There are many causes of infertility, but endometritis (inflammation of the endometrial lining of the uterus) is one of the major causes. This inflammation can arise for a number of reasons, including wear and tear from previous pregnancies, as a reaction to sperm and debris introduced at covering, and frequently as a result of bacterial infection.
As infection of the reproductive tract is a major cause of endometritis, preventing such infection is very important. If the mare has good reproductive tract conformation, natural protection is provided. However, many mares have poor conformation, and bacterial invasion of the reproductive tract will arise when the natural defenses fail. To assist in preventing this invasion, veterinarians use a procedure called the Caslick's operation to partially close off the mare's reproductive tract to the entrance of air and debris.
The reproductive tract of the mare can be divided into two regions. The outer part is made up of the cervix, vagina, and vulva, and the inner part is made up of the uterus, fallopian tubes, and ovaries.
The inner part of the tract must remain free of infection to ensure successful fertilization and embryo/fetal development. This is because within the endometrium lie the endometrial glands, which provide secretions that pass into the lumen of the uterus and nourish the early embryo. In the later stages of pregnancy, the endometrium also provides the surface to which the placenta attaches. Any inflammation of this area, therefore, interrupts these functions and, even if a mare becomes pregnant, makes it very difficult for her to maintain the pregnancy. Endometritis can be treated successfully, but by the time the mare owner is aware of the problem, significant and even permanent damage might have occurred. If the cause or causes of the endometritis are not addressed, then the problem will recur.
Therefore, one of the major functions of the outer part of the tract is to protect the inner part from bacterial invasion. This protection is provided by the means of three seals: the vulval, vaginal, and cervical seals.
The vulval seal is formed by the perineal area plus the vulva constrictor muscle in the walls of the vulva. When viewed from behind, the vulval lips in a well-conformed diestrous mare (not sexually receptive or in heat) should sit close together, providing an effective physical barrier to invasion by bacteria or dirt.
The vaginal seal is formed by the natural apposition (sticking together) of the walls of the vagina where it naturally collapses, and sits above, the floor of the pelvic girdle and by the hymen, if it is still present. The secretions within the vagina are naturally thick, especially during diestrus. This stickiness helps the walls appose, making the seal more effective. The secretions within the vagina also tend to be acidic, and as bacteria do not survive well in a low-pH environment, this further helps prevent bacterial invasion.
The last seal, the cervical seal, is formed by the tight muscle ring of the cervix.
Failure of Defenses
Failure of these three seals--especially the vulval and/or vaginal seals--predisposes the mare to a condition called pneumovagina, or vaginal wind-sucking, in which air is sucked in and out of the vagina through the open vulva. Carried along with the air are bacteria, which bombard the cervix, exposing it to high levels of contamination that it is often not able to cope with. This is especially true when the mare is in estrus and the cervical seal is naturally less competent (because it's open for passage of sperm).
Movement of bacteria into the higher, more susceptible parts of the mare's reproductive tract will result in infection. Infections such as contagious equine metritis (CEM) and others lead to endometritis and the high risk of infertility.
The series of seals is affected by both the stage of the mare's estrous cycle and her conformation. During estrus all three seals slacken as the muscles associated with the reproductive tract relax, especially the cervix. This relaxation eases intromission (entry of the stallion's penis) at covering, but it also decreases the competence of the reproductive tract seals and, therefore, increases the chance of bacterial invasion.
However, during estrus, the mare's system is dominated by high plasma estrogen concentrations that enhance her immunological response to invasion by foreign material, thereby partly compensating for the relaxation of the protective seals. This enhanced immunological response reduces the chance of uterine infection, despite the increased chance of bacterial invasion.
The major and most concerning factor that governs the effectiveness of the three seals is the conformation of the mare. The ideal conformation is achieved if 80% of the vulva lies below the pelvic floor. A simple test can be used to assess this: If a sterile plastic tube is inserted through the vulva into the vagina and allowed to rest horizontally on the vagina floor, the amount of vulva lying below this tube should be approximately 80% in a well-conformed mare.
If the mare's pelvis is too low, then the vulva tends to fall toward the horizontal plane. This not only reduces the effectiveness of the vulval seal, but it also sets the anus back from the vulva (see the photo above). This opens up the vulva to contamination by feces, increasing the risk of uterine infection.
A mare with naturally poor conformation generally has a low pelvis. This is most often seen in performance mares or Thoroughbred mares, as the hindquarter musculature required for athletic excellence tends to be associated with a low pelvis and high croup, which predispose a mare to a sloping vulva.
In addition, as the mare gets older--and especially if she is multiparous (has had two or more foals)--the whole inner reproductive system (cervix, uterus, and ovaries) tends to fall deeper into the abdominal cavity as the broad ligaments (the support structures for the uterus) become stretched. In the worst-case scenario, and especially if the mare's pelvis is low, this can lead to the vagina sloping down toward the cervix, allowing urine to collect in the cranial vagina (toward the front, near the cervix) rather than pass out through the vulva. This collected urine provides an excellent breeding ground for bacteria, which the cervix is eventually unable to prevent passing into the uterus. Such conformation problems are further exacerbated in mares in poor body condition.
