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Why Is My Mare Not Pregnant?

Infertility is described as the inability to conceive or carry a foetus to full term. It is the most common reproductive complaint of horse owners. It can be a difficult problem for the vet as recognition of the cause is frequently difficult. Accurate records may not be kept, the mare may change owners, the stallion may be infertile, or reproductive management may be poor. Recent advances in technology such as chilled or frozen semen have complicated and in some cases increased the incidence of infertility.

There are several factors that affect the mare's ability to conceive :-

  1. Age of mare –Younger horses have higher conception rates, after 12 yrs of age there is a significant drop in fertility. In older mares the conception rates may still be good but there is a much greater incidence of early embryonic loss. Early embryonic loss (up to day 35) is approximately 9% in mares under the age of 3 and in mares over 12yrs of age this rises to over 60%. Older mares also have a higher incidence of uterine cysts which reduce the space available for embryo implantation.
  2. Physiological effects – “silent” heats or failure to recognise a normal oestrous cycle, individual variations in cycle predictability, uterine inflammation (endometritis) due to either chronic infection or as an overreaction to semen in the uterus. Endometritis is the most common cause of early embryonic loss.
  3. Mare management – accurate timing for insemination / covering with respect to ovulation
  4. Stallion infertility / management – over use of the stallion (more than three coverings a day), failure to detect ejaculation.

An experienced stud vet can have a great influence in improving conception rates by recognising potential causes of infertility and addressing these even before an insemination or a covering takes place. For example, a pre-breeding check will have identified those mares whose vulval conformation is suboptimal (sloping in such a way that increases the risk of faecal contamination or where there is a poor seal between the vagina and the outside allowing sucking in of air and other contaminants). Most of the conformational problems can be corrected by a procedure known as Caslick's operation which involves stitching the top of the vulva to improve the integrity of the seal, this is usually done after insemination / covering. In some cases of chronic infection this minor surgery is done immediately after treatment of the infection to prevent an instant re-infection even before artificial insemination covering has taken place. An endometrial swab should be taken from each mare at the start of each breeding cycle to check for uterine infection so that any infections can be treated appropriately. Milder infections can be treated by “washing out” the mare after insemination or covering has taken place, this involves either uterine lavage or infusion of intra-uterine antibiotics. For more severe infections it may be necessary to “wash out” the mare for several consecutive days whilst she is in season thereby ensuring a clean environment for the next season which can be used.

For those mares who fail to show oestrous well or have abnormal cycle lengths hormonal manipulation can improve fertility. This usually involves a short course of progesterone to put the oestrous cycle on hold thereby priming the reproductive hormones. This is followed by an injection of prostaglandin which causes the removal of any inhibitory hormones remaining and most mares should come into season 3-5 days after this injection. Progesterones are also commonly used to decrease the incidence of early embryonic loss in those mares who have suffered from this previously or those considered to be at high risk such as older mares. In these cases the progesterone supplementation starts from around 3 days post ovulation and is maintained until a 45 day pregnancy is confirmed. At this stage the foetus takes over in production of adequate progesterone to maintain the pregnancy and the supplementation can start to be tapered off.

It is normal for mares to have a low grade inflammatory response in the uterus following mating or insemination. For most mares this is mild and transient and is self-cleared within 12 hrs. However, a significant number of mares have an over-reaction to foreign material (semen) in the uterus and produce persistent amounts of fluid which is probably the most common cause of early embryonic loss. These “fluid producing” mares are usually older, often have poor conformation and require careful attention to maximise chances of a successful pregnancy. There are some maiden mares (young or old) that develop mating-induced endometritis because the cervix does not open sufficiently when the mare is in season; the reasons for this are poorly understood. For the milder cases of fluid accumulation the hormone oxytocin can be administered which facilitates uterine contractions which aid in expelling the fluid. In the more severe cases fluid accumulation and uterine inflammation can be evident as little as 4 hours after mating / insemination. In these cases large volume uterine lavage is required, sometimes in adjunction with antibiotic therapy as well as oxytocin. This may need to be carried out several times after insemination for some mares to give them their best chance of conceiving.

Apart from endometritis, the most significant causal factor in failure to conceive is suboptimal timing of mating or insemination with respect to ovulation. Fresh semen should last 48hrs in the mare so natural cover can be carried out 2 days before ovulation and still be successful. For artificial insemination with frozen semen there is a much shorter window where the semen will be viable inside the mare so a much more precise timing of insemination is necessary. The length of time semen is viable varies widely between stallions but can be as little as 6 hours in the worst frozen semen and as long as 72 hrs in very good fresh semen. The use of drugs which ensure ovulation occurs within a predictable time frame is valuable in reducing labour, unnecessary coverings or repeated scanning.

Richard Stringer BVSc MRCVS of Stringer Equine Veterinary Practice