Dickson Varner, DVM, MS, Dipl. ACT, professor in the Department of Large Animal Medicine and Surgery at Texas A&M University, presented several cases of stallion infertility at Florida Association of Equine Practitioners (FAEP) Promoting Excellence Symposium, held Sept. 27-29, 2007, at the Atlantis resort on Paradise Island, the Bahamas. These scenarios shed some light on some of the typical and atypical reasons for this expensive problem.

Some of the cases Varner illustrated included issues with the quality and quantity of sperm and problems with the stallion's reproductive conformation, including congenital abnormalities (present at birth). He also discussed the impact of case history in assessing these horses. Varner looked for feedback on the cases from the assembled practitioners, quizzing the attendees on what steps they would take to diagnose and manage these stallions.

Varner noted that stallions get to remain intact and become sires because of their pedigree, conformation, and performance--not necessarily their reproductive ability. So it might be up to veterinarians to close the gaps to get them performing at their optimal level.

One case involved a stallion that was practically infertile at the beginning of the breeding season, but whose pregnancy rate improved to the high end of normal by the end of the year. The more mares this stallion covered, the better his fertility became.


Evaluating sperm for acrosome presence

Stallion sperm exposed to a fluorescent stain to evaluate the presence (yellow) or absence of the sperm acrosome. The sperm acrosome fits over the head of the sperm like a bathing cap, and opens to release enzymes that are necessary for fertilization to occur.

Varner said these cases can often be attributed to sperm plugging the ampullae (the thickened distal portions of the deferent ducts). Sperm production doesn't stop during breeding's off-season, and if the horse isn't given an opportunity to "clear the pipes," sperm can become lodged in the ampullae. Varner said this is most common in horses with larger testes, which create more sperm and perpetuate the problem. Varner estimated that he does phone consultations on or personally sees approximately 20 such cases every year. Luckily, treatment for these horses is generally simple and effective--massaging the ampullae, giving oxytocin and collecting from these horses regularly throughout the off-season can keep the ampullae from clogging.

In another case, a Quarter Horse stallion had abysmal pregnancy rates--out of books of 25 mares each year he was able to impregnate only two in two years. After much collecting and testing, Varner reached the conclusion that the horse's sperm were unable to acrosome react--a necessary step in a sperm's ability to fertilize an egg.

The sperm of affected horses do not develop pores between the surface membrane and underlying acrosomal membrane, thereby disallowing release of enzymes that are critical to penetrating the layers surrounding the egg. This condition is associated with an imbalance in the cholesterol/phospholipid ratio in the stallion, but the underlying cause has not been determined.

Varner noted that he has seen numerous such cases in the Thoroughbred industry and only a couple in Quarter Horses. One Quarter Horse recovered on his own, but none of the Thoroughbreds have recovered from the condition, which might have a genetic predisposition.

Varner was also called in to consult on a case involving a stallion that was recovering from a neurologic disorder. Only 4% of this stallion's sperm were normal. The veterinarians eventually concluded that the stallion's testes had become overheated during the time he spent in lateral recumbency while recovering from his disorder seven months earlier.

Varner said this is a good illustration of the time required for restoration of spermatogenesis following lengthy periods of illness or stress. If a horse's testes experience localized heat due to an injury, or if he goes through a febrile period, it can take months--or even a year--for production to return to normal levels.

Occasionally, a case will come along that's just plain weird, said Varner. He described one stallion that produced a crystal-clear ejaculate--no sperm at all. He eventually determined that the stallion had a congenital abnormality in his reproductive conformation--he had a bilateral closure of the ductus deferens (put simply: his pipes didn't go anywhere--they started at the testes and hit a dead end near the urethra). The testes were normal and producing sperm. A sample revealed that the stallion had 15 billion sperm per cc of fluid within the epididymides (a more typical count is 250 to 500 million)--but it couldn't go anywhere.

Take-Home Message

It pays to think outside the box when considering possible causes of stallion infertility. The stallion's reproductive conformation, quality of sperm, and history can all impact his performance in the breeding shed.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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