Embryo Transfer Brings Surrogate Mothering to Horses

For more than 20 years humans have reaped the benefits of surrogate mothering. Women who cannot maintain a pregnancy, or whose health would be endangered by pregnancy, can "donate" a fertilized egg to a surrogate recipient, who carries the baby to term.

In veterinary medicine, embryo transfer technology has been practiced for over ten years, and the procedure is increasing in popularity. Fertilized eggs are retrieved from donor horses and implanted in the uterus of recipient horses that can carry the foal to term.

According to Dr. Debra Sauberli, a board-certified theriogenologist at the University of Illinois Veterinary Teaching Hospital in Urbana, veterinarians will "flush" a fertilized egg out of a pregnant mare 7 to 8 days after ovulation.

To perform the flush a catheter is inserted into the uterus. The catheter has a cup that fits over the cervix to form a seal, and a special solution is flushed through the uterus, a liter at a time. The flushed fluid is then filtered to separate cells and debris from the solution. The small amount of solution that is left is set in a Petri dish and examined under a microscope. A trained eye will find the egg, no larger than a speck of dust, somewhere at the bottom of the dish.

Immediately after flushing, a donor's embryo may be packed in a transport tube filled with a special solution and then placed in a cooled transport container (the same kind used to hold semen samples from stallions for artificial insemination) or transferred directly to a recipient mare's uterus.

Most recipient mares are housed in recipient stations, businesses specifically providing surrogate mares. Eggs are usually sent same-day air to a recipient station, where a mare has been synchronized in her estrus cycle to specifically match the donor mare's estrus cycle.

Most donor eggs are inserted into a recipient mare's uterus trans-cervically, through an artificial insemination straw inserted through the cervix. Eggs can also be implanted surgically. Research suggests that the surgical procedure has a slightly higher success rate. Although the surgical procedure is relatively simple, Dr. Sauberli points out that it requires more time and involves more risk and expense.

Since the procedure is costly, requiring special training and equipment, donor mares are usually valuable show horses or racehorses that will produce valuable offspring.

There are numerous benefits of embryo transfer for performing horses and older mares. Show mares and racing mares can produce foals without interrupting their performance schedule. Donating an egg eliminates many risks associated with pregnancy, which may be important for performance horses or mares that cannot carry foals due to age, uterine problems, or lameness.

Embryo transfer can also increase the production potential of a single mare. A prized horse that donates multiple embryos to multiple recipients can pass on her genes to two or more foals a year (provided the horse registry allows this), rather than bearing a single foal.

University veterinary hospitals, private practice veterinarians, and specialized practices perform embryo flushing and transfer. Although most transfers to recipient mares are done at specialized practices called recipient stations, Dr. Sauberli says, "We are seeing more people who own both the donor mare and recipient mares. These recipient mares are usually grade horses, that is, not registered, possibly not purebred, or mares not of breeding quality. Instead of selling these excess horses, owners may keep them and use them as recipients." Over the years, techniques and equipment have improved, making "direct-transfer" more feasible, economical, and convenient.

By Kim Marie Labak, Information Specialist - University of Illinois, College of Veterinary Medicine