History

A 17-year-old 506-kg (1,113.2-lb) multiparous American Quarter Horse mare was referred to the Louisiana State University Veterinary Teaching Hospital Theriogenology Service because of a 1-year history of infertility characterized by nymphomania, anovulatory cycles, and an enlarged right ovary. The mare had reportedly produced 5 or 6 foals throughout her life, with the last foal born approximately 2 years before the present examination. The previous year, the mare was bred by natural mating, and pregnancy was diagnosed with transrectal ultrasonography and visualization of the fetal heartbeat. This pregnancy was lost before the fourth month of gestation, and irregular estrous cycles followed for the remainder of the breeding season. The irregular cycles were characterized by intermittent episodes of behavioral estrus (eg, frequent squatting stance, tail raising, and clitoral eversion combined with urination). Serial transrectal ultrasonographic reproductive examinations were performed by the referring veterinarian during that breeding season and the current breeding season; however, ovulation was not detected. The mare had consecutive anovulatory cycles, with multiple large follicles (diameter, ≤ 50 mm) on the right ovary. Rebreeding was not attempted.

On referral examination, the mare had vital signs within reference limits and was bright, alert, and responsive. The mare had a body condition score of 6/9, no observed abnormalities of the mammary glands, poor vulvar conformation, and sutures from a previous healed Caslick operation. These sutures were removed, and the mare was prepared for examination of the reproductive tract.

On transrectal palpation, the uterus and cervix had moderate tone. The left ovary was approximately 5 × 3 × 3 cm (length × width × height), contained multiple small follicles (diameter, 5 to 20 mm), had no corpus luteum, and had a readily palpable ovarian fossa. The right ovary was enlarged (approx 14 × 10 × 8 cm) and ventrally displaced, and the right ovarian fossa could not be felt. On transrectal ultrasonographic examination, the right ovary had multiple irregular cystic compartments containing anechoic to mildly echoic fluid (Figure 1). Luteal tissue was not seen. The 4 largest cystic structures had diameters of 41, 44, 55, and 56 mm. Small cystic structures (diameter, 3 to 15 mm) were evident near the poorly defined right ovarian fossa. Multiple endometrial cysts were present along the uterine horns and body. There was no endometrial edema or free fluid in the uterine lumen. Serum concentrations of testosterone (36.2 pg/mL; reference range, 20 to 45 pg/mL), inhibin B (18.1 pg/mL; reference range, 2 to 100 pg/mL), and anti–Müllerian hormone (AMH; 5.7 ng/mL; reference range, 0.1 to 6.9 ng/mL) were within reference limits for cycling, nonpregnant mares.

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Representative transrectal ultrasonographic images of the right ovary of a 17-year-old 506-kg (1,113.2-lb) multiparous American Quarter Horse mare referred to the Louisiana State University Veterinary Teaching Hospital Theriogenology Service because of a 1-year history of infertility, nymphomania, anovulatory cycles, and an enlarged right ovary. The ovary has multiple cystic compartments (asterisks) that range in diameter from 5 to 56 mm, with smaller cystic structures (diameter, 3 to 15 mm; arrows) near the poorly defined right ovarian fossa. The scale toward the right in each image is in centimeters.

Citation: Journal of the American Veterinary Medical Association 258, 7; 10.2460/javma.258.7.713

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Question

What are the main differential diagnoses for the mare's condition?

Answer

Epithelial inclusion cysts (EICs), persistent anovulatory follicles, granulosa cell tumor (GCT), granulosa-theca cell tumor (GTCT), and cystadenoma are the main differential diagnoses for the mare's condition.