The effect of poor conformation of the perineum might be alleviated by a Caslick's vulvoplasty procedure. This is a relatively simple and commonly used procedure developed by E.A. Caslick, DVM, in 1937 that seals the upper part of the vulva in order to reduce entry of foreign debris and, thus, bacterial invasion. The perineal area is locally anesthetized and the lips on either side of the upper vulva are trimmed. The resulting two raw edges are then sutured together and, with time, naturally heal together as an open wound would. The sutures can then be removed, resulting in a sealed upper part of the vulva.
The length of vulva that is sutured varies with the condition of the mare, but can be as much as 80%. The opening left at the ventral commissure (bottom of the vulva where the lips join) must be adequate for urination, but prevent the passage of air and feces into the vagina.
Does She Need a Caslick's?
If your mare has poor perineal conformation and a history of endometritis and/or early embryo mortality, a Caslick's operation, following treatment for any existing infection, might well be advised. R.R. Pascoe, DVSc, FRCVS, FACVSc, in 1979 assessed the conformation and incidence of endometritis in 9,000 mares, and as a result he suggested that mares should be allocated a Caslick Index. This index is derived by multiplying the angle of inclination of the vulva with the distance from the pelvic floor to the dorsal commisure (top of the vulva). It can be used to determine whether the occurrence of endometritis is likely and, hence, a mare's need for a Caslick's operation.
There is no doubt that the Caslick's is very successful in preventing the passage of air, feces, and associated bacteria into the reproductive tract and reducing the chance of endometritis developing. However, the use of a Caslick's operation and the breeding of mares that require such an operation have to be considered carefully.
For mares with a Caslick's procedure performed at the usual length (up to 80% of the vulva sewn up), the vulva needs to be cut and resutured for each breeding and for foaling. This means that for every foal she has, she will need to be cut and resutured at least twice. This will increase if she does not become pregnant the first time she is bred and has to be re-covered.
For mares with a short Caslick's (less than 30% of the vulva sewn up), it is possible for artificial insemination to be carried out without cutting the vulva. Potentially such mares need only be cut and resewn once for each foaling. Unfortunately, artificial insemination is not accepted by the Thoroughbred industry, as this breed has the highest incidence of poor perineal conformation and, hence, the highest Caslick Index. The majority of mares that undergo a Caslick's operation have to be cut and resewn for covering as well as foaling.
As might be expected, there is a finite number of times that a Caslick's operation can be performed, as the vulval lips become more fibrotic with time, and harder to cut and resuture. It is possible in theory to perform a more recently developed procedure, the Pouret operation, on mares where a Caslick's operation is increasingly difficult to perform. This attempts to realign the mare's vulva and anus, but it is a much more major and complicated procedure and, therefore, is rarely practiced.
The finite number of times a Caslick's operation can be performed obviously limits some mares' breeding careers of some mares. Hence, carrying out a Caslick's operation on a mare before it is really required, i.e., as a safeguard, is not advised if she is destined to be a career broodmare. If the mare is older and/or is intended to produce only a few foals, then the long-term consequences of a Caslick's operation are less important.
Apart from potentially limiting a mare's breeding career, another consideration is that the perineal and general conformation that predispose a mare to reproductive tract infections is inherited. Mares with poor perineal conformation are very likely to pass that poor conformation on to any filly foals and, hence, on to subsequent generations, perpetuating the trait within the population.
There is, therefore, a welfare consideration to make when performing a Caslick's operation. The equine industry has to decide whether the shorter-term financial gain of selecting breeding stock purely for athletic performance should be at the expense of the longer-term detrimental effect on reproductive ability and equine welfare.
- Caslick, E.A. The vulva and the vulval-vaginal orifice and its relationship to genital health of Thoroughbred mares. Cornell Veterinarian 27, 178-187, 1937.
- Davies Morel, M.C.G. Breeding Horses, Blackwell, Oxford, U.K., 2005.
- Davies Morel, M.C.G. Equine Reproductive Physiology, Breeding and Stud Management, CABI, Wallingford, Oxon, U.K., 2003.
- Pascoe, R.R. Observations of the length of declination of the vulva and its relation to fertility in the mare. Journal of Reproduction and Fertility Supplement 27, 299-305, 1979.
- Pouret, E.J.M. Surgical technique for the correction of pneumo- and arovagina. Equine Veterinary Journal 14, 249-250, 1982.
About the Author
Mina Davies Morel, PhD, is head of the equine group at the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University in the United Kingdom. She has particular interest in equine reproductive physiology and its application to stud management, and she is the author of a number of scientific papers and text books on the subject. She is a leisure rider and owner of Welsh Cob Section Ds.