Embryo Transfer an Involved but Viable Breeding Option

Horse owners with a mare that can't carry a foal to term need not give up on the idea of getting a foal from her. One potential solution is embryo transfer (ET), a technique that involves taking a fertilized egg from the desired mare and implanting it into another mare's uterus to carry to term.

It's also an option for owners with performance mares that would like a foal, but want to keep their mares in competition. Depending on a horse's registry, it is also a way that valuable mares can produce more than one foal a year.

"ET is actually a routine technique in equine veterinary practice," said Ahmed Tibary, DVM, PhD, Dipl. ACT, head of the large animal theriogenology program at the Washington State University College of Veterinary Medicine. "It can be used as an infertility treatment or for mares with severe lameness, metabolic disorders, or other health disorders for which pregnancy may be too risky. But most people use ET for performance horses, such as show, polo, dressage, or jumping horses."

" The key to success is getting a healthy embryo, and finding a healthy and reproductively sound recipient mare that can carry the embryo and is ovulating at the same time as the donor mare."
–Dr. Ahmed Tibary
While the technique might be routine, it is quite involved.

"It is a case-by-case situation," said Tibary, who has been performing ET since 1985. "The key to success is getting a healthy embryo, and finding a healthy and reproductively sound recipient mare that can carry the embryo and is ovulating at the same time as the donor mare."

The ET process begins like any other pregnancy. First, the donor mare should have a breeding soundness evaluation to evaluate her ability to ovulate and produce a fertilized embryo. Then she should be monitored during the cycle and bred close to ovulation to increases the odds of pregnancy and getting a viable embryo.

"The chance of getting an embryo in the first place can be difficult depending on the age and reproductive health of the donor, and on the semen quality and fertility of the stallion used," Tibary said. "In older mares or those that have reproductive problems, the odds of getting an embryo in a particular cycle may be well below 40%. In younger mares with no reproductive problems, there is a 70 to 80% chance of getting an embryo. To boost those odds, it is imperative to use high quality semen from a highly fertile stallion, in addition to close ultrasonographic monitoring of the mare in the immediate pre- and post-insemination period and ovulation to address any problems that may arise."

At the same time the donor mare is being monitored for ovulation, several potential recipient mares should be checked for ovulation as well. They should also have breeding soundness evaluations to make sure the recipients are healthy enough to carry the foal to term. The goal is to find a recipient that is ovulating the same day or one or two days after the donor. To help this happen, veterinarians can treat the mares with hormones to synchronize their ovulation. Another option is to use a mare that has had an ovariectomy after she has been prepared with hormones.

Embryo for transfer

An equine embryo at 7.5 days.

Seven days after ovulation has occurred in the donor, the embryo is flushed out through a Foley catheter inserted through the cervix, using a special protective solution. In most cases, the fresh embryo is transferred directly to the recipient mare with a conventional non-surgical technique similar to insemination.

"In some cases, you can freeze the embryo until you find a recipient, but the embryo might not survive that type of manipulation and the pregnancy rate drops dramatically unless a specific technique and stage of embryo is used," Tibary said. "If the embryo is transferred fresh, the chance of success is a lot better. Embryos can also be cooled and shipped to another state where a recipient is, but the pregnancy rate drops some."

Due to all these factors, ET can be an expensive and frustrating proposition.

"If the breeding is done at home, we give a synchronization schedule to the client and the attending veterinarian," Tibary said of WSU equine clients. "When the time comes for the ET procedure, the donor and recipient mare can be transported to WSU for just one day. If we monitor everything at WSU and breed the mare, the client needs to send the donor and two or three recipient mares for two to three weeks."

Beyond the ET procedure itself, other costs often include breeding soundness evaluations, hospital boarding fees, hormonal treatments to induce ovulation, semen or stud fees, and the management of mares post-breeding.

"The ET case load at WSU is relatively small," said Tibary, who has managed some of the largest stud farms in Morocco and Abu Dhabi before joining the faculty at WSU in 1998. "Overseas, I routinely did 50 to 80 ETs per year. To increase the caseload at WSU, we need to have a herd of recipient mares. There are several large operations that have recipient herds throughout the Northwest that sell or lease mares. But having a herd of quality recipient mares at WSU would be a big plus for us as a teaching and research institution and an asset for Pacific Northwest breeders and veterinarians.

"ET is currently a part of our teaching program for veterinary students, but there is a lot of research that can be done to further develop ET associated techniques. One is gamete transfer, which is only available at three or four labs in North America," he said.

WSU's theriogenology section provides a whole gamut of equine reproductive services including stallion and mares breeding soundness evaluations, infertility treatment, twin pregnancy management, pregnancy and fetal well-being evaluations, obstetrical and postpartum care, semen collection and freezing, and artificial insemination with fresh-cooled or frozen semen.

For more information about ET or other equine reproductive services, contact the WSU Veterinary Teaching Hospital at 509/335-0711 or Dr. Ahmed Tibary at 509/335-1963. For those interested in making a donation toward a recipient herd at WSU or the theriogenology program, contact Lynne Haley, director of veterinary development, at 509/335-5021.

Reprinted from the WSU College of Veterinary Medicine Equine News Winter 2009 issue.

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