The role of a particular type of ovarian tumor, the Granulosa Cell Tumor (GCT), was reviewed as a cause of unwanted or unusual behavior in the mare. GCTs are the most common ovarian tumor in the horse and can result in mares that have prolonged anestrus (no normal heat cycles), prolonged or intermittent estrus (time in which the mare is in heat), or aggressive stallion-like behavior. All of these abnormal behaviors are a result of the type of steroid hormone that is being secreted by the particular tumor cell that is present in the ovary.
Because there are also other conditions that can cause aggressive behavior (the authors provide a table that outlines other causes such as pain, underlying disease, and management factors), it becomes important to examine a “moody mare” carefully to arrive at the correct diagnosis. A thorough physical examination will direct the veterinarian as to what other diagnostics may be needed (rectal examination, CBC and blood chemistry, blood hormone levels, radiographs, ultrasound, etc.).
With respect to GCTs, an ultrasound of the ovary may reveal enlargement of one or both ovaries, and blood hormone profiles may show elevated blood testosterone. More recent work has suggested another hormone, anti-Mullerian hormone, may be more a more sensitive indicator for the presence of a GCT in the mare.
Removing the affected ovary or ovaries (ovariectomy) is the preferred method of treatment for mares with GCT, as this effectively removes the source of the abnormal hormone responsible for the unwanted behavior.
Scientific Article: Nout-Lomas Y.S. and Beacom C.L. Granulosa cell tumours: Examining the’moody’ mare. EVE Oct. 2015. 515-518.