Researchers Reverse Temporary Immunocastration

Stallions that do not fully recover breeding capacity after temporary castration via a vaccine that works against reproductive hormone GnRH can now be assisted through daily injections of buserelin, a GnRH agonist, said European researchers.

Equity, an anti-GnRH vaccine from Pfizer Animal Health currently marketed in Australia to control heat cycles in mares, can also be administered to stallions to temporarily "immunocastrate" them. The drug inhibits testosterone and can be used to improved behavior during competition season, for example. While most stallions receiving the Equity vaccine recover their full breeding capacity within a year, there is a risk that these horses could remain permanently castrated, said Marianne Vidament, DVM, researcher at the French National Stud and the French National Institute for Agricultural Research (INRA), and primary author of the study.

But now there's hope to reverse that process, at least temporarily, Vidament said. Intramuscular injections of buserelin every morning, starting two months before breeding season and continuing throughout the season, appear to restore both libido and fertility to pre-castration levels, she said.

In her study, Vidament focused on stallions which had been immunocastrated because they carried the equine viral arteritis virus. Within eight months, seven of the eight stallions no longer shed the virus, she said. However, three of the stallions experienced long-term castration, unable to breed the following season.

"Every time one of these stallions was injected with the buserelin, there was a significant jump in testosterone level within just a few hours," Vidament said. "The horse would regain his full reproductive interest and capability, whether for natural breeding or collecting with a dummy mount."

However, the effects were short-lived, as base hormone levels returned to their immunocastration state within 24 hours, she added.

The relatively low volume administered into a muscle makes it a simple, convenient, and inexpensive--albeit temporary--solution to the problem, Vidament said. Even after months of consecutive daily injections, the horses showed no negative side effects.

"The risk of long-term castration with immunocastration is far from being non-negligible," Vidament said, during the presentation of her research March 4 at the Equine Research Day in Paris. "But fortunately we now know that buserelin injections can help moderate that risk."

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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