Hormone-producing tumors in mares can cause changes in behavior that are varied and complicated. The quiet old mare suddenly becomes unpredictable and bad tempered, and may even show stallion-like behavior; another may stay in heat all the time.
These behaviors could signify a benign Granulosa Theca Cell Tumor (GTC), an abnormal growth of cells within the ovary. Most Granulosa Cell Tumors cause the ovary to enlarge, which is easily discovered by rectal palpation. Once the presence of an enlarged ovary has been established, a rectal ultrasound may be used to examine and measure the ovaries in more detail. Most GTC tumors will have either a multi-cystic look or a honeycomb appearance.
There is no definitive test for GTC tumors, but a blood sample with elevated hormone levels is a strong indication that one is present. Mares displaying stallion-like behavior usually have high levels of testosterone. In other cases, samples
may show high levels of progesterone. Inhibin is the latest hormone on the list.
When the ovary is relatively small, it may be removed vaginally in an operation called a colpotomy. When the ovary is larger, it may need to be removed through a flank incision, either with the sedated mare standing under local anesthetic, or lying on her side under a general anesthetic. Really monstrous tumors are removed through a mid-line incision under general anesthesia.
On occasion, the ovary of a mare can become abnormally large, while remaining physiologically normal. Such "autumn ovaries," as they have been called, are found towards the end of the breeding season.
Taken From Horse to Horse