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Granulosa-theca cell tumors can affect any mare but are most common in mares from five to 10 years old. Mares of all breeds may be affected. These tumors usually form on only one ovary, but hormones produced by the tumor inhibit normal activity of both ovaries.

Tumor-related hormonal disturbances keep some mares from cycling normally, while other mares show persistent signs of estrus. Because granulosa-theca cell tumors produce testosterone, a male hormone, affected mares sometimes develop cresty necks and stallion-like behavior.

Broodmare managers may notice that particular mares fail to come into heat, suggesting the presence of a granulosa-theca cell tumor. A veterinarian can usually diagnose the tumor by using rectal palpation and can advise treatment. Small tumors can often be surgically removed with good results, while removal of very large tumors is more dangerous due to the hormonal changes and their influence on metabolic function.

If mares are racing or performing in another discipline, lack of regular estrous cycles may not be noticed and the horses may not show obvious signs that the tumors are present. A granulosa-theca cell tumor may have been to blame when We Are, a Thoroughbred filly racing in France, was disqualified after winning a Group I race. Post-race laboratory tests showed the horse had a high level of testosterone. Her owner and trainer knew the filly had not received supplementary testosterone. After tests confirmed the hormone level was due to the tumor, racing officials did not impose sanctions, but simply disqualified the horse from her win in the Pour Moi Coolamore Prix Saint-Alary and also a race she won in early May.

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