Exploring Embryo Transplants

Equine veterinarians transfer embryos between mares on a regular basis.


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A recipient mare and her foal.

Photos courtesy of Royal Vista Southwest.

When transferring embryos was first tested on rabbits in the late 1800s, veterinarians couldn’t have known the impact this experiment would someday have on the equine industry. Today, equine veterinarians transfer embryos between mares on a regular basis, providing horse owners a wide range of benefits.

Many decades after the first successful embryo transfers, the cattle industry embraced the process in livestock reproduction. The first live calf from a frozen embryo transplant was born in 1973. Today, the cattle industry uses embryo transfer regularly.

The Horse Industry

After successful embryo transfer in cattle, the equine industry was quick to follow. The first foal produced by embryo transfer between mares was born in Japan in 1974. Ten years later, Colorado State University began experimenting with equine embryo transplants and produced twin foals from a bisected embryo transplant.

Older and younger 7-day-old embryos.

Today, state-of-the-art of equine embryo transfer is being performed regularly using the techniques of superovulation, transfer of fresh or cooled-transported embryos and vitrification (ultrarapid freezing) of embryos. Some specialized research laboratories and private reproduction centers are also producing embryos by oocyte transfer, sperm injection and nuclear transfer.

The emphasis today is in non-surgical techniques, which are easier to perform than those requiring surgery.

The Benefits

The benefits of equine embryo transplant are significant to mare owners. According to Pete Sheerin, DVM, a diplomate of the American College of Theriogenologists and owner of Nandi Veterinary Associates in New Freedom, Pa., embryo transfer can be performed on several categories of mares.

A mare in the process of getting flushed.

“The most common mares we see are the older problem mares,” he says. “These mares may have issues that prevent them from carrying a foal to term due to a torn or abnormal cervix, a uterus with a great deal of fibrosis, physical ailments such as arthritis, laminitis, or medical issues such as pituitary pars intemedia disease, metabolic disease, previous ruptured uterine artery, etc.”

Mares that are competing on the show circuit are also candidates if the client wants a foal but does not want to interrupt the mare’s career.

“We have bred mares while at a show and flushed them for an embryo,” says Dr. Sheerin. “We have flushed mares for an embryo the day following a race. We also see mares that the client wants multiple foals from in one year, perhaps by multiple stallions.”

Owners who don’t want to risk their mare’s health are also among those who take advantage of embryo transplanting, according to Terri Van Wambeke, DVM, CVA, CVMST, with TJ Holistic, a veterinary practice in Galt, Calif.

“Embryo transplanting is a benefit to these owners because it reduces the foaling risk in expensive mares,” she says.

The Process

The embryo transfer process begins with collection of the embryo from the donor mare six to eight days after ovulation. This is performed through flushing with a saline solution containing bovine or equine serum and an antibiotic.

Before collection, the mare is rectally palpated to detect follicular development. The perineal area is washed with surgical scrub, and a catheter is inserted into the vagina. The uterus is flushed and then irrigated to ensure no embryo remains inside the horse, and the fluid is collected.

Flushing is the method of choice for most veterinarians performing this procedure because surgical transfer is expensive and carries a higher risk, according to Dr. Van Wambeke.

“Flushing is a proven and widely available low–risk practice that can be done farm-site, has reasonably good success rates when performed by well trained personnel, and is relatively inexpensive when compared to surgical embryo transplanting,” she says.

After flushing, the embryo is then detected in the fluid visually, with or without a microscope. At the Equine Reproduction Laboratory at Colorado State University, the embryo recovery rate using this method is 75 to 80 percent.

Transferring the embryo involves preparing the receiving mare in the same way that the donor was handled. The embryo is placed in an artificial insemination rod and inserted into the uterus by hand.

The Issues

Although equine embryo transplantation is a relatively easy procedure, practitioners are likely to come across several problems.

“Issues that arise are usually failure to collect an embryo or failure to obtain a pregnancy after transfer,” says James W. Bailey, DVM, with Royal Vista Southwest, a private equine reproduction clinic in Purcell, Okla. “Embryo transfer is a matter of paying attention to multiple details. It’s important for donor mares to be in the best health possible, free of uterine infections, non-infectious endometritis, fluid accumulation, etc.”  Dr. Bailey also emphasizes the need for quality semen, and timely ovulation after insemination, preferably in the first 24 hours.

“Evaluate the flushing technique to be sure proper uterine filling is occurring and all fluids are recovered with uterine massage during the procedure,” he says. “Failure to obtain pregnancies in the recipient mare is many times a result of improper preparation of the recipient mare. Also ensure good quality materials are being used, and non-traumatic and aseptic transfer technique is practiced.”

According to Sheerin, the most common issue he encounters is uncertainty in date of ovulation.

“Everything we do is based upon ovulation, so it is imperative that we know the date,” he says. “Because of this, the donor mare needs to be examined daily by rectal palpation and ultrasound.”

Sheerin explains that estimating the day of ovulation decreases the success rate of the procedure. “Mares that double ovulate can pose a challenge as to when they need to be flushed if the ovulations are asynchronous,” he says. “I use ovulatory inducing agents such as hCG or SucroMate (deslorelin) to induce ovulation so that I am not dealing with asynchronous ovulations. I also use these agents when breeding a mare with a single follicle as I want to breed the mare as close to ovulation as possible.”

Sheerin also sees challenges pertaining to management of the recipient herd.

“Sometimes there is poor communication by the mare owner with respect to ovulation of the donor mare,” he says. “I would like to know when the mare is in heat, and be notified as soon as the mare’s veterinarian has confirmed ovulation. This helps me make sure that we have a recipient mare lined up with the donor mare.”

The Costs

The cost of equine embryo transplant can be a factor for some owners. It may run as much as $4,500, which includes the purchase of the recipient mare.

According to Sheerin, most programs charge a nonrefundable enrollment fee, which allows the recipient mare to be purchased and maintained, a fee for the flush and another for the transfer. If the recipient becomes pregnant, owners will be billed a purchase or lease fee.

Many facilities want the recipient mare back when the foal is weaned, as they know the mare’s history. The mare refund is a usually a fixed amount, or may be returned with a credit on subsequent recipients.

“There are a variety of ways it is done, so owners considering embryo transfer should call several facilities to determine their fees,” says Sheerin. “In addition to fees, the mare owner should also consider the facility’s  success, the expertise of the veterinarian performing the service and how long they have been providing the service. Like many other things, cheapest is not always the best.” 

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