Equine Embryo Transfer

Knowing the time of ovulation is very important, so the mare should be scanned throughout her cycle. She should also be monitored and managed for fluid during her cycle and following insemination. The cleaner and healthier the mare’s uterus is, the healthier the embryo will be and its chance of sticking in the recipient mare greatly improved.

Generally, the flush date is seven to eight days post-ovulation. The earlier day is chosen if the mare is young, and the semen is fresh or chilled, and the later day is chosen if the mare is older and the semen was frozen, or if previous attempts have been successful on a particular day.

Day of flush relates to the size of the embryo and, surprisingly, smaller, younger embryos are less fragile than larger, older ones. If you flush too early, however, the embryo may not yet be released into the uterus and so will be missed.

Another possibility is if, at that time, there is no embryo, which will be because the mare has failed to conceive (for various reasons).

The flush of the embryo

For the flush, the donor mare is put in the crush, lightly sedated, her tail wrapped and her perineum cleaned thoroughly.

A Foley-type balloon-tipped catheter is passed through the cervix and inflated, which locks the catheter in the uterus. The catheter is attached via ‘y’ tubing to the pre-warmed embryo flush medium and a filter cup, which will catch the embryo.

The fluid flows into the uterus and inflates it. With the help of rectal ultrasound, it is established the fluid has reached up both uterine horns. A gentle massage of the uterus encourages the fluid around the uterus and then the fluid is allowed to flow out passing through the filter cup.

Between flushes, the cup is disconnected and the contents checked for an embryo under a microscope. If one is found in the first flush and the mare hasn’t had a double ovulation – and hence a chance for a second embryo – then the catheter is removed and the mare can go back out. If, however, the embryo is not there on the first flush, the practitioner will use several more bags of flush to try to retrieve an embryo.

The embryo transfer 

If an embryo is recovered, it is given a grading based on its size, shape and developmental stage. Whilst the grading is subjective, it does help with predicting the likelihood the recipient mare will hold the pregnancy. Grade 1 is best and grade 4 is very unlikely to survive.

The embryo is washed to remove debris or micro-organisms, more washes occur if the flush fluid was dirty, suggestive the donor mare’s uterus was not healthy and, therefore, capable of adversely affecting the embryo.

The recipient mare is prepared for the transfer. First, a rectal examination and ultrasound establish her uterus is quiet and fluid-free, she has a clear CL (corpus luteum) and a tight cervix – all of which indicate her progesterone level is suitable for pregnancy. Then, she is cleaned with a thorough surgical scrub of her perineum, sedated and given drugs which, depending on the practitioner’s preference, may include an anti-inflammatory, antibiotics and hormones.

The embryo is drawn up into a straw in a fluid bubble and the straw is put into a transfer gun, which is in a sterile plastic sleeve. The gun is passed through the vagina to the cervix where the sleeve is pulled back. The gun is manipulated through the cervix into the uterus with as least disturbance to the cervix or uterus as possible. This is the hardest part of the whole procedure. Then, the embryo is pushed out of the straw and the gun is gently withdrawn.

The recipient broodmare 

It is very exciting to retrieve an embryo, but implanting it into a suitable recipient mare is crucial to success.

The general preference in Australia and at Illawarra Equine Centre is to use Standardbred mares aged between three and 10 years. We try to choose nice types, good size, calm temperaments and catchable, with a perfect uterus and vulva conformation.

The mares are very well managed year round. They are all wormed regularly, have their feet and teeth attended to, and are caught and tied up most days as they are scanned routinely every second day to follow their cycle.

When a donor mare is close to insemination, the recipient herd is checked to find two mares that will ovulate preferably 1-2 days after the donor mare. These mares’ cycles are followed to determine the day of ovulation and their uteruses are checked for fluid as there should be none at the time of implantation – that’s why there are always two mares prepared as a back-up is necessary or the embryo will not survive.

Following the transfer of the embryo, the recipient mare is managed as best as possible to minimise stress. She is kept in the same paddock with her buddies and fed the same.

Pregnancy test after embryo transfer

Five days after the transfer, we perform the first pregnancy test. It is always tremendously satisfying to see that round black circle on the ultrasound screen and to make an equally happy phone call to the client.

Further pregnancy tests are performed until 45 days when the recipient mare generally goes home to her new owners. We encourage owners to treat the recipient mare as well as they would their own broodmares – regular worming, dental care, vaccinations, hoof care and good handling will all be necessary to ensure the outcome is a healthy foal.

Success of embryo transfer

Whilst ET has progressed in its success rate, the factors affecting success should be considered by every mare owner before embarking on what is an expensive program.

The most important factors are:

• The donor mare’s age – a younger mare being better than an older one – her uterine health, fertility and time of season all play a big part.
• The stallion’s fertility and quality of semen whether it is fresh, chilled or frozen.
• The recipient mare’s reproductive health, the day of ovulation, stress levels and quality of management throughout the pregnancy.
• The expertise needed for insemination, post-breeding follow-up, flush, transfer and recipient mare management.