Results

A presumptive diagnosis of EIC was made. Owing to the extensive cystic ovarian condition, a right unilateral ovariectomy was performed with a standing laparoscopic technique. To allow exteriorization of the enlarged ovary, one of the largest cystic structures had to be aspirated, yielding approximately 100 mL of dark-brown serous fluid (Supplementary Figure S1, available at: avmajournals.avma.org/doi/suppl/10.2460/javma.258.7.713). The cystic fluid was centrifuged, and cytologic examination of the sediment revealed numerous RBCs and microscopic debris. The removed right ovary (after fluid aspiration) was approximately 13 × 7 × 5 cm and had 2 focal areas of dark pigmentation on the tunica albuginea near the caudal ovarian pole (Figure 2). Most of the ovarian parenchyma was occupied by multiple cystic structures (diameter, approx 5 to 56 mm). The 4 largest cysts contained amber to dark-brown serous fluid, and the smaller cysts contained red-tinged serous fluid. Multiple small cysts overlaid the ovarian fossa.

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Photographs of the enlarged, cystic right ovary removed from the mare described in Figure 1. A—Dorsal aspect of the ovary. The ovarian parenchyma is occupied by multiple cystic structures (asterisks; diameter, approx 5 to 56 mm), and 2 focal areas of dark pigmentation (arrows) on the tunica albuginea are evident near the caudal ovarian pole. The ruler in the image is in centimeters. B—Ventral aspect of the ovary. Multiple smaller cysts (arrowheads) are near the ovarian fossa. C—One of the largest cystic structures has been incised, revealing its contents of amber to dark-brown serous fluid (asterisk). Smaller cysts (arrowheads) are near the ovarian fossa. D—A resected portion of the ovary showing small cystic structures (arrowheads) near the ovarian fossa.

Citation: Journal of the American Veterinary Medical Association 258, 7; 10.2460/javma.258.7.713

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The ovarian tissue was fixed with neutral-buffered 10% formalin, and tissue sections were routinely processed. Histologic examination revealed that the grossly apparent cystic structures in the ovarian parenchyma were each lined by a single layer of columnar to cuboidal epithelial cells, some of which were ciliated, and that the epithelial lining was surrounded by a dense layer of collagenous stroma (Supplementary Figure S2, available at: avmajournals.avma.org/doi/suppl/10.2460/javma.258.7.713). The cranial pole of the ovary contained multifocal to coalescing areas of acute necrosis, hemorrhage, and occasional hemosiderin-laden macrophages, often in perifollicular areas. Also in the cranial pole, there was a small cluster of acinar structures lined by cuboidal to columnar epithelium with scarce cilia. Most acinar lumina were filled by homogeneous eosinophilic secretion. Small follicles lined by well-differentiated granulosa cells were also seen. There were rare fluid-filled spaces lined by degenerated cells near the ovarian fossa, suggestive of recent follicular atresia or ovulation. These findings were consistent with multifocal EICs.

Discussion

Epithelial inclusion cysts, also known as fossa cysts or germinal inclusion cysts, are nonneoplastic, nonfollicular cystic structures that may occur beneath the lining of the ovarian fossa in mares.¹ The term germinal inclusion cysts is considered inadequate because the ovarian fossa in mares does not contain germinal epithelium.² Microscopic EICs are common histologic findings in postpubertal mares and generally do not cause clinical signs.¹ However, EIC accumulation and enlargement may occur in older mares and lead to complete obstruction of the ovarian fossa, impairing ovulation and oocyte transport.² In severe cases, occupation and destruction of the entire ovarian parenchyma may occur.² Similar structures are seen in ovaries of other species, including bitches and women, and are generally small and diffusely distributed throughout the subepithelial surface of the ovaries.² Because of the unique anatomic structure of the cortical and medullary ovarian tissue in mares, EICs are limited to the ovarian fossa in mares.²

