Sterilizing stallions temporarily

Sterilizing stallions temporarily can help with feral population control, equine venereal disease management, and behavior issues. But it’s risky business, because sometimes the sterilization becomes permanent. Some researchers have worked to find ways to reverse immunocastration in stallions that received anti-GnRH injections and didn’t become fertile again naturally. But a group in Austria has been focused on a very different approach: GnRH receptor downregulation.

“If GnRH receptor downregulation would work, it would be a great alternative to GnRH immunocastration because of the better reversibility,” said Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute, in Neustadt, Germany, and professor of artificial insemination and embryo transfer in the Vetmeduni veterinary school Department for Small Animals and Horses, in Vienna, Austria.

Both methods modify the body’s use of GnRH—gonadotropin releasing hormone. The brain’s hypothalamus releases GnRH into the body, which signals the reproductive organs to function; this causes mares to cycle and stallions to produce sperm.

With the anti-GnRH vaccine, antibodies are stimulated to overact against GnRH, making the hormone ineffective. In GnRH receptor downregulation, however, horses receive a sort of overdose of synthetic GnRH that more or less short-circuits the body’s GnRH receptors. Essentially, the receptors are no longer receptive to the hormone, and they don’t respond to its calls to activate the reproductive system.

Aurich used a different approach—a synthetic GnRH called deslorelin, which has been shown to work well in dogs. The research team tested deslorelin implants in Shetland pony stallions over an 11-week period to see how they affected their libido, sperm motility, sperm membrane integrity, sperm DNA fragmentation, estrogen receptors, blood testosterone levels, and anti-Mullerian hormone (AMH, a sex hormone) concentrations. They divided the ponies into treatment groups to compare different dosing regimens. The scientists also checked hormone responses to certain hormonal challenges to see how the implants affected basic hormone responses at the pituitary gland level.

They found that deslorelin at high doses reduced the pituitary gland’s response to GnRH, she said. But it didn’t affect the rest of the parameters significantly. The stallions still had good-quality sperm and healthy libido throughout the experiment, regardless of dose.

“In the present study we could show that the treatment with the deslorelin implants results in clear downregulation of the GnRH receptor, but, surprisingly, gonadal (testicular) function was almost not affected,” Aurich said, adding that other studies have shown similar results in cattle. “Deslorelin treatment does inhibit the pulsatile release of some reproductive hormones, but there is still a certain degree of gonadotrophin release that is sufficient for stimulation of gonadal function.

“These implants are not, therefore, useful for reduction of sexual behavior and gonadal function in stallions, which is in contrast to GnRH immunocastration that obviously results in much more complete cessation of gonadal function (although there are some individual differences),” she continued. “However, it would still be very interesting to have a method for a reliable and really transient reduction of gonadal function in stallions.”

The team is now looking at how the deslorelin implants might affect ovarian function in mares, she said.

The study, “Effects of implants containing the GnRH agonist deslorelin on testosterone release and semen characteristics in Shetland stallions,” was published in Animal Reproduction Science.