Mare Breeding Problems: Make Room for Baby

One of your main objectives whether you own or work with broodmares should be to produce the maximum number of live, healthy foals from the mares bred during the previous season. Perhaps the biggest obstacle to achieving this aim is the "problem" mare. The biggest category of problem mare is the so-called "dirty" mare, or mare with a persistent infection that keeps her from either getting pregnant or maintaining a pregnancy. Commitment from the mare owner, breeding farm staff, and veterinarian is needed to successfully handle this mare. The owner should be made aware of the problem at the outset and be given a realistic expectation as to the chance of success.

This article will consider the dirty mare and how to provide effective management to help get her in foal. Particular emphasis will be on the mare susceptible to persistent post-breeding uterine infection.

Failure to Conceive

Many mares which cycle, but fail to conceive, have infections in their reproductive tracts--hence they are sometimes called "dirty" mares. While the term dirty mare is in common usage in breeding, it is not always appreciated that there are several causes of a mare being persistently infected.

All dirty mares either have, or are at risk of, inflammatory changes involving the endometrium (lining of the uterus). This is the reason that dirty mares are said to have endometritis (inflammation of the endometrium). The uterus is the place where the early pregnancy will develop, so mares with endometritis might either fail to conceive or undergo early pregnancy loss.

Dirty mares fall into the following categories:

  • Mares with acute infectious endometritis;
  • Mares with chronic (long-term) uterine infection, often with scar tissue; and
  • Mares susceptible to persistent mating-induced endometritis.

Acute Infectious Endometritis

An important cause of infection can be poor vulval shape. In the normal mare, the vulva provides the first effective barrier to protect the uterus from ascending infection. In the normal mare, the vulval lips are full and firm and meet evenly in the midline, and there is 80% or more of the vulval opening below the brim of the pelvis (Figure A, above). If the vulval seal does not work properly, sucking of air and contamination into the vagina can occur (Figure B). The initial vaginitis can lead to acute uterine infection, resulting in a dirty mare.

Diagnosis of endometritis can be made by collecting concurrent endometrial swab and smear samples during early heat for bacteriological culture and cytological (cell structure) examination, respectively. This allows time for resolution of the problem prior to breeding, and maximizes the chances of pregnancy.

The ideal technique should ensure that the swab enters the uterus and collects bacteria from only the uterine lumen (cavity). It is important to ensure that the method of swabbing does not introduce bacteria into a previously normal uterus. A positive culture result, with no evidence of inflammatory cells in the smear (usually neutrophils), is likely to be due to contamination during collection.

Acute infectious endometritis is found most frequently in older mares which have had several foals. Such mares have a breakdown in uterine defense mechanisms that allows the normal genital flora to contaminate the uterus and develop into persistent endometritis.

The approach to treatment most favored by practitioners has been the infusion of various antibiotics--dissolved or suspended in water or saline--into the uterus. The number of treatments required depends on individual circumstances, but daily infusions for three to five days during estrus works well in most cases.

In addition, predisposing causes to the persistent endometritis, such as defective vulval conformation, should be addressed. For example, a Caslick's procedure (stitching shut of the upper vulvar lips, see Figure C above) can help prevent windsucking and contamination in a mare with poor vulvar conformation.

Chronic Uterine Infection

These mares can have a wide range of degenerative changes (fibrosis, scar tissue, and glandular degenerative changes), and the infection has often been present for a long time.

Successful treatment of this category of dirty mare is difficult. Improved fertility after endometrial curettage (removal of growths from the wall of the uterus) has been reported; however, this has involved the use of mechanical and chemical agents (namely povidone-iodine and kerosene) that cause endometrial necrosis (tissue death). This treatment, apart from being of questionable value, can cause irreversible damage, including adhesions. The treatment does not selectively go after growths--rather, it attacks the entire endometrium. There is no known treatment for fibrosis or scar tissue. The degree of fibrosis or scar tissue relates to fertility--the more scar tissue or fibrosis, the poorer the fertility.

Repeated daily lavage with 0.5-0.8 gallons (2-3 liters) of hot (122°F, 50°C), sterile, isotonic saline has been suggested as a method of reducing the size of the lymphatics (vessels that drain fluid from tissues) and thereby the whole uterus. The prognosis for fertility remains poor no matter what treatment is used. Mares with lymphatic drainage problems are typically seen as delayed uterine clearance mares because they cannot effectively remove excess fluid via the lymphatics.

Persistent Mating-Induced Endometritis

Persistent mating-induced endometritis (delay of clearance) has come to be recognized as the major reason for failure of mares to conceive. This means it is the most important category of dirty mare.

The mare has a normal inflammatory response to semen, whether from live cover or artificial insemination. Semen is deposited directly into the uterus when mares are bred. This means that bacterial and seminal components--as well as debris--contaminate the uterus. This then results in uterine inflammation.

It was previously thought that the inflammatory response to breeding was due to bacterial contamination of the uterus during insemination. It is now accepted that sperm themselves--as well as bacteria--are responsible for the acute inflammatory response in the uterus after insemination.

A transient uterine inflammation following insemination is useful and serves to clear the uterus of excess sperm and debris associated with breeding. In most mares, this transient endometritis resolves spontaneously within 24-72 hours so that the environment of the uterine lumen is ready to support the embryo when it descends into the uterus five to six days after ovulation.