The lining of the cranial aspect of the ovarian fossa is composed of pseudostratified columnar ciliated epithelium that likely originates from the paramesonephric ducts, given its similarity and continuity with the fimbriae of the uterine tube infundibulum.¹ A sharp transition to a simple cuboidal to nonciliated squamous epithelium is seen in the caudal aspect of the fossa, contiguous with the mesothelium that surrounds the remaining ovary.¹ Multiple epithelial-lined invaginations occur in the ovarian fossa, and EICs appear to originate from the separation of these invaginations from the tubular gland-like clefts, likely associated with successive disturbances of the fossa lining during repeated ovulations throughout the mare's reproductive life.^1,2 Similar to the fossa epithelium, EICs are lined by either squamous, simple cuboidal, or pseudostratified columnar and often ciliated epithelial cells.¹ Mitotic figures are rare. Although small EICs are supported by normal ovarian stroma, large EICs are often surrounded by dense collagenous tissue with fibers parallel to the epithelial lining.¹ Eosinophilic secretion is often seen in the lumina of EICs and is similar to secretions commonly found in the fossa and fimbriae.^1,2

In general, ultrasonographic or gross examination distinction between EICs and antral follicles may be difficult. Histologically, antral follicles are characterized by a cumulus oophorus, multiple layers of granulosa cells, a basement membrane, and theca interna and externa, whereas EICs are lined by simple, often ciliated epithelium that may be surrounded by dense collagenous tissue,² as observed in the mare of the present report. Atretic antral follicles, however, may pose a challenge for histologic identification because most cell layers degenerate and slough off into the follicular antrum, and a single epithelial layer remains lining the follicle wall.^1,2 Similarly, histologic examination revealed rare fluid-filled spaces lined by degenerated cells near the ovarian fossa in the ovary removed from the mare of the present report.

Epithelial inclusion cysts are differentiated from paraovarian cysts on the basis of anatomic location. The latter are epithelial-lined cystic structures commonly found in the tissues adjacent to the ovaries in mares. Most paraovarian cysts are remnants of the embryonic urogenital system, including fimbrial cysts (hydatid of Morgagni), epoophoron, paroophoron, and mesonephric duct cysts.² The fimbrial cysts are remnants of the paramesonephric ducts located in the cranial aspect of the infundibular fimbriae.² Remnants of the mesonephric ducts or tubules include the epoophoron and paroophoron located in the cranial and caudal aspects of the ovary, respectively, and the mesonephric duct cysts are located in the mesovarium.² Paraovarian cysts are generally not associated with impairment of ovulation or oocyte transport.

Differential diagnoses for ovarian enlargement in mares include persistent anovulatory follicles, ovarian hematomas, supplementary corpora lutea, and neoplasia. The diagnosis is generally based on history, clinical signs, transrectal palpation, ultrasonography, serum hormone concentration analyses, ovariectomy, and histologic examination.³ Ovarian biopsy, exploratory laparoscopy, and advanced imaging modalities have also been suggested for inconclusive cases.³ Ultrasonographically, a multicystic or honeycombed appearance is often seen in ovaries of mares during the vernal and autumnal transition or in mares with GCTs, GTCTs, or cystadenomas.^4,5

Large anovulatory follicles are a physiologic feature of the vernal and autumnal transition periods in mares. These anovulatory follicles normally regress as the mare progresses to regular cyclicity or anestrus.^2,4 However, preovulatory follicles may fail to ovulate during the physiologic breeding season, often leading to ovarian inactivity and prolonged interestrous intervals.⁴ Most persistent anovulatory follicles become hemorrhagic and luteinize; however, some will remain as transient follicular structures and regress.^2,4 Ultrasonographically, hemorrhagic or luteinized anovulatory follicles have echogenic particles in the follicular fluid and progressive accumulation of echogenic strands of fibrin.⁴ Luteinized anovulatory follicles are responsive to the administration of prostaglandin F_2α. For the mare of the present report, findings on ultrasonography were not suggestive of follicular hemorrhage or luteinization.

