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Ovariectomy is indicated for the removal of a diseased ovary. It is most often performed in mares that have a suspected or confirmed tumor of the ovary and the most common type is called a granulosa theca cell tumor. This can cause the ovary to achieve a variable size (usually 3 to 4 times normal) and produce hormones that can convert the mare outwardly to look and behave like a stallion. Although these tumors are typically benign, they can cause temporary or permanent reduction in fertility because the tumor can suppress the function of the other ovary. Clinical signs can vary widely and some mares might appear unchanged and even get pregnant. Surgical removal should be done early in the disease process before the tumor achieves gigantic proportions and before the suppressive effect on the opposite ovary becomes permanent. Diagnosis can be made by palpation per rectum, ultrasound examination, and hormone assays.

The Surgical Process

Standing laparoscopic ovariectomy

Removal of the ovary can be performed by laparoscopy with the mare standing or anesthetized, or in the case of large tumors, removal can be performed as an open procedure through a ventral midline incision with the mare on her back under general anesthesia. There are other options for approaching the diseased ovary, depending on the size of the ovary, the preference of the surgeon, and the cost. Laparoscopy is used in most cases and belongs to a branch of surgery called minimally invasive surgery. The advantage that it offers is completion of a procedure through very small incisions, using specialized instrumentation, that leaves the patient at low risk for post-operative pain and complications. Consequently, the surgery can be done during a very short hospitalization and with rapid return to normal use.

Many mares with normal ovaries are presented for bilateral ovariectomy because of the perception of excessive and unwanted female behavior. This is similar to castrating an intact male horse because of the need to eliminate undesirable behavior. Unfortunately, the procedure does not always achieve the desired purpose in mares, and owners need to consider this. The reason for this is that some mares will continue to produce estrogen from another source, possibly the adrenal glands, and the problem will persist after ovariectomy. In fact, “jump” or mount mares are ovariectomized so that the continued production of estrogen will make them appear to be permanently in estrus. Then they will stand for the stallion and the stallion can be collected for such purposes as artificial insemination or breeding soundness examination.