Selected Topics in Reproductive Pathology: Mare I

I. The Enlarged Ovary

1. Introduction

A mare is considered “normal” if she has regular estrous cycles, conceives, carries the foal to term, undergoes parturition, raises the foal, and continues to have regular estrous cycles. When any of these processes are associated with pathologic conditions, infertility may ensue. This article addresses common pathologic conditions that may disrupt these processes and require veterinary intervention.

2. The Enlarged Ovary

Normal ovarian function is essential for fertility and reproductive efficiency. Any ovarian disorder or pathology that disrupts the estrous cycle causes loss of time and the potential for pregnancy. Ovarian enlargement has been identified in barren, pregnant, and maiden mares during routine reproductive exams as well as in mares that present for anestrus, behavioral abnormalities, and colic.

Ovarian enlargement is usually a unilateral condition that can have numerous etiologies that are both non-neoplastic (hematoma, hemorrhagic anovulatory follicle, and abscess^1–3 ) or neoplastic (granulosa theca cell tumor, dysgerminoma, teratoma, teratocarcinoma, cystadenoma, and cystadenocarcinoma^4–9 ). Clinical signs, ultrasonographic findings, hormonal assays, laparoscopy, biopsy, and exploratory surgery can aid in differentiating these disorders.³ 

3. Ovarian Hematoma

An ovarian hematoma results from excessive hemorrhaging into the follicular lumen after ovulation.¹⁰ As one of the common causes for an enlarged unilateral ovary, it is imperative to differentiate it from neoplastic disease (i.e., granulosa cell tumors).

Ovarian hematomas can be found in maiden, barren, and foaling mares, although in the authors’ experience foal heat ovulations tend to be the most prone to formation. Clinical presentation can include the presence of an enlarged ovary during a routine reproductive examination, abnormally aggressive behavior, or colic as a result of pain from the weight of the ovary.¹

A diagnosis can be made by utilizing different techniques. Ultrasonographic evaluation of the enlarged ovary reveals hyper- or hypoechogenic homogeneous blood within the lumen of an extremely large follicle or a “spider web” appearance that results from a fibrin strand formation that spans the diameter of the structure. Theses hematomas can vary in size, ranging from 50 mm to the size of a melon.

The affected ovary has an ovulation fossa that can clearly be identified upon transrectal palpation. The contralateral ovary is of normal size and function, with mares continuing to cycle normally. Endocrine profiles are normal.¹

Signs of colic or aggressive behavior can be explained by the tension placed on the proper and broad ligaments from the increased size of the ovary.¹ In most instances, the hematoma will regress in size and eventually return to normal functionality as long as the ovarian parenchyma is not compromised because of the hemorrhage or increased pressure.³ The area of hemorrhage is limited to the ovarian stroma, and bleeding from the ovary is rare.

Treatment aims at suppressing estrus to allow time for the resorption and regression of the hematoma.¹ This can be accomplished by either orally administering altrenogest (0.044 mg/kg body weight) for 15 days (3 in 1) or injecting 150 mg of progesterone and 10 mg of 17ß-estradiol once daily for 10 days and administering prostaglandin F₂α on the last day (3 in 1). Alternatively, a long-acting progesterone and estradiol can be used. The hematoma should spontaneously regress over time, during which a normal interovulatory period should be identified.¹¹

If the hematoma becomes excessively large, it can destroy the remaining normal parenchyma and leave the ovary nonfunctional, in which case removing the hematoma may become necessary.³ A standing-flank laparotomy approach can lead to identifying the ovary, draining the hematoma, and removing and visually ligating the ovarian vasculature.¹²

4. Hemorrhagic Anovulatory Follicles

Hemorrhage into the dominant follicle with ovulation failure occurs when the follicle does not rupture or collapse (Fig. 1).¹³ After acute hemorrhage, the contents of the follicle are organized; on most occasions, this is accompanied by luteinization of the follicular wall.¹⁴
 

Hemorrhagic anovulatory follicles (HAFs) have also been called autumn follicles,¹⁵ persistent anovulatory follicles,¹⁶ and hemorrhagic follicles.²

The presence of the hematoma can make the ovary sensitive upon palpation, functionally compromised, and enlarged. Although the overall HAF incidence is 5% to 8% of the mare population, it can be a frustrating disorder because normal cyclicity is affected.^16,17 It has been suggested that the induction of estrus with prostaglandin substantially in creases the likelihood of developing HAFs.^14,18 With prostaglandin-induced luteolysis, progesterone drops rapidly, allowing for the removal of the negative feedback of progesterone on the luteinizing hormone (LH) that causes an early rise during the beginning of follicular deviation.¹⁸ This LH surgecould interfere with the intrafollicular metabolism of prostanoids and the proteolytic enzymes that are necessary for ovulation and follicular collapse.¹⁹ An increase in HAF incidence has been reported after the use of human chorionic gonadotropin to induce ovulation.²⁰ However, the mechanism responsible for this increase is not clear.¹⁴

Diagnosis can be difficult because HAFs can appear to be similar to a normal evacuated follicle and corpus hemorrhagicum (CH) (or hematoma) during an ultrasonographic transrectal examination.¹³ A differentiating feature that may help is the thickness of the luteal border, with an HAF <3 mm in contrast to >5 mm in CHs.¹³ Vascular perfusion has also been identified as being different in HAFs versus CHs. Hormonal assays are usually normal palpation of the ovulation fossa is possible with the contralateral ovary of normal size and consistency.

Treatment can be difficult. If the ovarian structure is an HAF and has luteinized tissue with an associated elevation in progesterone, administering prostaglandin may illicit regression; otherwise, time will be needed for the structure to regress naturally with the development of a new follicular wave. The use of progesterone and estradiol with ovarian hematomas as described previously can also aid in HAF regression, with a return to cyclicity.

5. Ovarian Abscess

Primary ovarian abscessation is rare.^21,22 It has been described as being associated with the aspiration of follicular “cysts.”³ Diagnosis is based on history and the ultrasonographic appearance of homogeneous inspissated tissue mass.

Ovariectomy via flank laparotomy is the treatment of choice because it avoids possible laceration or contamination of the abdominal cavity. If the affected area of the ovary is small, treatment with the appropriate antibiotics may be possible. The difficulty lies in identifying the bacteria without invading the ovary. [...]