Embryo transfer (ET) in horses is a topic that I am getting asked about with increasing frequency. The concept of it and the potential possibilities it opens up seems to intrigue people. However, there are many factors to consider and although in some breeds and disciplines it has now become routine it is not something that can be done on a whim next time your mare is in season – your vet needs forewarning! The recent economic climate has left the UK with a lot of “spare” horses, but sadly not every unused / unwanted mare is going to be able to find a useful alternative life as a surrogate.
ET involves using a surrogate or recipient mare to carry and raise a foal that was removed from the donor mare as an embryo only a few days old. If the donor mare is in competition she would ideally have an “easy” week or two between being in season and being “flushed”. Other than those in competiton, the advantages of ET mean that foals can be born from exceptional older mares that would be unlikely to have a healthy pregnancy or very young mares not mature enough to carry a foal yet and allows genetically superior individuals to have an increased number of offspring.
Most practices offering ET will have a number of surrogates they can line up in synchrony with the donor, or you may wish to provide your own. There are a couple of specialised ET centres in the UK that run herds of potential surrogates and can provide one at the right stage of the cycle at very short notice if, for example, 2 embryos are flushed from the donor mare and only one surrogate at home has been prepared, or if the alignment has not gone perfectly. Surrogate hire will cost an additional £1200-£1500 for the year (hence a lot of people try and provide their own “spare” mare). These surrogates should ideally be between 4-10 years old and proven breeders. Ideally they cycle with some predictability so that synchronising the cycle with the donor mare is easy. The surrogate should ovulate around 48hrs after the donor (anywhere between a few hours and 4 days is acceptable). Surrogates should also be swabbed in the same way as any mare being prepared for cover / AI to ensure that there is no bacterial infection in the uterus which will prevent success.
The following process is an outline of what the vet will be doing and looking for with each ultrasound scan. Most vets will offer this as a package and ideally have both mares at the practice so they can be scanned as often as necessary with no extra cost.
- Observe donor mare in season or induce season.
- Induce surrogates into season.
- Follow donor through the oestrous cycle. Sswab uterus when cervix first opens (to check for infection) and order semen / cover when ready.
- Continue following surrogate(s) and swab them when cervix opens.
- Inseminate / cover donor.
- Check donor has ovulated at correct time, repeat insemination / covering if not yet ovulated.
- Induce ovulation in the surrogate(s).
- Confirm no mare has produced any uterine fluid in reaction to the semen
- Start the donor mare on the drug “regumate” which helps keep her cervix as tightly closed as possible which facilitates the embryo flush.
- Eight days post ovulation, flush the donor and check for an embryo/ embryos. Remember, at this point you cannot tell otherwise if the donor is pregnant so the flush is speculative.
- If an embryo is found, transfer to to the surrogate. Some vets would start the surrogate mare on regumate also at this stage to increase the chances of implantation of the embryo.
- Give the donor an injection of prostaglandin to bring her back into season. This prevents the possibility of an unwanted pregnancy if there are any “missed” embryos (this is very unlikely).
- Day 14 or 15 post ovulation (of the donor) the surrogate can be scanned to check that the embryo has implanted successfully.
Anecdotal evidence suggests that success rates and the chance of recovering multiple embryos is significantly enhanced by the addition of fertility aid feeding supplements to the mares and we do so for all of ours.
Embryo recovery rates average 75% and approximately 75% of embryos introduced to the surrogate will implant successfully as a healthy pregnancy. So, overall the chances of a foal from one attempt at ET is around 50%. The whole process including the stud fee, livery, surrogate hire and the vets package will cost you in the region of £4000.
We at Stringer Equine Veterinary Practice have a small herd of surrogate mares available for hire and offer ET packages at our premises in Exeter or at Langaller Stud, Newton Abbot. For more information please contact our office.
Richard Stringer BVSc MRCVS of Stringer Equine Veterinary Practice