Horse-Breeding Problems: Priapism

A stallion’s prolonged erection is not a joking matter.

TV commercials instruct viewers that a physician should be contacted immediately if an erection lasts longer than four hours after taking certain medications. We’ve all heard jokes and politically incorrect comments regarding persistent erections in men after taking these medications. What about a persistent or inappropriate erection in a stallion? Is there a concern? The answer is yes, and it’s not funny. “Priapism,” or persistent erection, in humans is described as an erection that lasts for more than four hours beyond sexual stimulation or an erection that is unrelated to sexual stimulation. In most cases, emergency treatment is required to avoid permanent damage.

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In stallions, an erection is attained following an increase in blood flow into the corpus cavernosum of the penis, and the erection subsides when blood exits the structure. Failure of blood to exit the corpus cavernosum within a reasonable time period can lead to blood clotting, formation of fibrous tissue and permanent loss of normal penile function, leading to impotence and infertility. An 11-year-old American Quarter Horse stallion, actively used for both live cover breeding and western performance, was once admitted to the clinic for evaluation of front-leg lameness. Physical exam and radiographs revealed osteoarthritis and a chip fracture in his knee. Arthroscopic surgery was recommended to remove the chip and clean up the joint. Pre-surgery blood work and clinical examination were normal. The stallion was sedated, and general anesthesia was induced. The anesthetized stallion developed an erection approximately 45 minutes into the surgical procedure. Cold compresses were applied and the penis was massaged in an unsuccessful attempt to reduce the erection and replace the penis into the prepuce. Intravenous administration of benztropine mesylate did not result in significant change in the erection.

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A decision was made to drain the blood from the corpus cavernosum. Two large-bore needles were placed into the penis, one needle near the end of the penis just behind the glans penis and one needle at the base of the penis. Blood was drained from the penis, and the corpus cavernosum was subsequently lavaged with heparinized sterile saline to remove additional blood and prevent clot formation. Thankfully, the penis rapidly decreased in turgidity as the stagnant blood was removed, and was replaced into the prepuce. A purse-string suture loosely placed around the prepuce ensured that the penis remained within the sheath during recovery from anesthesia. After recovery, the sutures on the prepuce were removed. The horse was fitted with a sling to prevent the penis from exiting the prepuce for three days. In stallions, penile problems associated with medications are primarily limited to administration of the tranquilizer acepromazine. “Ace” has been associated with paralysis of the penis in stallions.