Evaluating Reproductive Health in Mares

The reproductive system of the horse has evolved over time with specific protective functions that are necessary for fertility. The horse and pig are the only domestic animals in which semen is deposited directly into the uterus, in contrast to ruminants and small animals where semen is deposited in the vagina and the sperm cells migrate through the cervix. There is, therefore, considerably more contamination of the uterus of mares and sows with the seminal plasma, excessive sperm cells, and external contaminants in the normal breeding process.

In horses, the sperm cells that will fertilize the egg (ovum) migrate rapidly through the uterus and into the opening of the oviduct, where fertilization occurs and where the embryo develops and remains for four to five days. The balance of the ejaculate in the uterus produces an intense inflammatory reaction, which is essential for initiating uterine defenses, but must be dealt with by the mare in order for pregnancy to be established. Sperm cells within the uterus are responsible for recruiting specific white cells (neutrophils) that ingest and kill the excess sperm, any bacteria, and other contaminants. The inflammation also results in fluid production by the lining of the uterus (endometrium). In addition, the uterus is stimulated to contract and expel its contents through the open cervix.

All of these actions are intended to clear the uterus of the contamination, dead sperm cells, used-up neutrophils, and the fluid. All of this must be done within five days after breeding, so that the embryo finds a compatible environment when it emerges from the oviduct. Any significant inflammation that persists will damage the embryo and most likely destroy it.

In young, healthy mares, these processes are very efficient, and the loss of embryos due to inflammation is negligible. As the mare ages and defense mechanisms become less efficient, conception rates begin to decline. When the defenses are no longer capable of handling the challenge, bacteria colonize on the endometrium and produce a persisting inflammation. Mares which reach this stage are termed "infected" and the situation must be reversed before fertility can be restored; otherwise the bacteria will keep the uterus inflamed. For the purposes of this discussion, we will refer to the categories of mares as the following:

Resistant mares have no uterine inflammation prior to breeding--their uterine defenses are adequate, and they are capable of clearing post-breeding inflammation within a few days.

Susceptible mares have reduced uterine defenses. They are normal prior to breeding, but their uterine clearance is reduced. They remain inflamed too long for the embryo to survive, but typically clear up before the next cycle. This condition is referred to as post-mating endometritis.

Infected mares have persistent inflammation and positive uterine cultures from bacteria or secondary organisms such as yeasts.

Mare Examination

Obviously, there are different schemes to consider in order to establish management plans for maximum fertility for mares in each of these categories. In order to classify the mares and make decisions on management, information that will help evaluate the health of the reproductive tract must be collected. Your veterinarian has a number of tools and procedures to accomplish this evaluation. They include:

Physical Examination--This could include a) Inspection of the external genitalia of the mare to determine if there are anatomical problems that could contribute to contamination of the uterus and visual inspection of the vagina and cervix with a speculum; b) Manual examination of the entire tract by palpation per rectum; and c) direct manual palpation of the cervix.

The owner or handler of the mare can make significant contributions to assessing the physical qualities of the external genitalia. By being familiar with your mare, changes in her external anatomy will become evident as she goes through stages of her cycle and the changes associated with aging. These observations can be extremely helpful in assessing problems. So, start looking under the tails of mares on a regular basis, and become a good observer and recorder.

Figure 1 on page 72 shows the external anatomy of a young mare with excellent perineal conformation. (The perineal area includes the vulva, the anal area, and the tissues in between.) Note that the alignment of the anus and vulva is perpendicular. In Figure 2 on page 72 note the differences in these structures in a mare which has lost some of the muscle tone and support for her perineum due to aging and delivering multiple foals. The vulva has changed from a perpendicular alignment to one with a noticeable slope. This change usually is an indicator of potential contamination of the reproductive tract, since an upright and toned perineum is a major factor in the exclusion of air, feces, and other contaminants from the vaginal tract and eventually the uterus. When the mare is in heat the vulva relaxes, compounding the problem for the mare with too much slope.

Visual inspection of the vagina and cervix through a vaginal speculum is an important component of all reproductive examinations. Inflammation of the uterus can often be reflected in changes in color of the membranes (increasing angry reddening) or the evidence of inflammatory discharges. Care must be taken to examine the area soon after dilating the vagina with air, since that act will induce local irritation and redden tissues rather quickly.

