Horse-Breeding Hazards: The Granulosa Cell Tumor

Identify the symptoms and treatment for the most common type of ovarian tumor in mares.

Mares can develop several different types of tumors of the ovary, but by far the most common ovarian tumor is the granulosa cell tumor. These tumors almost always involve one ovary, are slow growing and do not spread to distant sites in the body. If a mare exhibits behavioral changes, such as aggressive or stallion-like behavior, or if she exhibits prolonged periods of heat or estrus, a granulosa cell tumor might be present.

A granulosa cell tumor can usually be diagnosed by an ultrasound examination. An affected ovary will be enlarged and will often have a classic, honeycombed internal structure. However, the tumor might also look like a solid mass or a single large cyst. The opposite ovary will typically be small and inactive. The presence of one enlarged ovary and one small inactive ovary along with classic behavioral changes is usually sufficient for a diagnosis. A blood sample can be submitted to a diagnostic laboratory for evaluation of the hormones inhibin, testosterone and progesterone.

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Hormone Levels Inhibin levels are higher than normal in about 90 percent of mares with known granulosa cell tumors. Testosterone levels are elevated in only 50 to 60 percent of affected mares and usually only in mares exhibiting stallion-like behavior. Testosterone will only be elevated if a large number of a second cell type (theca cells) is present in the tumor, making it a granulosa-theca cell tumor. Progesterone levels are not normally elevated in mares with granulosa cell tumors, because the tumor does not produce progesterone, and mares with granulosa cell tumors do not usually continue to cycle. If blood progesterone levels are high, it suggests the ovarian enlargement is not caused by a tumor. Mares with granulosa cell tumors do not commonly continue to develop follicles and ovulate from either the affected ovary or the opposite ovary.

Treatment for Tumors in the Mare

Granulosa cell tumors are usually surgically removed because the tumor can cause abnormal behaviors and will certainly affect follicular development on the opposite ovary. The entire ovary containing the tumor can be removed by a ventral midline surgery, flank surgery or, with small tumors, through a vaginal incision. At some veterinary facilities, the tumor can be removed by use of a laparoscope.

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It might take six to eight months or more for significant follicular development and ovulation to occur on the opposite ovary following removal of the tumor. The good news is that after surgery, the vast majority of mares will resume normal reproductive function, cycle every 21 days, become pregnant and carry foals to term with just one ovary.

Stallion-Like Behavior

Stallion-like behavior can also be observed occasionally in normal mares in the middle of pregnancy due to production of testosterone from the fetus. Prolonged or irregular estrus periods are routinely associated with the spring transition period in normal mares. Not every behavioral change is indicative of the presence of an ovarian tumor. If an unusual behavior persists or is unexplained, consult your equine veterinarian.