A healthy equine embryo. | Courtesy Dr. Ryan Ferris
Veterinarians routinely perform embryo transfers to obtain foals from valuable mares that are unavailable to carry a pregnancy for a variety of reasons. Maybe the mare’s owner wants her to produce more than one foal a year. Maybe she’s physically unable to carry a foal to term. Or, perhaps she’s a performance mare that’s actively competing.

There are common challenges, however, that practitioners face when completing these procedures. Ryan Ferris, DVM, MS, Dipl. ACT, owner of Summit Equine, in Newberg, Oregon, described three and how to overcome them during the 2017 American Association of Equine Practitioners convention, held Nov. 17-21 in San Antonio, Texas.

While he was an assistant professor at Colorado State University’s (CSU) Equine Reproduction Laboratory, Ferris and colleagues performed three studies during the 2016/17 breeding season to address the following issues.

1. What media should veterinarians use for embryo recovery?

“The options for embryo recovery fluid can be overwhelming,” Ferris said, referring to what the veterinarian uses to retrieve the embryo from the uterus. Clinicians at CSU typically use a complete flush media (a ready-to-use solution that doesn’t require additions of serums or antibiotics), whereas in many other countries practitioners use Ringer’s lactate solution or Hartmann’s solution, which veterinarians in the United States also use during uterine lavage.

During the 2016 breeding season Ferris and his team alternated using complete flush media (78 embryo recovery attempts) or Hartmann’s solution (77 embryo recovery attempts) on every other mare. They recovered embryos in 46.2% of attempts with complete flush media and 57.1% of attempts with Hartmann’s solution.

On Days 14 and 25 they performed pregnancy exams on recipient mares and found that results with the complete flush media and Hartmann’s solution groups were essentially the same: 80.5% and 80.8% pregnant, respectively, on Day 14, and 78% and 76.6% pregnant on Day 25.

These results indicate that veterinarians have multiple options for media type for embryo recovery. “Ambulatory vets can carry one type of media (Hartmann’s solution) the for both uterine lavage and embryo transfer,” said Ferris, adding that Hartmann’s solution might also be more cost-effective.

2. What options are available if a cycling recipient mare is not available?

“One of the more important factors affecting the likelihood of establishing a pregnancy after embryo transfer is donor-recipient synchrony,” said Ferris. In other words, the donor and recipient mares’ estrous cycles must be in synch.

Ideally, the recipient ovulates one day before to three days after the donor does. However, many clinics just don’t have a large enough pool of cycling recipient mares to match the donor population, particularly when it’s early in the breeding season. In these situations, veterinarians commonly administer hormones to recipient mares to mimic the rise of natural estrogen from growing follicles and rising progesterone levels after ovulation. But how does this effect pregnancy rates?

To find out, Ferris and his team transferred:

  • 65 embryos into naturally cycling recipient mares;
  • 40 embryos into noncycling mares treated with two days of estrogen to mimic estrus, followed by five to seven days of progesterone to support the pregnancy; and
  • 14 embryos into naturally cycling mares early in estrus (without a dominant follicle) treated with the aforementioned hormone therapy.

Ferris said there was no statistically significant difference between pregnancy rates among all groups.

“Estrogen and progesterone in noncycling recipient mares can result in a similar pregnancy rate as recipients with natural ovulation,” he said. “Hormone therapy can be especially advantageous for matching a specific recipient with a donor mare.”

3. Does the recipient have enough progesterone to support pregnancy?

The mare’s corpus luteum (the structure formed after ovulation) produces progesterone that’s critical to maintaining pregnancy. The minimum level of progesterone needed to support a pregnancy is approximately 2.5 ng/mL. It’s been assumed that recipient mares’ progesterone levels five days after ovulation are above 4 ng/mL, said Ferris.

To confirm this, his team evaluated progesterone production in 456 recipient mares with normal detectable ovulations five days post-ovulation. They found that 5.5% had levels below 4 ng/mL and 0.9% had levels below 1 ng/mL, indicating a corpus luteum never formed.

“Normal corpus luteum function after ovulation does not occur in a small percentage of mares,” said Ferris.

Overall, he said, “these three techniques can allow clinicians greater flexibility to overcome common challenges in embryo recovery and recipient mare management.”