Endometrial Biopsy: a tool to evaluate fertility in the mare
By Andrés de la Concha-Bermejillo DVM, MS, PhD
With over 800,000 tests run annually, TVMDL encounters many challenging cases. Our case study series will highlight these interesting cases to increase awareness among veterinary and diagnostic communities.
Infertility in the mare results in significant economic losses to the horse industry every year. The causes are varied and include uterine infections and uterine inflammation, hormonal abnormalities, development of defective ova in old mares, twinning, and other causes. Endometritis occurs in up to 60% of barren mares and is by far the most common cause of infertility in the mare, frequently resulting in failure to conceive or in pregnancy loss.
Mares are often presented to the veterinarian for a breeding soundness examination (BSE) to determine the likelihood a particular mare or a group of mares to carry a foal to term. This may be either as part of a pre-purchase assessment, for infertility work-ups, or as a routine on all open or maiden mares prior to the next breeding season. In addition to rectal palpation and ultrasound and endoscopic examination, a complete BSE should include ancillary laboratory work including uterine cultures, uterine cytology, endometrial biopsies and in some cases, hormonal profiles. The Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) has a group of professionals with ample expertise in laboratory assessment of mare infertility.
The endometrial biopsy is the most frequent laboratory method used to evaluate fertility in mares and to assess the mare’s prognosis for successfully carrying a foal to term. Endometrial biopsies are assessed and graded (grades I, IIa, IIb or III) based on the frequency, type and degree of inflammation, glandular nesting and periglandular fibrosis. Preferably, endometrial biopsies should be taken at the base of one of the uterine horns and be no less than 1 cm in length.
Grade I: Mares classified as grade I have a normal endometrium or mild, focal inflammation or fibrosis. Mares in this category have a > 80% chance of conceiving and carrying a foal to term.
Grade IIa: Mares classified as grade IIa have mild to moderate inflammation of the endometrium and/or multifocal areas of periglandular fibrosis with 1-3 layers of fibroblasts surrounding glands or an average of < 2 fibrotic glandular nests per 5 mm linear field. Mares in this category have a > 50 to 80% chance of conceiving and carrying a foal to term.
Grade IIb: Mares classified as grade IIb have multifocal areas of moderate inflammation and/or multifocal fibrosis with 4 or more layers of fibroblasts surrounding glands or an average of 2-4 fibrotic glandular nests per 5 mm linear field. These mares have a >10 to 50% probability of conceiving and carrying a foal to term.
Grade III: Mares classified as grade III have severe inflammation and/or diffuse fibrosis with an average of 5 or more fibrotic glandular nests per 5 mm linear field. These mares have a10% or less chance of conceiving and carrying a foal to term.
In the histopathology report, the type of cell in cases of endometritis is also recorded. The presence of neutrophils indicates active inflammation likely resulting from a bacterial infection. As the inflammation becomes chronic, neutrophils are replaced by lymphocytes. The presence of eosinophils in the endometrium suggests that a mare has pneumovagina, and/or is pooling urine into the vagina (sinking vagina) making her susceptible to uterine infections. These mares eventually develop glandular nesting and periglandular fibrosis. Pooling urine into the anterior vagina is a common cause of infertility in the mare. It is a problem of either poor conformation or poor muscle tone and incomplete involution of the tubular genital tract.
Endometrial biopsies also are assessed for the stage of the estrous cycle. Endometrial pathology is better evaluated during the physiological breeding season in mares that are cycling and are in estrus or diestrus. Endometrial atrophy occurs as a normal finding during winter anestrus and vernal transition. When this occurs during the physiologic breeding season, it is indicative of severe pathology and is most frequently seen in aged mares with reduced ovarian activity.
Submission of uterine swabs for culture, fresh samples for endometrial cytology along with fixed endometrial biopsies is highly desirable because it provides additional information for the interpretation of histologic changes.
To learn more about this case, contact Dr. de la Concha, veterinary pathologist at the College Station laboratory. For more information on TVMDL’s test catalog, visit tvmdl.tamu.edu.
Buczkowska J, Kozdrowski R, Nowak M, Sikora M: Relationship between uterine biopsy score, endometrial infection and inflammation in the mare. Tierarztl Prax Ausg G. 44:158-163, 2016.
de la Concha-Bermejillo, Kennedy PC. Prognostic value of endometrial biopsy in the mare: a retrospective analysis. J Am Vet Med Assoc. 181:680-681, 1982.
Kenney, R.M. Cyclic and pathologic changes of the mare endometrium as detected by biopsy, with a note on early embryonic death. J Am Vet Med Assoc. 172:241–262, 1978.