Embryo Transfer in Practice

CHAPTER 176 Embryo Transfer in Private Practice



Juan José Vázquez


Embryo transfer in horses is an assisted reproduction technique that has the potential to improve reproductive performance in mares. It is widely used in conjunction with artificial insemination in ambulatory practice, veterinary clinics, breeding farms, and specialized embryo transfer centers.


The worldwide popularity of embryo transfer is explained by the possibility of registering more than one foal a year from a given mare, the acceptable pregnancy rates obtained with this technique during the past 20 years, and the potential for optimizing the genetics of high-quality mares, particularly mares that are continuing to perform. The procedure also makes it possible to obtain foals from mares that cannot carry a pregnancy to term because of reproductive and nonreproductive problems. Embryo transfer is a growing trend in the equine industry and is being recognized by more studbooks, registries, and breeding associations throughout the world. In this chapter, the general concepts and strategy of embryo transfer in practice and information to assist the practitioner in the planning of such a program are described.



STALLION AND SEMEN SELECTION


As in any breeding program, the quality and fertility of the stallion or the sperm to be used in an embryo transfer program are very important. It should be anticipated that embryo recovery rates are similar to per-cycle pregnancy rates for each stallion. Generally, in an embryo transfer program, the use of cryopreserved semen decreases the embryo recovery rate but, in some instances, the embryo recovery rate from stallions with high-quality post-thaw semen is similar to that obtained with use of chilled or fresh semen. In our practice, we find lower recovery rates when using cryopreserved semen, compared with fresh-chilled semen, but no differences are observed in pregnancy rates after transfer. These observations confirm those reported in the literature.



SELECTION OF DONOR MARES


It is emphatically important to communicate with the client regarding their expectations for the donor mare in an embryo transfer program. Donor mares are chosen for genetic merit or performance ability, not for fertility. Considerations essential in the selection of a donor mare are age, reproductive history, reproductive health, activity, and availability.



Age


Mares have been used as donors as early as 1 or 2 years of age. These mares are selected solely on the basis of their genetic traits, even before entering a training or performance career. This type of donor mare is typically highly fertile, and good recovery rates can be expected, provided that cyclicity has been initiated. The most common donor mares in practice are older and have already proven themselves as sport or performance horses. Generally it is more difficult to retrieve embryos from older mares, often because of uterine or tubal disease. Aged oocytes also contribute to lower fertilization rates and early embryonic losses, which may occur before the usual embryo collection dates at 7 to 8 days after ovulation. It is generally agreed that a 30% embryo recovery rate can be expected in mares over 18 years of age.



Reproductive Health


A good reproductive history and a thorough clinical reproductive examination must be performed before entering donor mares into an embryo transfer program. Mares with a history of infertility or subfertility and those that fail to conceive through a complete breeding season are poor candidates as embryo transfer donors. Low recovery and transfer rates can be anticipated from such mares. For mares with a history of chronic or infectious endometritis, all diagnostic and therapeutic methods should be performed before embryo transfer attempts are undertaken. Frequent embryo collection attempts in a single season or in subsequent breeding seasons can also lead to chronic inflammation of the uterus and may result in complicated reproductive management of the mare and a lower embryo recovery rate.



Physical Activity of the Donor


Many embryo donors are performance and sport horses with a full schedule of performance activities that allows little time for prolonged residence at an embryo transfer facility. This may interfere with timely breeding and the embryo retrieval schedule; hence, success rates may be poor despite optimal reproductive quality of the mare. A recent study of mares in a controlled exercise program described a low embryo recovery rate that was likely caused by heat stress. This finding was similar to embryo recovery rates obtained in cattle under heat stress. In contrast, there are no differences in embryo recovery rates between mares performing in different sport venues. In our population of mares, a detrimental effect of active training on recovery rates from mares has not been observed. In fact, we believe the optimal ages (4 to 10 years) and generally nulliparous status of these mares make them excellent candidates for donor mares in an embryo-transfer program.



RECIPIENT MARE SELECTION


Without a doubt, recipient mare selection is the most important influence on the success or failure of an embryo-transfer program. The optimal candidate should therefore be selected carefully. It is important to have access to a good reproductive history, but this is sometimes difficult to obtain. A thorough reproductive examination should be performed.


Young, fertile mares between 4 and 10 years of age that are nulliparous or have had one uncomplicated parturition are ideal. However, it is frequently difficult to obtain the ideal mare, and mares with fertility problems may be offered as recipients. In this situation, maiden 4-year-old mares are preferred because they have had little opportunity to develop reproductive problems. In addition, postpartum mares can be used as recipient mares. In our hands, recipient mares are used for more than one season, and embryos are generally transferred into the mares after the second postparturition estrus (30 days after foaling). No difference has been reported in pregnancy or embryo loss rates between nulliparous mares and those receiving the embryo after the first postpartum estrus. However, the latter type of mares should have had a normal parturition and the uterus must be free of uterine fluid at the foal-heat reproductive examination.


It is mandatory that recipient mares not have medical or locomotor problems that might affect pregnancy outcome, and good body condition is essential. It is useful to introduce new recipient mares to the herd during the autumn or winter season of the year before implanting is planned so that they can be handled, acclimatized to the environment, and have the opportunity to develop good condition and body score.


At present there is debate about the influence of recipient mare on size and behavioral traits of the offspring. In a recent study some preliminary results show that the recipient mare does not play a significant role in size, conformation, and developmental characteristics of the embryo transfer product. However, it is advisable to use recipient mares that have similar body characteristics to those of the donor mare.

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Embryo Transfer in Practice

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