Mare Breeding Soundness Examination

Historically, mares have one of the lowest average conception/birth rates of any of the domestic species (somewhere in the neighborhood of 60-70%). Excellent breeding and overall management of mares and the blessing of high-quality semen from a fertile stallion can result in higher rates on many farms, but in general, the enjoyment and thrill of successfully breeding a mare often are accompanied by some frustration and disappointment when she fails to settle right away, resorbs the fetus, or aborts a celebrated pregnancy. With an eye toward identifying potential chances for success or reasons for failure, many mares are presented to a veterinarian for a breeding soundness examination (BSE). The BSEs most often are performed on mares as part of a pre-purchase evaluation, for infertility work-ups, prognostically prior to urogenital tract surgery, or as a matter of routine on all open or maiden mares prior to the next breeding season (in the latter case, the benefit of identifying a problem and correcting it prior to the start of the next breeding season is obvious).

Poor Vulvar Conformation

An example of poor vulvar conformation. Photo by Christine Schweizer, DVM

The mare BSE, in general, includes the recording of a thorough and detailed history, an overall physical examination and a detailed examination of the reproductive tract. The latter would include an assessment of perineal conformation and health, rectal palpation, ultrasound of the cervix, uterus, and ovaries, a vaginal examination, a uterine culture and cytology, and a uterine endometrial biopsy. Other ancillary diagnostics also can be performed as indicated.

A complete history is necessary in order to interpret the status of the mare as she is presented. General information about her age, previous athletic use, and overall health status is obtained. Records of previous illnesses or injuries and medications are important (knowledge of steroid or other hormonal treatments is particularly useful). Determination of vaccination and worming status also is a good idea (tetanus prophylaxis needs to be current in order for the examination to be performed safely).

A more pointed reproductive line of information then is pursued. First, the mare's history and present status as a maiden (never been bred), open (previous foal or foals, but has not been bred back since last foaling), or barren mare (failed to conceive/carry a pregnancy after having been bred) are determined. A chronology of previous foalings and details concerning past problems, including dystocias (difficult deliveries), sick or premature foals, retained placenta, etc., are recorded, as is any evidence of previous embryonic loss or fetal abortions as well as how those were determined and whether a reason for the loss was identified.

Then, especially when dealing with an infertility work-up, a thorough documentation needs to be made as to fertility status of the stallion(s) to which she was bred; number and dates of cycles on which she was bred; last possible exposure to a stallion; interestrus interval (time between heat periods); length and character of heat periods; teasing/palpation/ultrasound records; method of breeding, i.e., natural cover, artificial insemination (fresh, cooled, or frozen); and results/findings on previous reproductive examinations. This last line of questioning begins pointing out potential areas of trouble.

For example, the normal length of time from the beginning of one heat period to the start of the next is approximately 21 days. An abnormally shortened interestrus interval could indicate an endometritis (uterine infection or inflammation) problem. Conversely, an abnormally lengthened interval might indicate that she is conceiving, then losing the pregnancy early on (early embryonic loss). Most importantly, these last questions help to determine the level and quality of the breeding management of the mare. A completely normal mare which is improperly managed has not been given a fair chance. Remember, the more we intervene in the natural breeding process (i.e., artificial insemination with cooled or especially frozen semen), the more intensive the management of the mare needs to become.

A general physical examination then is performed to assess the mare's overall health and to identify any problems that might interfere with this individual's ability to conceive, carry, give birth to, and raise a healthy foal. Overall body condition is important since mares which are too fat or too thin might have difficulty conceiving and carrying a foal to term. Chronic conditions such as laminitis (founder) or chronic obstructive pulmonary disease (heaves) can prevent a mare from physically being able to meet the demands of late pregnancy if the conditions are severe enough.

Assessments of breed quality and "typiness" of the mare are not the function of this examination, but overt, conformational flaws (such as a "parrot mouth," for example) that might be inherited should be identified. The size and symmetry of the pelvis relative to the mare's overall body type, as well as the slope of her croup, should be assessed since these can impact the mare's overall fertility (i.e., a mare with a narrow or damaged pelvis might have difficulty during delivery, or a mare with a very flat croup might be more prone to endometritis).