Ancillary examinations include the uterine culture, uterine cytology, and uterine biopsy. Special procedures include ultrasonic scanning of the uterus and rarely, visual examination of the uterus with an endoscope. These procedures are clearly the responsibility of the veterinarian, but you need to understand the fine points of all of these procedures in order to appreciate the approaches that will be necessary.

Proper perineal conformation
Poor perineal conformation

This mare exhibits normal perineal conformation with vertical alignment of the vulvar lips. The vulvar lips stay closed to the environment, preventing contamination.

This is the characteristic inward-sloping perineum of an older mare that has borne multiple foals. This conformation predisposes a mare to contamination of the cervix and uterus.
Guarded swab, capped
Guarded swab, extended
This guarded swab can be placed into the uterus without contaminating the swab with material from other parts of the tract. Once in the uterus, the swab is extended past the cap to collect a sample. It must be retracted back into the guard to prevent contamination on the way out.

Uterine Culture--Since the external portions of the mare's reproductive tract, and the environment, can contaminate a bacterial culture, care must be taken to culture only the uterus in this procedure. If a positive bacterial culture is obtained, it is imperative to know its source. A culture swab passed though the vulva and vagina is easily contaminated. There are many bacteria on the skin of the perineum, in the vestibule of the vagina, and occasionally even on the surface of the cervix. These might make a culture grow organisms whether or not there is actually a bacterial population in the uterus. A positive culture obtained from these areas can lead to a mistaken diagnosis, which could lead to inappropriate treatment of a normal uterus, potentially creating a problem.

One approach to avoid this mistake is to use a guarded swab, an example of which is shown in Figure 3 on page 72. The swab is passed through the vagina and cervix, usually via a speculum, and into the uterus while encased in the sterile plastic tube. Then the swab is pushed through the cap, the uterus is sampled, and the swab withdrawn well back into the tube. Of course, the swab must then be handled carefully to avoid contamination in the process of applying it to the culture medium.

There is general agreement that cultures of the uterus are more reliable when taken while the mare is in heat. At that time, the cervix is relaxed and the uterus will be moist from mucus secretions, which will make recovery of bacteria more predictable. The most common bacteria isolated from the uterus of mares are ones that grow in an aerobic (oxygenated) environment. The four bacterial species most frequently associated with endometritis when isolated in pure culture are Streptococcus zooepidemicus, E. coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae. The key word here is endometritis (inflammation of the uterine lining). If there is no evidence of that inflammation, then bacterial recovery must be viewed with suspicion. Signs supportive of presence of active inflammation include vaginal discharge, reddening of the vaginal tract, and presence of inflammatory cells in uterine samples.

Uterine Cytology--Since we have defined endometritis as inflammation of the uterus, one clear bit of evidence of inflammation is the presence of neutrophils (phagocytic white blood cells; for more information on this, see "The Equine Immune System" in the March 2002 issue of The Horse, article Quick Find #3344 at www.TheHorse.com) within the uterus. Keep in mind that massive numbers of neutrophils migrate to the uterine lumen (internal space) as a result of any contamination, including breeding. If uterine defenses are not functioning efficiently, inflammation persists and neutrophils can be detected within the lumen.

This is the basis for using uterine cytology as an indicator of inflammation. At the time the uterus is cultured, it is relatively easy to obtain a sample of uterine fluid. The sample might be taken from a swab, or from small amounts of sterile saline used to flush the uterus. A practical alternative is to use the cap from the guarded swab used in the culturing process. The sample can be stained with a rapid technique and examined immediately with a microscope. The presence of neutrophils in the sample is a reliable indicator of endometritis. Also, the presence of uterine epithelial cells (cells from the uterine lining) confirms that the sample has come from the uterus. Both types of cells are seen in Figure 4 on page 72.

In cases of low-grade inflammation, cytology can be of great value. In fact, if a reliable technique indicates the absence of neutrophils, the culture becomes less critical in the evaluation.

Other tests for detecting inflammation include ultrasonic scanning of the uterus for fluid and endometrial biopsy, a more complex technique that has more diagnostic implications.

Managing for Maximum Fertility

Resistant Mares--Breed them at the appropriate time.

