Bluegrass Equine Reproduction Symposium: Stallions

A four-day seminar focusing on reproduction education for veterinarians was hosted by Hagyard-Davidson-McGee veterinary firm in Lexington, Ky., Oct. 23-26 This is the fifth year for a fall seminar hosted by the group, which now alternates with a critical care symposium in odd-numbered years.

The first day offered a series of wet labs on pathology, embryo transfer, fetal developmental monitoring, and hysteroscopic examination and insemination. Following are reports on some of the lectures.

Stallion Anatomy and Physiology

Lectures the following day mainly covered stallion reproduction topics. Tom Little, DVM, the resident veterinarian at Gainesway Thoroughbred farm in Lexington, discussed the stallion’s reproductive anatomy. Little said that familiarity with normal anatomy and physiology “is essential to recognizing clinical problems and formulating rational therapeutic approaches.”

Little discussed the various features of the stallion’s reproductive organs and accessory sex glands. He said vasculature (amount of blood vessels) has an influence on the sizes of a stallion’s testes. And that the size of the testes can be used to estimate daily sperm output based on research done by Charles Love, DVM, PhD, Dipl. ACT, who also was a speaker on the program.

As all breeders know, a stallion has the ability to regulate the temperature in the testicles by drawing them closer to the body or letting them be farther away from the body. Also, the outermost layer of skin contains sebaceous glands and an “exceptionally high number of sweat glands,” noted Little. “These impart an oily feel to the surface of the scrotum and aid in thermoregulation.”

The next layer in is the tunica dartos, “which consists of smooth muscle and fibroelastic tissue,” said Little. “This layer lines both scrotal pouches, including the median septum, and controls the degree of relaxation in the scrotal wall.” The next layer is the scrotal fascia, which allows “considerable mobility” of the parietal tunic and testis within the scrotum. The fourth and innermost layer is the parietal vaginal tunic, which, according to Little, surrounds each spermatic cord, testis, and epididymis and is continuous with the parietal peritoneum at the vaginal ring.

Another important feature of the stallion’s testes is the vaginal cavity, which is a small space that contains a small amount of serous fluid that serves to lubricate and facilitate movement of the testis within the parietal vaginal tunic.
Another anatomical feature of stallions is the cremaster muscle. During breeding, the cremaster pulls the testicles “out of harm’s way” (see image).

Little listed the accessory sex glands as the seminal vesicles, ampullae of the deferent ducts, prostate gland, and bulbourethral glands. He said secretions from the accessory sex glands contribute about 95% of the ejaculate volume of what is collectively called seminal plasma. “The specific functions of seminal plasma, other than as a vehicle for sperm transport to the mare, have not been established,” he added.

Disorders of the Accessory Sex Glands

Dickson Varner, DVM, MS, Dipl. ACT, of Texas A&M University, said that in general, stallions are amenable to transrectal palpation and ultrasonography, although appropriate restraint is important. He said is usually is not necessary to go deeper than the elbow to palpate or use ultrasound in a stallion. Varner said it sometimes is necessary to tease a stallion before palpation or ultrasound, and video endoscopy can be useful.

Developmental anomalies of the accessory sex glands are rare in stallions, Varner noted, as are tumors. He added that bacterial infections of these glands also are uncommon. However, he said bacterial infections are noteworthy “because of their persistent nature, effect on fertility, and proclivity for venereal transmission.”

One problem that generated much attention was spermiostasis, or plugged ampullae. The ampullae are the thickened distal portions of the deferent ducts, noted Little in his previous talk. In the adult horse, they are 15-20 mm in diameter and approximately 20-25 cm in length. The ampullae converge over the neck of the bladder and pass under the isthmus of the prostate, where they taper sharply to form excretory ducts.

Some stallions, said Varner, have an impaired ability to get rid of spermatozoa from this area when they have been sexually rested. “As a result, spermatozoa accumulate in the epididymis and deferent ducts, including the terminal portion of the ductus deferens, the ampullae; hence the designation of the condition--plugged ampullae.”
Indications that this problem is occurring are normal or large testicles, no sperm in the ejaculate, or a high incidence of detached heads from ejaculated sperm (50-80%). Sperm also might have an abnormally high concentration, and sperm might be ejaculated in clumps.