However, if the endometritis persists more than four or five days after ovulation, this is incompatible with embryonic survival. These mares which fail to clear these inflammatory responses are referred to as susceptible, and they often develop a persistent endometritis.

The physical ability of the uterus to eliminate bacteria, inflammatory debris, and fluid is known to be the critical factor in uterine defense. It is a logical conclusion that any impairment of this function, i.e. defective uterine contractility, renders a mare susceptible to persistent endometritis.

Detection of Susceptible Mares

This can be difficult, as there might only be subtle changes in the uterine environment that aren't readily detected by current diagnostic procedures. Many mares show no signs of inflammation before mating, but will fail to resolve the inevitable endometritis following mating. Use of ultrasound to detect uterine fluid has proved useful to identify mares with a clearance problem (see image at right).

Excellent management is necessary prior to breeding these types of mares:

Hygiene--Good hygiene at foaling is essential, and all mares should be thoroughly examined postpartum for the presence of trauma that might compromise the physical barriers to uterine contamination. Gynecological examinations, particularly of the vagina, should be performed as aseptically (sterile) as possible. Pay attention to hygeine at mating by using a tail bandage and washing the mare's vulva and perineal area with clean water (ideally from a spray nozzle to avoid the need for buckets).

Timing of Breeding--Breeding should occur at the optimal time, and the number of breedings per cycle should be restricted to one to reduce the chance of infection. These mares need very close monitoring of estrus by rectal palpation and ultrasonography. The use of ovulation induction agents is strongly recommended to ensure they are only bred once. Prediction of ovulation can also be made easier by not breeding these mares too early in the year, i.e., before they have begun to cycle regularly. If feasible, the use of artificial insemination can help reduce (but not eliminate) the inevitable post-breeding endometritis.

Ultrasound Evaluation of the Uterus--Ultrasound can detect intraluminal uterine fluid, in addition to conventional techniques or endometrial cytology and bacteriology before breeding. Even if cytology and bacteriology have been negative before breeding, mares susceptible to post-breeding endometritis usually accumulate fluid in the uterine lumen for more than 12 hours after breeding.

Prior to Breeding

A single breeding must be arranged one to three days before the anticipated time of ovulation. It is my experience that most stallion spermatozoa are viable in the mare's reproductive tract at least 48-72 hours after breeding. They can also maintain viability in a proper container for this period of time. Early breeding allows more time for drainage of fluid via an open cervix and also utilizes the natural resistance of the tract to inflammation during estrus. It allows sufficient time to flush the mares more than once before ovulation if necessary.

In the author's opinion, treatment for endometritis is ideally performed before ovulation. Progesterone concentrations rise rapidly in the mare after ovulation, uterine contractility decreases, and the cervix closes. Progesterone decreases the immune response to inflammation, making the mare more susceptible to infection. Thus, any post-ovulation treatment has an increased risk of uterine contamination. In addition, uterine fluid is less likely to drain if the cervix is beginning to close.

After Breeding

Oxytocin can be given every four to six hours after breeding to cause uterine contractions to rid the uterus of unwanted fluid. Ultrasound examination of the uterus the day after breeding can be performed to assess the amount of intrauterine fluid. If more than 0.8 inches (2 cm) of fluid is present, lavage of the uterus with 0.3-0.5 gallons (1-2 liters) of warm, sterile saline using a uterine flushing catheter is performed (see photo at right). During lavage, oxytocin should be given to stimulate uterine contractions. In cases where the cervix has failed to relax adequately, digital dilation of the cervix--with scrupulous attention to cleanliness--is indicated. This is followed by infusion of antibiotics. Further doses of oxytocin are given every six hours until the mare is re-examined the next day.

In some mares, the slower release of prostaglandin might be useful in addition to oxytocin. A single prostaglandin injection should be given some six to eight hours after the first oxytocin injection.

The mare is re-examined the following day and oxytocin treatment repeated if fluid is still present. Only rarely will a second infusion of antibiotics or lavage procedure be performed due to the risk of uterine contamination.

An important concept is to treat in relation to breeding and not wait for ovulation.


A dirty mare, once inseminated or bred, should not only be checked for ovulation, but also for fluid accumulation in the uterus. That is one of the most reliable clinical signs of susceptibility to post-breeding endometritis. If a mare is recognized as being susceptible to this problem, intensive post-breeding monitoring and treatment is necessary to improve the chances of conception.


Asbury, A. Evaluating Reproductive Health in Mares. The Horse, April 2002, 71-76. Article #3432 at

Evers, S. The Infertile Mare. Article #4551 online.

About the Author

Jonathan F. Pycock, B. Vet. Med, PhD, DESM, MRCVS

Jonathan F. Pycock, BVetMed, PhD, Dipl. ESM, MRCVS, operates Equine Reproductive Services, a first opinion and referral private equine practice based in Yorkshire, England. He has published many papers and book chapters on a variety of equine reproductive topics, and edited the book Equine Reproduction and Stud Medicine. His main interests include ultrasonography, breeding the problem mare, and artificial insemination. Currently, he is evaluating the use of oxytocin and depot oxytocin as a post-breeding treatment for mares.

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