Intracytoplasmic sperm injection (ICSI) is an assisted equine reproduction technique in which a single sperm is injected directly into a mare’s egg in the laboratory to produce an embryo.
Once considered a niche technology associated with human fertility treatments, ICSI is now increasingly common in sport horse breeding. The modern breeding technique allows breeders to produce embryos from valuable mares, maximise limited semen supplies and preserve genetics from stallions long after their death.
ICSI is used alongside ovum pick-up (OPU), a procedure in which eggs are collected from a mare’s ovaries. Together, these techniques form part of a process known as in-vitro embryo production (IVEP).
How does ICSI work in horses?
1. Egg collection (OPU)
Eggs are collected from the mare’s ovaries using an ultrasound-guided needle while the mare stands under sedation.
2. Fertilisation
In the laboratory, a single sperm is injected directly into each mature egg.
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3. Embryo development
The fertilised egg develops into an embryo over around seven to 10 days.
4. Transfer or freezing
The embryo is transferred to a recipient mare to carry the pregnancy, or frozen and stored for future use.
5. Gestation
Although fertilisation occurs in the laboratory, the foal develops normally in the uterus of a mare.
What does ovum pick-up (OPU) involve?
The procedure is carried out with the mare standing under sedation. An internal ultrasound probe allows the vet to visualise the ovary and guide a needle into each follicle to collect the fluid and oocyte (egg).
Typically 15 to 20 follicles may be collected during a session. The eggs are then matured in the laboratory before fertilisation.
OPU is generally well tolerated by mares and can be repeated every three to four weeks, provided sufficient follicles develop.
The technique can be particularly useful for:
mares that cannot carry a pregnancy
older mares with reproductive challenges
competition mares that remain in work
Because eggs can be collected in a single procedure, disruption to training schedules is usually minimal.
Why is ICSI used in horse breeding?
ICSI has become increasingly popular because it allows breeders to:
maximise limited frozen semen supplies
breed from stallions that have died
produce embryos from mares still competing
breed from mares unable to carry a pregnancy
Because only a single sperm is needed for each egg, one straw of semen can be used many times.
More than two decades after the technique was first introduced in horses, there is currently no scientific evidence suggesting that ICSI-conceived foals develop differently from those conceived naturally.
What is the difference between ICSI and IVF?
ICSI is a specialised form of in-vitro fertilisation (IVF).
In conventional IVF, eggs and many sperm are placed together in a dish and fertilisation occurs naturally. In ICSI, however, a single sperm is injected directly into the egg.
Conventional IVF is not reliably successful in horses, which is why ICSI is used instead. Success rates vary depending on factors such as mare age, egg quality and semen quality, but laboratories commonly report embryo development rates of around 60 to 70%.
Is ICSI available in the UK?
ICSI was initially more widely available in North America and continental Europe, where specialist laboratories were established earlier.
In the UK, a number of veterinary reproduction centres now perform ovum pick-up, with eggs transported to specialist laboratories for fertilisation. Domestic laboratory capacity has expanded in recent years, making the technique increasingly accessible to UK breeders.
As a result, ICSI is now commonly used to expand access to valuable bloodlines and increase flexibility in breeding programmes.
Is ICSI safe for horses?
More than two decades after the technique was first used in horses, there is currently no scientific evidence suggesting that foals conceived through ICSI develop differently from those conceived naturally.
Can a competition mare produce a foal using ICSI?
Yes. Eggs can be collected from mares while they remain in training or competition. The embryo is then transferred to a recipient mare to carry the pregnancy.
Why is ICSI used instead of IVF in horses?
Conventional IVF does not work reliably in horses, so ICSI is used instead, with a single sperm injected directly into the egg.
Can semen from a dead stallion be used for ICSI?
Yes. Because only one sperm is needed per egg, ICSI allows breeders to maximise limited frozen semen supplies, including semen collected before a stallion’s death.
Oscar joined Horse & Hound in October 2023 and is the magazine’s dressage editor and sports manager, overseeing coverage of equestrian sport. After studying equine science at Myerscough College, he spent four years working for leading dressage rider Emile Faurie, competing at the 2015 National Dressage Championships and travelling with the yard to CDIs including Aachen and Saumur. He holds a master’s degree in Literature from York St John University (2021).