A thorough assessment of the normalcy of the mammary gland also should be made. The halves of the glands should be symmetrical with no heat, redness, or abnormal swellings or discharge (equine mammary tumors and mastitis are not common, but do occur). Teats should be anatomically normal, and the whole udder should accurately reflect the mare's lactational status at the time of the examination (an enlarged gland in a supposedly dry mare can indicate a problem). Location of the gland also should be assessed. An udder that appears to be "pulled forward" (in front of the stifle) from its normal position up between the mare's hind legs is indicative of a previous prepubic tendon rupture, and such a mare is not capable of carrying a pregnancy.

A detailed examination of a mare's reproductive tract begins with an evaluation of the mare's perineal region. First, the tail, vulva, and hind legs are visually examined for any abnormal-looking, fresh, or dried discharge (pus, blood, excessive urine staining, etc.). The vulva and anal region then are carefully inspected for any abnormalities, including scarring or tears (indicative of past foaling injuries), swelling or masses (melanomas in this region are especially common in gray mares, and they can cause problems if numerous and/or large). Whether or not the mare has had or presently has a Caslick's procedure (see The Horse of May 1997, page 28) also should be determined at this point.

A final and important assessment needs to be made in regard to the overall conformation of the perineum and vulva. The vulva should normally be in an upright position in a straight line with the anus. Greater than two-thirds of the vulvar length should be below the level of the floor of the pelvis, and the vulvar lips should meet evenly to form a seal. Asymmetry in the vulvar lips or any deviation in the angle of the vulva so that it slopes forward and is pulled above the floor of the pelvis increases the likelihood of fecal contamination of the vulva, compromise of the vulvar seal, and chronic aspiration of air and contaminants into the vagina (pneumovagina or "wind sucking"). Some mares are born with poor perineal conformation, but mares which have had multiple foals, are older, and/or are thin might start out normal, then develop these changes.

The examiner also should part the vulvar lips and listen as he/she does so to see if there is an inrush of air ("the windsuck test"). A positive finding indicates the seal of the vestibular-vaginal ring within also is compromised, making the mare more prone to pneumovagina. Chronic pneumovagina sets the mare up for inflammation and ascending infections of the cervix and uterus, and it is a common cause of infertility that is readily treatable with a Caslick's, provided no permanent damage has been done to the endometrium (lining of the uterus).

With the vulvar lips parted, the clitoris should be examined to make sure it is normal in size and appearance (the clitoris might be enlarged in some pseudo or hermaphroditic conditions). Where there is an indication (i.e., if the mare recently has been imported to this country or exposed to a recently imported stallion, or has a history of a profuse discharge), the clitoral sinus can be cultured for contagious equine metritis (CEM). This disease is considered to be "foreign" to the United States, but outbreaks occasionally have occurred.

A thorough rectal examination of the mare's cervix, uter-us, and ovaries is next performed, utilizing both manual palpation and ultrasound of all the structures for a direct assessment of the reproductive tract. Care should be taken to ensure the safety of both the veterinarian and the mare during this procedure, and proper restraint of the mare and gentle technique on the part of the clinician are essential.

The equine uter-us is an approximately "T" shaped organ, with the uterine body forming the "base" and the left and right uterine horns forming the "crossbar." The ovaries are located at the tips of each horn and are "kidney bean" shaped, with a distinct ovulation fossa palpable on each normal ovary. The ovaries and uterus are suspended within the pelvis on each side by the broad ligaments. During the spring/summer breeding season, normal mares should be cycling. Care is taken to determine what ovarian structures are present (follicles, corpora lutea), stage of cycle, and whether there are any abnormalities (inactive, small ovaries, mass lesions, loss of ovulation fossa, etc.). During the winter anestrus, the ovaries normally are small and inactive. Uterine tone and thickness are carefully assessed during the exam, and note is taken of the quality of the uterus, depending on the stage of the cycle and the time of year.

