Clinical findings, pathological changes and endocrinological secretory patterns in mares with ovarian tumours

The clinical, structural and functional characteristics of ovarian tumours were studied in 10 mares. Proliferation of the granulosa cells of large and small follicles and the presence of various numbers of Leydig-like cells in the theca interna were the predominant microscopic findings in 9 of the mares. All 9 of these animals had higher than normal (P less than 0.005) concentrations of testosterone in the peripheral plasma; 2 animals which exhibited male behaviour had testosterone concentrations greater than 100 pg/ml and the largest number of Leydig-like cells in the theca. Mares with lower testosterone concentrations (7/9) exhibited anoestrus or intermittent oestrous behaviour. The predominant microscopic changes in the other mare involved the proliferation of Sertoli-like cells; oestrogen concentrations were highest in this mare. Testosterone values were normal when no Leydig-like cells were observed in the theca. The unaffected ovary of all 10 mares was small and inactive. Eight mares resumed normal cyclic ovarian activity between 115 and 393 days after the removal of the affected ovary.