Veterinarians at the Smithsonian’s National Zoo performed the first successful reverse vasectomy on a Przewalski’s horse (E. ferus przewalskii; E. caballus przewalskii—classification debated), pronounced zshah-VAL-skeez. Przewalksi’s horses are a horse species native to China and Mongolia that was declared extinct in the wild in 1970.

Currently, there are approximately 1500 of these animals maintained at zoological institutions throughout the world and in several small reintroduced populations in Asia. This is the first procedure of its kind to be performed on an endangered equid species.

The genes of Minnesota—the horse who underwent the surgery—are extremely valuable to the captive population of the species, which scientists manage through carefully planned pairings to ensure the most genetically diverse population possible. The horse was vasectomized in 1999 at a previous institution so that he could be kept with female horses without reproducing. He came to the National Zoo in 2006.

While surveying the captive North American population of Przewalski’s horses, scientists realized Minnesota’s genetic value. Based on his ancestry, he is the seventh most genetically valuable horse in the North American breeding program. Zoo scientists were confident that if they could successfully reverse the vasectomy, Minnesota would be able to sire a foal through natural mating.

“The major challenge we faced was that this procedure had never been performed on an equid, let alone a critically endangered species,” said Dr. Budhan Pukazhenthi, a reproductive scientist at the National Zoo’s Conservation and Research Center in Front Royal, Va. “We had to develop all new protocols ourselves.”

The team sought the expertise of Dr. Sherman Silber, a St. Louis-based urologist who pioneered microsurgery for reverse vasectomies in humans and had been successful in vasectomizing and then subsequently reversing vasectomies in South American bush dogs at the St. Louis Zoo.

“Although our team is very experienced in horse anesthesia and surgery, by using the specialized professional skills of Dr. Silber, we greatly increased the likelihood of success,” said Dr. Luis Padilla, associate veterinarian at the Conservation and Research Center.

Silber, working with the Zoo’s team of veterinarians and reproductive scientists, first performed the operation on Minnesota in March 2007. That procedure proved unsuccessful, possibly due to the presence of scar tissue or the fact that the horse was positioned on its side, making it difficult to perform the surgery. Silber was confident that if the horse could be placed on its back, the procedure would be a success.

Laying an anesthetized horse on its back for a prolonged period of time can be challenging due to their size and physiology. Veterinarians decided it could be done, but only if the surgery time was kept to a minimum. In October 2007, the team operated on Minnesota again—completing the procedure in an hour. Six months later, the Zoo’s veterinarians and reproductive scientists collected a semen sample from the horse that indicated the procedure had been a success.

“I’ve always dreamed of using my expertise to contribute in some way to wildlife survival,” said Dr. Silber. “It also was exciting to pioneer a new procedure for which humans were the ‘test animal.’”

National Zoo scientists hope to pair Minnesota with a suitable female later in the coming months. His genes will infuse genetic diversity in a Przewalski’s horse population that is based on genes from only l4 original animals. National Zoo scientists are researching ways to improve fertility and produce more offspring in the aging, captive population. Bolstering the population translates into more horses for future reintroduction programs, essential for a critically endangered species. Currently, National Zoo scientists are working in remote areas of China using radio collars and Geographic Information System technology to map the movements of Przewalski’s horses reintroduced by Chinese colleagues into their former habitat.

This breakthrough also has important implications for how endangered species in captivity are managed. The new knowledge could allow males and females of a species to be exhibited together but temporarily prevented from producing offspring if the Species Survival Plan—a cooperative breeding program among zoos—does not recommend them for breeding.