Can Embryo Transfer Success Rates be Improved?
Embryo transfer success rates are recognized as having a variable outcome dependent upon technician technique as well as mare suitability. While the flush process is usually fairly stable in results and less subject to variability, the subsequent transfer often does not maintain that same stability of success between all practitioner groups. Ramirez et al. performed a retrospective review which compared usage of the standard manual transfer with a technique using cervical forceps. The review also looked at how the operator experience was affected by the difference in technique.
573 transfers spread over 12 breeding seasons were compared – 345 using conventional manual methods and 228 using a bovine cervical forceps and Polansky speculum technique. 167 of these were performed by operators considered to have low or moderate experience, while the remaining 406 transfers were performed by operators with high experience.
102 mares underwent embryo flushes performed at 8 or 9 days post-ovulation. The retrieved embryos were transferred into recipient mares aged between 3 and 12 years old. No medications were used during any of the transfers. Subsequent embryo loss rates after the pregnancy confirmation stage did not statistically differ between the two groups.
There was significant difference in transfer success rates (as evidenced by pregnancy maintenance rates at the first post-transfer evaluation 12-14 days post-ovulation) between the two experience levels when using the standard manual technique (57% vs. 72%). With the forceps technique, there was no statistical difference (84% vs. 86%). Overall, there was a cumulative statistical difference, with the forceps technique yielding 85% compared to 65% for the standard technique (195/228 vs. 236/345 respectively).
This work confirms previous work published by Cuervo-Arango et al. which demonstrated increased embryo transfer success rates using the forceps designed for the purpose by Drs. Wilsher and Allen (termed “Wilsher forceps”) of the UK’s Equine Fertility Unit. The similar confirmation in the current review of the same effect being seen with the more easily-obtained bovine cervical forceps rather than the somewhat scarcer Wilsher forceps is beneficial to the transfer technician in the field.
Our own commentary at Equine-Reproduction.com, LLC – where we have been using Wilsher forceps for years – is that we have seen an increased embryo transfer success rates using the forceps technique. The ability to straighten even tortuously convoluted cervixes to allow ease of transfer is most convenient, and undoubtedly the reduction in transient prostaglandin release, as well as reduction of possible contamination compared to other methods, is beneficially related to the increased pregnancy rates identified. This method also allows the use of a regular insemination pipette as the transfer device, rather than a ½-ml semen straw, which in turn permits transfer of larger (as indicated, day 8 or 9) embryos. This is advantageous in that there is potentially a higher flush and identification success rate.
(Ramirez H, Belmar F, Cuervo-Arango J, Losinno L. 2023. Embryo transfer technique affects pregnancy rates in recipient mares. JEVS 125:104672)
1: Cuervo-Arango J, Claes AN, Stout TA. 2018. Effect of embryo transfer technique on the likelihood of pregnancy in the mare: a comparison of conventional and Wilsher’s forceps-assisted transfer. Vet Rec 15;183(10):323.