Ask the Vet: Embryo Transplant

Written by Madison Seamans, DVM, MS, Kuna, ID

Question: I have a 21-year-old FQH mare that has never been bred. Is it possible to harvest her eggs and do an embryo transplant?

Answer: Maybe. This is really two questions. Embryo transfer involves breeding the mare, flushing the fertilized ovum seven days after ovulation and transferring it to a recipient mare. Harvesting eggs involves removing unfertilized ova (eggs) from the ovary, and supplying the sperm cells via one of several different techniques.

Let’s start with embryo transfer. In young, fertile mares, flushing embryos is a relatively simple procedure that is offered by many veterinary practices. The mares are bred naturally or artificially, their cycle closely monitored via ultrasound so the uterus can be flushed seven days after ovulation. The harvested embryo is placed in the uterus of a recipient mare that has ovulated on about the same day, but not bred. This procedure is quite successful if both mares have normal reproductive systems. In older mares, however, this can become a challenge.

Reproductive efficiency begins to decline in mares after the age of about 13 years. Although a twenty-one-year-old mare may be in perfect health in every other way, her reproductive competence is limited.

Producing a viable embryo is a process that may be more difficult than it sounds. The mare’s ability to get pregnant is first dependent on her ability to produce fertile ova (eggs). Stallions produce billions of new sperm cells most every day of their adult lives. Mares, however, are born with all the eggs they will ever have, so an aged mare has aged ova that may not function as well as they did earlier in her life. In addition, she must have normal anatomy and physiology of her cervix, uterus and uterine tube. Aging can compromise all or part of this process, and thus prevent the production of a viable embryo.

There are many tests that can be performed to determine potential fertility in mares. A culture of the uterine fluids during the first two days of heat can detect the presence of infection that will not affect her general health, but will greatly impair fertility. An endometrial biopsy of the lining of her uterus can give us more information about reproductive health. Hormone analysis can be of some use to determine her ability ovulate normally and support an embryo during early development. However, the most accurate test will be to breed her and see if an embryo can be flushed. In mares that cannot produce an embryo, another option is available.

Oocyte transfer: A needle is inserted into the structure on the ovary, which contains the egg (The ovarian follicle) and the fluid containing the ovum is aspirated and placed into a dish containing culture medium. Ova harvested in this manner can be used for in vitro fertilization (test tube babies), or transferred to the uterine tube of younger mare to be bred in the same cycle (gamete intrafallopian transfer, the “GIFT” procedure).

During the recent decade, our understanding of equine reproduction has greatly expanded. With this new knowledge, technology has been developed to prolong the productive years of our favorite mares, and helped to preserve valuable genetic resources for the equine industry. These procedures are not cheap and the success rate in older mares is not high–there are no guarantees–but they do provide options for mare owners that previously had none.

Madison Seamans, DVM, MS, Kuna, ID