Granulosa cell tumors and GTCTs are the most common neoplasms involving ovaries in mares.^3,4,5,6 These gonadostromal tumors usually result in unilateral ovarian enlargement with multiple cystic structures and a poorly defined ovarian fossa.⁴ The cysts are lined by multiple layers of granulosa cells surrounded by ovarian stroma that may contain theca cells. Necrosis and hemorrhage are often seen, and the contralateral ovary is generally inactive because of inhibin-induced suppression of follicle-stimulating hormone. Affected mares may have behavioral abnormalities (eg, nymphomania and stallion-like behavior^4,6), and endocrine testing (eg, measurements of serum testosterone, inhibin, progesterone, and AMH concentrations) is valuable for diagnosis. Serum testosterone concentration is high in approximately 55% (41/75)⁶ of mares with GCTs or GTCTs and may reflect the theca cell component of the tumor.^5,6 Serum inhibin concentration is high in approximately 85% (64/75)⁶ of mares with GCTs or GTCTs, and because of the downregulation of follicle-stimulating hormone secretion, the contralateral ovary is often inactive. Further, serum progesterone concentration is invariably low (< 1 ng/mL).^4,6 Serum AMH concentration is typically high in mares with GCTs or GTCTs and may rise before concentrations of inhibin and testosterone rise.⁶

Ovarian cystadenomas are benign epithelial tumors that may originate from the epithelial lining of the ovary or ovarian fossa, subsurface epithelial structures, rete ovarii, or gonadal ridge.⁷ Because of the histologic similarity with EICs, it is believed that cystadenomas may also originate indirectly from EICs.⁵ Histologically, the cystic structures of cystadenomas are lined by simple cuboidal to low-columnar epithelial cells that have rare mitotic figures and may or not be ciliated.⁵ The contralateral ovary does not seem to be impaired, and cystadenomas may be compatible with adequate fertility and pregnancy.⁵ Further, cystadenomas may be endocrinologically inactive or associated with a high serum testosterone concentration.⁵ Ovarian cystadenocarcinomas and tumors originating from germ cells, including teratomas and dysgerminomas, are rarely seen in horses.⁴

In the mare of the present report, serum concentrations of testosterone, inhibin B, and AMH were within reference limits for cycling, nonpregnant mares. The nymphomania reported in this mare could have been associated with a low serum concentration of progesterone, with or without a high serum concentration of estradiol. Mares with GCT or GTCT usually have estradiol concentrations within reference limits for cycling mares, even when nymphomania is reported^4,5; therefore, measurement of estradiol concentration is generally not included in the diagnostic plan. For the mare of the present report, findings on transrectal palpation, ultrasonography, and endocrinologic assessments were suggestive of EICs. No abnormalities were identified in the mare's contralateral ovary, which contained follicles (diameter, 5 to 20 mm) and no corpus luteum, and the referring veterinarian had not documented any ovulation from the contralateral ovary during the past breeding season. Thus, unilateral ovariectomy was elected for a conclusive diagnosis and as an attempt to restore the mare's potential fertility.

Although EICs are often incidental histologic findings, accumulation and enlargement of EICs may result in sub- or infertility because of obstruction of the ovarian fossa or complete destruction of the ovarian parenchyma. Epithelial inclusion cysts should be suspected in mares with multicystic ovarian enlargement and should be differentiated from physiologic ovarian enlargement, paraovarian cysts, cystic neoplasms (eg, GCT, GTCT, and cystadenomas), and nonneoplastic ovarian disease (eg, hemorrhagic or luteinized anovulatory follicles). In mares with unilateral involvement and an apparently functional contralateral ovary, unilateral ovariectomy may be considered for conclusive diagnosis and treatment, as performed in this mare.

Outcome

The mare recovered without complications from surgery and was discharged 2 days postoperatively, with instructions to the owner for stall rest and surgical site care. In addition, the mare was to be reevaluated by the referring veterinarian within 2 weeks. Approximately 14 days after surgery, the mare was determined to be in estrus and was mated. The referring veterinarian diagnosed pregnancy and monitored the mare's gestation until a fetal heartbeat was observed with transrectal ultrasonography. However, on a recheck examination at approximately 8 months of gestation, the mare was no longer pregnant. No sign of abortion had been seen by the owner.