Has you mare changed her behavior, is she acting aggressive, stallion like or exhibiting markedly prolonged periods of heat or estrus. Exceptions to changes in behavior are pregnant mares and transitional mares. Pregnant mares can exhibit stallion like behavior due to the production of testosterone from the fetus. Transitional mares exhibit prolonged or irregular estrus periods during the spring. Consequently not every behavioral change is necessarily indicative of a granulosa cell tumor. Granulosa cell tumors are usually slow growing and do not spread to distant sites in the horse’s body.
The presence of a granulosa cell tumor can usually be diagnosed via a reproductive ultrasound exam. The affected ovary will be enlarged, have a honeycomb appearance, however sometimes the ovary can be a solid mass or a single large cyst. The opposite ovary is usually very small and inactive. However, a recent report by Sherlock, Ellis, Bergen, Withers, Fews, and Mair (2016) revealed that 25% of horses in their study had granulosa cell tumors in both ovaries. Additional proof or diagnostics involves a blood sample for hormonal analysis. Classically affected mares have low serum progesterone and high testosterone (50 – 60% of mares). If the mare has a high serum progesterone concentration, it suggests that the ovarian enlargement is caused by something other than a granulosa cell tumor. Inhibin (90% of mares) and anti-Mullerian hormone are the diagnostic hormones examined in the blood. Authors Sherlock, Ellis et al. (2016) determined that only 60% of mares with histologically confirmed granulosa cell tumors demonstrated elevations in serum inhibin concentrations.
Mares with granulosa cell do not commonly continue to develop follicles and ovulate from either the affected ovary or the opposite ovary. The opposite or unaffected ovary becomes inactive due to the production of hormones from the affected ovary.
Treatment involves surgical removal of the ovary. At Arizona Equine Medical and Surgical Centre, tumors are routinely removed via a “colic” type incision. The mare undergoes general anesthesia, and her abdomen is clipped and prepared with a sterile scrub. The surgeon makes a colic type incision and the ovary is readily removed via this incision. Postoperative care is similar to a horse that has undergone colic surgery. The abdominal incision takes several months to heal and requires stall rest for the mare. The “normal” or unaffected ovary may take 6 to 8 months or more to develop follicles and ovulate. Following removal of the tumor, the majority of the mares will resume normal reproductive function, cycle every 21 days, become pregnant and carry a foal to term with only one ovary.
C.E. Sherlock, K. Lott-Ellis, A. Bergren, J.M. Withers, D. Fews, and T.S. Mair. Granuolsa cell tumours in the mare: A review of 52 cases. Equine Veterinary Education28(2): p 75-81.
Images used under creative commons license – commercial use (4/29/2016) Ed Uthman (Flickr)