Varner said this problem usually can be solved by “serial ejaculations” or collection of 10-20 ejaculates over a one- to two-week period. Massage of the affected ampulla(e) prior to semen collection might assist in dislodging semen in the plugged lumina(e). Varner said administration of oxytocin or prostaglandin F2 alpha prior to breeding or semen collection might aid in correcting the problem.

“Relapse is likely if an affected stallion does not ejaculate regularly (two to three times weekly),” said Varner. “Spermiostasis that results in persistent azoospermia may require catheterization of the deferent ducts under general anesthesia, followed by forced antegrade removal of the luminal contents by fluid pressure.”

During the conference, several speakers noted they had made more diagnoses of cases of plugged ampullae in recent years.

Discoloration of Semen

Love, also of Texas A&M, said that evaluation of semen quality should not just include motility, morphology, and total sperm numbers. He said the clinician also needs to evaluate the gross character of the semen sample, which includes the color and consistency.

“The type of cells present in the ejaculate might include sperm, red blood cells, white blood cells, epithelial cells, fragmented cells, or urine crystals,” said Love. He said one of the first characteristics of the semen sample should be color, which can range from clear to opaque-white or brownish-gray-black, to pink, red and yellow. “The normal color is primarily due to the concentration of sperm cells, but can also be affected by epithelial cells and frangmented cells,” said Love.

He listed some potential causes of the variety of color of stallion semen. Clear semen could be caused by ejaculation failure, complete blockage of the ampullae, low total sperm numbers, and excessive lubricant in the artificial vagina.

Brown-gray-black semen usually is a result of epithelial cell and smegma contamination of the ejaculate, noted Love. Inadequate washing of the penis can result in this “brackish” appearance to semen.

Opaque semen usually has a very high sperm concentration, but also can be associated with the presence of large numbers of white blood cells. Love said stallions at sexual rest will have a larger number of sperm in the tail of the epididymis than stallions which are on a frequent ejaculation schedule. These excess sperm might be fine, or they might have reduced motility caused by excessive storage time in the epididymis.

Red semen usually means there is blood in the ejaculate. The amount can vary from microscopic levels to “overt” amounts of blood, said Love. It is critical to isolate the source of the blood for a diagnosis. The causes can include penile abrasions, red blood cells from a mare when a dismount sample is taken from live covers, and urethral tears.
Yellow semen usually is a result of urine in the ejaculate (urospermia). He said it is unclear why some stallions occasionally have urine in ejaculates, but clinical work-ups should be done to rule out problems.

One--or Two--Strands of DNA?

Love also discussed sperm chromatin structure assay, which is used to evaluate the ratio of single (abnormal) and double strands of DNA in individual sperm using fluorescent dye. “An increase in the amount of single-stranded DNA in fresh-frozen sperm has been associated with reduced fertility in a variety of species, including humans, boars, bulls, and stallions,” said Love. “In addition, some stallions that exhibit a high level of compromised DNA in fresh semen also have an accelerated rate of decline in DNA quality when the sperm are chilled and stored over time.”

The result of this test is plotted on a graph (see graph). This graph can help veterinarians in seeking a cause for reduced fertility. Cells which have abnormal DNA are plotted to the right on the graph, and are calculated as %COMP (percent cells outside the main population). Love said an increase in %COMP is correlated with decreased fertility.

This test gives a new way to look at semen and fertility.

Question and Answer Session

Love said that if the %COMP is low, then you are okay just doing one test (the stallion’s number of abnormal cells is low). However, if the number is high, he recommended doing another test just to make sure something didn’t happen in transport of the semen.

A question concerning horses with high %COMP and an association with early fetal loss prompted Love to say that this problem was looked at initially in humans, and researchers felt that there was a higher early embryonic loss associated with high %COMP. But, Love said he doesn’t feel this holds true for stallions. “It’s a logical assumption, but I’m not sure it happens,” he said.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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