Susceptible Mares--The key to improving uterine clearance after breeding involves techniques that will improve or speed up the evacuation of inflammatory materials from the uterus and return it to a condition favorable to the embryo by five days after ovulation. Fortunately, it has been shown that mechanical approaches such as saline lavage can safely begin as early as six hours after breeding. By that time, the sperm cells potentially involved in fertilization are safely inside the oviduct and protected from the inflamed uterus. Large-volume lavage with sterile saline or other electrolyte solutions, carried out properly, can be used repeatedly for two or three days after ovulation without fear of injuring the embryo. Combinations of drugs, such as oxytocin and/or some prostaglandins, can improve the benefit of lavage by enhancing the contractions of the uterus. Discuss the treatment of these mares with your veterinarian and rely on his/her experience.

Infected Mares--Mares which have previously been characterized as susceptible might become progressively more non-receptive to treatment and end up with persistent bacterial endometritis. When lavage and similar treatments fail, and cultures remain positive between heat periods, antimicrobial use is indicated. Specific antibiotics for the bacteria involved are recommended as long as the medications are not overly irritating in their own right. Combining uterine lavage with antibiotic therapy is logical. Removal of as much of the bacteria and debris by lavage should precede introduction of the antibiotics.

A major problem arising from treating the uterus with antibiotics is the introduction of resistant bacteria at the time the primary organisms are being treated. A classic example is the secondary infection with yeasts or resistant Pseudomonas strains after treatment of a simple primary Streptococci infection with penicillin. These types of infections become very difficult to control and often cause persistent fertility problems in broodmares.

Of further concern is the treatment of mares which are really not infected. As discussed earlier, it is possible to mistake contaminating bacteria for pathogenic ones (those that cause disease) and treat with antibiotics when none are needed.

Bacteria such as E. coli and Streptococci are plentiful in the environment, on the skin of the perineum, and in fecal material. Very careful evaluation for inflammation is critical in decisions to treat these mares--false positives can be a problem.

A sad example of this was referred to me years ago. A maiden filly had been cultured before breeding. The results of the culture indicated a few colonies of E. coli. A decision was made to treat her with a high-powered antibiotic and skip breeding her on that heat. The next time she came in heat, there was another organism growing on her culture. The rest of the breeding season was spent treating this mare. She never got bred that year, or ever, due to the terrible adhesions that had developed in response to all of the irritating medications.

Other Management Considerations

There are many simple things that can be done to improve the chances for future foals from marginal mares. Some of the abnormalities of anatomy mentioned early in this article are correctable. The simplest, and probably the most effective, procedure is suturing the lips of the vulva (Caslick's operation) in mares which are beginning to aspirate air and other things through the compromised vulva. This sort of problem is exaggerated when mares lose condition.

Other anatomical problems that are manageable include damage to the reproductive tract in the foaling process, including tears in the cervix. Urine pooling in older mares can be dealt with surgically. Since susceptible mares have problems clearing contaminants, there might be things in the breeding routine that will reduce contamination. Good breeding hygiene might add foals to a mare's produce record by the end of her career.

What Does That Test Mean?
The basic elements of the diagnostic puzzle are represented in the following chart.

Physical Findings

Culture Results
Cytology Findings
Diagnosis and Comments
Gross inflammation and/or discharge
Repeated isolation of bacteria
Numerous neutrophils


Numerous neutrophils
No growth
Numerous neutrophils Endometritis (failed to isolate the cause)
Bacteria isolated
Few or no neutrophils Normal uterus (bacteria are contaminants)
No signs of inflammation
Few or no neutrophils
Normal endometrium

Using these guidelines to classify mares as resistant, susceptible, or infected requires that these findings be taken in the context of each mare's history of cycling, breeding, and foaling. Positive evidence of inflammation at least five days after any breeding or examination procedures is significant. Retained fluid in the uterus (as seen by ultrasound) five days or more post-breeding is a clear indicator of susceptibility.--A.C. Asbury, DVM, Dipl. ACT

Mansmann RA, McAllister ES, eds. Equine Medicine and Surgery, 3rd ed. Santa Barbara: American Veterinary Publications, 1982.

About the Author

A.C. Asbury, DVM

A. C. (Woody) Asbury received his DVM from Michigan State University in 1956, then spent 21 years in California in breeding farm practice and at UC Davis. He joined the faculty at the University of Florida in 1977 and was involved in teaching, research, and administration until 1996. An Emeritus Professor at Florida, he lives in Kentucky, where he and his wife are developing a small farm.

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