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For those of us involved in the equine breeding industry, it is obvious that artificial insemination (AI) is set to play an ever-increasing role in the future. One of these areas of growth will be in the technique of deep uterine insemination (DUI) when using frozen semen.

Conventionally, mares are inseminated with a minimum of 500 million progressively motile (live) sperm. This is known as the insemination dose.

The semen is infused via a catheter into the middle of the mare’s uterus (womb). The uterus is T-shaped, and is made up of a first part called the body and two horns (left and right). Once the semen has been put into the uterus, the sperm travel up one of the horns until they reach the tip. There, they arrive at the entrance to the oviduct or uterine (fallopian) tube.

Once sperm have passed through this junction and are in the oviduct, they can fuse with and fertilise the female oocyte or egg. Very few sperm from the millions inseminated make it up the uterine horn and into the oviduct.

The ovaries, one on each side, are situated close to the opening of the oviducts. When ovulation occurs, an egg is released, usually from only one of the ovaries, and begins to travel down the oviduct towards the uterus. Veterinary examination, especially ultrasound scans, will detect the enlarging follicle and enable a confident prediction to be made as to which side ovulation will occur and when, so that insemination can maximise the chance of fertilisation.

Current thinking is that the number of sperm reaching the oviduct may be between 100 and 1,000. Once this was discovered, researchers investigated how few sperm may be necessary to achieve fertilisation when placed directly at the tip of the uterine horn during insemination. This interest stemmed from the increase in use of frozen semen. Semen from some stallions is notoriously difficult to freeze or in very short supply and it would be of tremendous benefit if mares could become pregnant as a result of inseminating much lower doses of sperm.

This is called “low dose insemination”, and since the sperm are inseminated much further into the uterus than with conventional AI, the other term used to describe it is “deep uterine insemination” or DUI.

There are two methods for inseminating the sperm deep into the uterus. The first is to use a special catheter (long plastic tube), which is guided up the uterine horn by placing one hand in the rectum of the mare and slowly advancing the catheter.

The second method is to place an endoscope in the uterus and actually see the tip of the uterine horn. The semen can then be inseminated via a special catheter inserted down the endoscope.

Using a rectally-guided catheter

The mare should be prepared for DUI in a clean, well-lit environment; stocks for restraint are essential.

A special catheter is used. This is long enough to reach the tip of the uterine horn while still having one end protruding externally. The catheters have a rounded tip at the end, so that they can be advanced up the uterine horn without catching on the folds that line the uterus. It is very important that the catheter reaches into the uterine body and does not remain obstructed in the cervix.

This passage through the cervix is not always easy. By rectal palpation, the vet can feel the catheter within the uterus and guide it deeper into the uterus than can normally be achieved. The catheter should be gently pushed until the tip is at the very tip of the uterine horn. This is where the sperm should be deposited. Using this technique, satisfactory pregnancy rates have been achieved with sperm numbers of 50 to 100 million.

Using an endoscope

Endoscopes are long, flexible tubes with a bright light source at the end. They allow internal structures to be viewed and are commonly used in an examination of the horse’s respiratory tract.

It is possible to place an endoscope into the uterus of a mare through the vagina and cervix in much the same way as an insemination catheter. Air is then passed through a channel in the endoscope into the uterus, which allows the inside of the mare’s reproductive tract to be seen.

The operator can gently steer the tip of the endoscope up the uterus until the entrance of the oviduct (fallopian tube) is reached. A special narrow catheter can be passed down a central channel within the endoscope, and the tip of the catheter exposed beyond the end so it can be seen. By careful steering of the tip of the endoscope, the catheter can be placed very close to the entrance of the oviduct.

Semen is then blown out of the catheter directly on to the surface of the papilla. Using this technique, acceptable pregnancy rates can be achieved using as few as five million sperm. This represents a hundredfold decrease in the usual number needed.

By depositing the sperm so close to the site of fertilisation, the distance they need to travel is reduced. In addition, the exposure to the potentially hostile uterine environment is reduced. These are the two most likely reasons sperm numbers can be so drastically reduced.

The future

The implications of DUI for the equine breeding industry need to be evaluated carefully. A theoretical concern is that this reduces the natural selection process by which only the “fittest” sperm travelling the whole way on their own succeed in fertilising the egg.

Obviously, using a catheter to carry out DUI is much easier and cheaper than using an endoscope. In our practice, we routinely inseminate mares with frozen semen using the DUI technique by catheter. If the client wishes us to reduce the number of straws of semen used or the semen is in very short supply, we will use an endoscope.

This veterinary feature was first published in Horse&Hound (28 April, ’05)

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