equine embryo transfer techniques compared

Equine embryo transfer is a delicate and an expensive process. Not only is the embryo itself fragile, but the traditional technique can be challenging even for experienced veterinarians. To help reduce the risk of pregnancy failure, scientists developed a transfer technique that’s more likely to lead to a healthy pregnancy regardless of the technician’s experience level. But does that new method improve pregnancy rates? Researchers recently compared the two to find out.

The “Wilsher” technique, in which forceps grasp the cervix with the aid of a vaginal speculum, results in a higher successful pregnancy rate of up to 10% compared with experienced practitioners using the conventional technique and up to 30-40% with less experienced practitioners, said Juan Cuervo-Arango, DVM, PhD, of the Utrecht University Faculty of Veterinary Medicine, in The Netherlands.

“I think the success of this technique is due to the fact that veterinary surgeons spend less time fiddling around with the cervix trying to pass the embryo transfer pipette as they do during the conventional technique, because the forceps straighten the cervix and makes it easier to pass the pipette,” Cuervo-Arango said. “It’s not totally foolproof, since the operator needs to know about the anatomy of the cervix and know how much force to apply to the forceps, but of course it requires much less experience.”

Embryo transfer switched from a surgical procedure to a vaginal procedure, passing through the cervix, in the 1990s (mainly for welfare reasons, to prevent putting mares through repeated surgeries for breeding), he said. But pregnancy rates appeared to be strongly correlated with the technician’s experience. That’s why a research group at Cambridge University, in the U.K., developed a new method aimed at a more reliable outcome, regardless of experience. Twink Allen, BVSc (Syd), PhD (Cantab), ScD (Cantab), DSc (h.c.mult), DESM, Dipl. ECAR, FRAgSE, FSB, FRCVS, CBE, and Sandra Wilsher, BSc (Hons), PhD, first presented the Wilsher technique in 2004.

The traditional embryo transfer technique involves placing the embryo inside a tiny plastic straw that is then placed within a transfer pipette. The pipette must squeeze through the opening of the tightly closed cervix, which separates the vagina from the uterus, protecting the uterus from outside contaminants. Once the pipette has passed through the cervix, the operator can place the embryo in the uterus where it will continue with its mobile phase around the uterine surface, the endometrium, to fixate eight to 10 days later and develop into a healthy pregnancy.

The difficulty, however, is getting through that cervical canal with the pipette, said Cuervo-Arango. Pipettes can damage the cervical mucosa or endometrium if moved too rough, or bacteria and other contaminants can pass through with the pipette, especially if the cervix has to be manipulated significantly to get the pipette through, which would be detrimental for the embryo, as inflammatory products are released. The pipette can also be placed poorly, with the embryo ending up outside the endometrium.

The Wilsher technique benefits from the use of a vaginal speculum and special cervical grasping forceps, he said. These forceps essentially straighten up the tightly closed cervix so it’s easier to get the pipette through. It led to success rates of up to 80% in its early studies, Cuervo-Arango said.

However, the original Wilsher technique requires three operators, he said. And that, plus the need to purchase the specific materials, might have prevented it from “catching on” with embryo transfer technicians, even 14 years after its development.

Cuervo-Arango and colleagues recently decided to test the technique to see how it compared with the traditional method. They modified the technique slightly, to make it require only two operators: one to hold the pipette wearing a headlamp to illuminate the cranial vagina and the other to hold the forceps and vaginal speculum.

They evaluated pregnancy statuses in recipient mares undergoing one of two embryo transfer techniques. Three operators having moderate to high experience transferred 179 embryos using the traditional manual method. Four operators with little or no previous experience transferred 170 embryos using the Wilsher technique.

They found a 90% pregnancy rate in the Wilsher group, compared to a 70% pregnancy rate in the traditional group (with 50% in the operator with less experience compared to 79% for the operators with high experience), he said. That difference was particularly interesting considering that the group of less experienced operators had a higher success rate, he added. Furthermore, that group’s success rate was not related to experience level—meaning the 10% of failures weren’t necessarily associated with less experience, but to pre-existent poor embryo or endometrial quality.

Meanwhile, pregnancy loss before term didn’t differ between the groups, hovering at around 8%, Cuervo-Arango said.

“These results really didn’t surprise me,” he said. “I knew the success rates could be increased (by an improved technique), since most embryos (>90%) are healthy at the moment of transfer and therefore should result in a healthy pregnancy as long as they are transferred properly in a healthy recipient mare.”

While the Wilsher technique in the study required two operators, it’s possible to do the method as a single operator, Cuervo-Arango said.

“I know of a colleague that does it on his own,” he said. “However, I don’t think is recommended since the mare is likely to move during the procedure even if she is sedated (which we always recommend), and then that would negatively affect the procedure and decrease the chances of success.”

The study, “Effect of embryo transfer technique on the likelihood of pregnancy in the mare: a comparison of conventional and Wilsher’s forceps-assisted transfer,” was published in the Veterinary Record.