It is well established that mares decline in fertility with advancing age; even mares within the window of optimal reproduction sometime suffer from reduced fertility. An important contributor to these problems is endometrial disease. Considering that a major goal of equine breeding operations is to enhance reproductive efficiency, diagnosing and treating endometritis and other forms of endometrial disease are major concerns.

A number of procedures are available to the veterinarian to evaluate an infertile or subfertile mare. These include visualization and palpation of the reproductive tract, ultrasonography, culture, hormonal profiles, and endometrial biopsy. Endometrial biopsy can yield information not obtainable by other procedures and can be an important part of a comprehensive infertility workup or breeding soundness examination. The information provided by endometrial biopsy should not be interpreted alone but in conjunction with a complete history and thorough examination.

Endometrial biopsy involves passing a long forceps through the cervix into the uterine cavity and taking small pieces of endometrial tissue for histopathologic examination. Typically, the uterine body, each horn, and any areas detected to be abnormal are biopsied. The tissue samples are preserved in fixative, processed, and sections placed on glass slides for microscopic examination. The technique is easily performed and relatively safe. Sedation is not routinely required.

The mare’s endometrium is composed of an epithelial surface lining the inside of the uterus and numerous glands that lie beneath the surface. Ducts connect the glands to the endometrial surface. These glands secrete fluid with a number of functions essential for pregnancy. A loose stroma occupies the space between the glands and contains blood and lymphatic vessels. The endometrium is subject to diseases that disrupt its normal anatomy and function, rendering the mare subfertile or infertile. The pathologist or theriogenologist examining the biopsy estimates the stage of the estrous cycle and identifies any abnormalities within the endometrium. A normal endometrial biopsy in a subfertile mare points the practitioner to other causes of infertility. The major changes revealed by endometrial biopsy include inflammation of the endometrium, dilated glands and lymphatics, and fibrosis. These changes can be mild or severe and the inflammation can be acute or chronic in duration. Fibrosis is permanent and, if extensive, can carry a poor prognosis for pregnancy