Now is a good time to determine whether an ovarian problem might be the reason a broodmare is barren

by Brent Kelley, D.V.M.

NOW THAT the breeding season is all but over, the time is perfect to address reproductive conditions of barren mares. Some mares might have problems with their ovaries that could require time to fully resolve, so waiting until next breeding season might be too late to get them back in foal.

Several reproductive problems occur with the mare's ovaries. Some are of no consequence, but others affect fertility and must be addressed.

A mare has two ovaries, located at the tips of the uterine horns. A normal ovary in the breeding season is kidney shaped and about two inches long. In the winter, when they are not active, each ovary is about one inch long.

Enlarged ovaries

The most common causes of enlarged ovaries in the mare are tumors, hematoma, pregnancy, and anovulatory follicles. The three main categories of ovarian tumors that occur in the mare are sex cord-stromal tumors, epithelial tumors, and germ cell tumors.

Sex cord-stromal tumor: The most common sex cord-stromal tumor in the mare is the granulosa cell tumor, which is usually benign, although malignant granulosa cell tumors have been reported. The tumor produces hormones; because of this, the opposite ovary does not produce hormones and it is usually small and inactive.

Mares with granulosa cell tumors show abnormal behavior. There may be prolonged anestrus (the time between cycles), persistent or intermittent estrus (nymphomania), or stallion-like behavior. Mares with stallion-like behavior have elevated testosterone levels and may develop a crested neck, heavy musculature, and an enlarged clitoris.

Diagnosis when the mare is usually not pregnant is based on clinical signs, rectal palpation, ultrasound examination, and serum hormone concentrations. Treatment consists of removing the affected ovary, which is a simple and quick procedure. The opposite ovary will return to normal in about six or seven months, although occasionally it may take up to 11Ú2 years. A mare can readily conceive with one ovary.

Epithelial tumors: Cystadenomas are rare and appear on the surface epithelium (covering of the area) of the ovulation fossa (the site where ovulation occurs). They are benign and are not hormonally active, although elevated testosterone levels have been reported in a few cases. The opposite ovary is normal in both size and activity.

The mare with an epithelial tumor exhibits no abnormal behavior, and diagnosis is based on palpation and ultrasound examination. The affected ovary is large and contains many cysts. Even though these tumors are benign, it is recommended that the affected ovary be removed.

Germ cell tumors: There are two types of germ cell tumors: dysgerminomas and teratomas. They are unilateral and hormonally inactive, and the opposite ovary functions normally.

Dysgerminomas are potentially malignant and frequently metastasize, or travel, to the abdominal and thoracic cavities. Teratomas are benign and are usually diagnosed on routine breeding soundness examinations. Removal of the affected ovary is recommended with germ cell tumors. The prognosis is good for teratomas, but poor for dysgerminomas. Chest X-rays and examination of abdominal fluid may be helpful in determining if metastasis has occurred.

Hematomas: This is a common cause of enlarged ovaries in the mare. They are believed to originate from a normal ovulation. There are no outward clinical signs, and an affected mare continues to cycle normally. The hematoma may reach 35 or 40 centimeters in diameter and is found on rectal palpation. On ultrasound examination, the hematoma resembles a corpus hemorrhagicum (a blood clot at the site of ovulation). A hormonal assay, which measures the circulating hormones in the blood, will differentiate a hematoma from a granulosa cell tumor.

Hematomas will usually regress over a period of a few weeks, although occasionally one will last for several months. In this case, normal ovarian tissue may be destroyed and the ovary becomes nonfunctional.

Pregnancy: Beginning about the 40th day of pregnancy, secondary corpora lutea (hormone-producing structures) are formed as equine chorionic gonadatropin is produced. They are usually bilateral. They regress at about six months of pregnancy. When there are multiple corpora lutea, the ovaries are enlarged. The mare may show mild stallion-like behavior.

Anovulatory follicles: Anovulatory follicles are those that do not rupture and release their eggs. This condition might occur in the transitional period leading into the breeding season. Hemorrhagic (bloody) follicles also can occur, but these are usually in the fall. Diagnosis is based on palpation and ultrasound examination.

Both anovulatory and hemorrhagic follicles regress over a period of time and no treatment is necessary.

Small ovaries

Bilaterally small ovaries may be caused by severe malnutrition, hypothalamopituitary dysfunction, immaturity of the mare, seasonal anestrus, old age, anabolic steroid use, and gonadal dysgenesis (when the gonads do not develop properly).

Severe malnutrition: Poor nutrition leading to severe weight loss can affect the emission of gonadotropin releasing hormone (GnRH) from the hypothalamus. The ovaries are small, hard, and inactive. Addressing and correcting the problems of poor nutrition and weight loss will bring the ovaries back to life.

Hypothalamopituitary dysfunction: The hypothalamus and pituitary glands do not function properly. A portion of the pituitary gland called the pars intermedia may be affected with neoplasia (cancer) or adenomatous hypoplasia (smaller in size), resulting in equine Cushing's disease. Signs include a long, curly coat, pot belly, increased urination and water intake, and small, inactive ovaries. As Cushing's is usually a condition of old age, the prognosis for a return to reproductive production is poor.

Immaturity: A filly reaches puberty on the average at 18 months of age, with a range of ten months to 24 months. Before that, the ovaries are small and nonfunctional. Allowing the filly to mature is the only treatment. Seasonal anestrus: About 75% of all mares are seasonally anestrus, which means they stop cycling in the late fall and do not cycle for about six months. During this time, the ovaries are small and inactive. No treatment is necessary, but placing mares under artificial lighting will cause their ovaries to start up again.

Old age: This is an inconsistent cause that does not affect all mares. Older mares often do not begin cycling in the spring until a few weeks later than younger mares, so an artificial lighting program can be helpful.

Anabolic steroid use: With repeated administration of anabolic steroids while in training, ovarian activity ceases, the ovaries become small and firm, and mares become studdish. This situation occurs for variable amounts of time after the use of the anabolic steroids has stopped, but eventually the ovaries return to normal function. Use of anabolic steroids should be avoided in fillies and mares intended for breeding.

Gonadal dysgenesis: Chromosomal abnormalities are responsible for this. The condition might be the reason a mare has never gotten in foal, does not have normal estrous cycles, and has small, inactive ovaries, regardless of the time of the year. There is no treatment.

Other ovarian problems

Persistent luteal activity: A corpus luteum that does not resolve at 14 to 16 days post-ovulation might have several causes: ovulation late in diestrus (the time between two cycles), early embryonic loss, the administration of nonsteroidal anti-inflammatory drugs that inhibit prostaglandin release from the endometrium, inadequate release of prostaglandin from the endometrium, and the inability of the endometrium to release prostaglandin.

An affected mare shows no signs of estrus for up to three months. Luteal tissue (corpus luteum) is found on the ovaries by palpation or ultrasound examination and serum progesterone levels are elevated. The administration of exogenous prostaglandin is usually an effective treatment.

Parovarian cysts: These are located around the ovary and may be mistaken for follicles on an ultrasound examination. They have no effect on the estrous cycle or fertility.

Epithelial inclusion cysts: These may interfere with ovulation and may cause the ovary to be nonfunctional. If fertility is affected, the ovary should be removed.