Tower Equine

Veterinary Surgeons


Uterine Fluid - January 2010

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Picture the scene. Your mare is at the AI centre, and all appears to be going well. The vet reports that she has a good follicle, and that breeding is planned for the next day. Twenty-four hours on and the news is still good – the semen has arrived, it looks encouragingly frisky under the microscope, and the follicle is just about ready to pop. By the next day however, the news is more mixed. The mare has ovulated as planned, but, suddenly, she has a large amount of fluid in her uterus. The talk is of uterine flushes and oxytocin, but the vet’s tone of voice gives you a sudden sinking feeling in your wallet. This brilliant breeding plan of yours is turning out slower and more costly than you planned….



Recognise the story? Most breeders will, because fluid in the uterus is probably the most common reason for a mare not getting in foal. But why is fluid such a problem, and what can we do about it?


Each mating, natural or artificial, causes inflammation in the mare’s uterus. Sperm are foreign invaders as far as the uterus is concerned, and so are all the penile bacteria that get deposited at the same time. One single spermatozoon may get lucky and fertilise the egg, but the rest all die – that’s over ten billion dead sperm in a natural mating.  Faced with such an invasion the uterine defence force swings into action, releasing proteins and cells that result in the production of fluid. This phase is normal and beneficial, as it helps flush away all the debris. The fluid drains out of the open cervix or is sucked away by the lymphatic drains in the uterine lining. The fertilised egg meanwhile is still wafting serenely down the fallopian tube, and won’t even reach the uterus until five days after mating. Within 48 hours of mating, a healthy uterus will have physically flushed itself out, and be as clean as a new nursery, ready to receive the embryo.


The problems come with mares in which the fluid doesn’t drain away properly. Sometimes the cervix is fibrous or scarred and, like an old door with rusted hinges, it can’t open fully to let the fluid out. That’s not all - often the uterine lining is thickened and scarred, the lymphatic drains have collapsed, and the enlarged and saggy uterus has all the muscle tone of a pot-bellied pub boozer. The end result is that drainage fails and a pool of infected fluid remains. The embryo, still only pinprick sized, floats down into the uterus and vanishes into the stagnant pool, never to be seen again.


Now before you can prevent such an outcome, you have to identify fluid prone mares. On initial inspection, a problem uterus can look fine and it is only after insemination that fluid retention becomes an issue. A difficult breeding history is an obvious pointer, as is a mid-teens mare that has never had a foal. The uterus stays much healthier if it is used, and a fourteen year old mare that has had six foals is a much better breeding proposition than a similar aged maiden.


Fortunately, fluid is easy to pick up on the ultrasound scanner, and the normal AI centre regime of frequent scanning means that most cases are quickly identified.

Once we know a mare has post-mating fluid problems there is a great deal we can do to tackle it. The more matings, the greater the challenge to the weakened uterine defences so a problem mare should be inseminated only once, at the optimum time. AI can be of benefit here, because of the reduced number of sperm and the antibiotics in the extender that help keep infection down. Once the mare has been inseminated, the vet can then get in quick and give the uterus a helping hand. Flushing out with several litres of warm sterile saline can be done as early as four hours after insemination. The sperm that are good swimmers will be safely in the fallopian tubes well before then, and the quicker the rest are hosed out the better.

Oxytocin is another potent weapon in the stud vet’s cupboard. This drug causes the smooth muscle of the uterus wall to contract, pushing fluid out through the cervix. It is often followed up by an infusion of antibiotics into the uterus.


Uterine Fluid drugs


How much fluid is present and at what stage of the mare’s cycle it occurs is also important. Early in estrus a little fluid can collect before the cervix is relaxed and open enough for it to drain. This fluid is often sterile, and clears of its own accord. More than two centimetres depth of fluid is not good news at any time, and especially not when a mare is out of season.


Successfully getting rid of uterine fluid after mating is one of the most important ways in which a stud vet can improve a mare’s chances of getting in foal. Oxytocin, uterine flushing and antibiotics are all part of the treatment plan which needs to be individually tailored to suit each mare. The repair of underlying vulval or cervical problems is also vital, as is the use of high quality, highly motile semen. The combination of a mare with fluid plus sperm that need armbands to swim is a recipe for a long and expensive stud season, so for the most difficult mares make sure you choose a very fertile stallion. For the same reason, frozen semen may be less suitable for these mares because it tends to result in more inflammation and more fluid than chilled or fresh semen.


We still have a great deal to learn about ‘the problem mare’, and new research into the uterine response to inflammation will hopefully lead to new treatments and improved conception rates.


Roger Lee MA VetMB CertEM (Stud) MRCVS