine behavior, depending on which cells in the testicle are affected by the tumor.
“The most common thought as to why tumors develop is that because the testicle is retained, it has a different temperature environment. 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,” says Tibary. They may contain connective tissue, hair, muscle, teeth, or bone.
The testicle’s reproductive tissue provides ideal conditions for this misdirected tissue growth. In these instances, a teratoma (usually
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