For instance, the uterus in cycling mares is soft and edematous during estrus (follicular phase), toned and tubular during diestrus (lu-teal phase), and especially toned up during early pregnancy. During the winter anestrus, the uterus is thin and atonic. Likewise the small, tubular cervix undergoes some similar changes in palpable tone, depending on the stage of the cycle (i.e., firm and closed during diestrus/pregnancy, softening and edematous as the mare comes into estrus, and soft and hard to detect when the mare is close to ovulating during mid- to late estrus). It is important that the tone of the tract matches the stage of the cycle as indicated by structures present on the ovaries.

Abnormal correlation between ovarian structures and tone quality might indicate potential problems (for instance, a mare which fails to soften her cervix during estrus is often more prone to pooling fluid in her uterus and can have an increased risk of becoming infected when bred). Any uterine abnormalities, such as palpable thickenings or masses, as well as endometrial cysts or free uterine fluid identified on ultrasound, are noted.

It is important at this point to stress that our rectal examination also is performed to determine whether or not the mare is pregnant before proceeding on to perform invasive uterine diagnostics (a leading reason why a mare isn't cycling is that she is pregnant!). Lastly, before withdrawing his/her hand from the mare's rectum, the veterinarian will make an assessment of pelvic diameter and symmetry. The presence of a healed fracture or other abnormality might decrease pelvic size and create obstructive difficulties should the mare foal.

Internal Exam

The interior of the reproductive tract is the next part to be examined. This procedure is begun by examining the vagina and cervix both visually with a speculum and manually by direct palpation per vagina. As we move on to more invasive diagnostic procedures, care must be taken to perform everything in as clean a manner as possible so as not to cause contamination of the reproductive tract (and in particular the uterus). Sterile lube, gloves/sleeves, and instruments are used, the tail is wrapped and tied out of the way, and the entire perineum is carefully cleaned, rinsed free of soap residue, and dried prior to the initiation of the intra-vaginal procedures.

Much can be learned about the condition of the reproductive tract via visual exam. Just as the palpable tone of the tract changes with the stage of the cycle, so does the appearance of the vaginal mucosa and external cervical os or opening (i.e., pink, moist, edematous/relaxed in estrus; pale pink, dry, and closed in diestrus/pregnant). Abnormalities such as vaginal urine pooling, foreign material, and abnormal exudates also are readily identified.

Mares with chronic pneumovaginas might have "froth" in their anterior vagina, and the mucosa will appear red and inflammed. Likewise, any inflammation deeper within the tract might be reflected by increased hyperemia (redness) of the vaginal and cervical mucosa. In a post-foaling mare, bruising of the cervix or vagina will be discernable, but tears or deficits within the cervix and scarring and/or defects (such as recto-vaginal fistulas) in the vagina often are more readily identified and better defined by direct manual palpation both in post-foaling and cycling mares.

Cervical tears occur most commonly during foaling and can result in defects in the external os and/or body of the cervix that are palpable as thin or missing areas. If the resultant damage is bad enough, it can prevent the cervix from forming a tight seal during diestrus, and during pregnancy the mare might have a tendency of developing ascending endometritis/placentitis and pregnancy failure.

Assessment of the cervix is best made when the mare is in diestrus and the cervix is at its tightest. Surgical repair of the cervix is indicated in mares with cervical damage that prevents formation of a suitable seal. Direct palpation of the cervix usually is performed after uterine culture samples have been gathered in order to decrease possible contamination of the endometrium with vaginal flora prior to sampling.

It is widely held to be true that the equine uterus is normally a sterile environment. Normal bacterial contamination occurs at breeding and at foaling, which healthy mares quickly clear prior to an infection becoming established. Mares which have compromised uterine defense mechanisms (i.e., poor perineal conformation, pneumovagina, damaged cervix, tendency to pool rather than clear uterine fluid, etc.) or which have problems that increase the likelihood of uterine contamination (urine pooling, rectovaginal fistulas, etc.) thus are more prone to having bacteria take up residence.

Bacteria of particular pathogenic significance include beta-hemolytic streptococci, Klebsiella, Pseudomonas and hemolytic E. coli. Yeast or fungal organisms also are found in mares with endometritis conditions. It is important to remember, however, that culture findings need to be interpreted based on uterine cytologic findings. The vulva, vagina, and external cervical os are not sterile locations, and contamination of the culturette during the sampling process can occur. Careful technique and use of a guarded culturette swab decrease the incidence of contamination.

The uterine cytology from a normal, non-infected or non-inflammed mare contains very few or no neutrophils (white blood cells). In general, however, infected/inflammed mares have high numbers of neutrophils on cytology. Where there are no signs of inflammation on cytology, the finding of just a few bacterial organisms on a culture is, therefore, most likely insignificant. Many clinicians advocate estrus as the best time to obtain culture samples since it is easier to pass through the relaxed cervix at this time and uterine resistance to infection contamination is believed to be at its best, thereby increasing the likelihood that only significant organisms will be found. In addition to the uterine swab technique, uterine cultures can also be obtained by endometrial biopsy samples.

Uterine biopsy samples might be the most objective means we have of assessing a mare's future prognosis for successfully carrying a foal to term. Endometrial samples are obtained using a sterile "alligator" biopsy forceps that is passed vaginally through the cervix and into the uterine lumen. Histological examination is performed on the fixed tissues, and they are assessed and graded based on the presence and degree of inflammation, periglandular fibrosis, cystic glandular degeneration, and lymphatic stasis in the endometrial tissue. Interpretation of findings also is affected by stage of estrus cycle, time of year, age of mare, and reproductive history. For example, the endometrial glands would be expected to appear inactive in December, but not in June. Also, some wear and tear are expected in older, multiparous mares, and mild changes can be forgiven to some degree.

Based on the pathologist's interpretation, the mare's biopsy sample is assigned one of four standard biopsy grades: I, IIA, IIB, and III. These grades are interpreted as a mare's chance (based on that sample's findings) of successfully conceiving and carrying a foal to term under proper management conditions. The higher the number, the worse her prognosis, so that in general a Grade I mare has an approximately 70-90% chance, a Grade IIA mare has approximately a 40-70% chance, a Grade IIB mare has approximately a 10-50% chance, and a Grade III mare has a less than 10% chance of her endometrium being able to support a pregnancy successfully to term.

A basic BSE on a mare is complete once the biopsy has been performed. Additional ancillary diagnostics such as uterine endoscopy, hormonal profiles, karyotyping, etc., can be performed as indicated by the basic examination's findings.

In conclusion, it is important to remember that a breeding soundness exam estimates a mare's potential to be able to conceive and carry a foal to term. Specific problems identified during the course of the examination can render a very guarded prognosis, but we can almost never state emphatically that a mare is for certain sterile (as a matter of fact, I have found that mares which have been dubbed "sterile" almost always manage to produce a foal at some later date just out of spite!). The opposite also is true. Even when all examination findings are within normal limits, the best we can say is that a mare is a "satisfactory potential breeding prospect."

A given mare's future success as a broodmare is at the mercy of many factors, not the least of which is the stallion to which she is bred and how well she is managed. Ultimately, the proof of any mare's reproductive soundness is a live, healthy foal nursing at her side.

About the Author

Christine Schweizer, DVM, Dipl. ACT

Christine Schweizer, DVM, Dipl. ACT, is presently a lecturer in theriogenology at Cornell University. She authored Understanding The Broodmare, published by Eclipse Press and available at or by calling 800/582-5604.Dr. Schweizer is a 1990 DVM graduate of the NYSCUM at Cornell University. She practiced in Cazenovia, N.Y. prior to an Equine Medicine and Surgery Internship at Rochester Clinic, Rochester, N.H. She next did a Residency in Theriogenology back at Cornell. Dr. Schweizer is also a Diplomate in the American College of Theriogenologists.

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