OVARIAN TUMORS Patrick M. McCue DVM, PhD, Diplomate ...

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OVARIAN TUMORS Patrick M. McCue DVM, PhD, Diplomate ...

OVARIAN TUMORSPatrick M. McCueDVM, PhD, Diplomate American College of TheriogenologistsMares can develop one of several differenttypes of tumors of the ovary, but by far themost common ovarian tumor is called thegranulosa cell tumor. These tumors almostalways involve one ovary, are slow growing,and do not spread to distant sites in the body.Presence of a granulosa cell tumor may besuspected if a mare exhibits behavioralchanges, such as aggressive or stallion-likebehavior or if a mare exhibits markedlyprolonged periods of heat or estrus. As areminder, stallion-like behavior can also beobserved occasionally in normal mares inthe middle of pregnancy due to productionof testosterone from the fetus. Prolonged orirregular estrus periods are routinelyassociated with the spring transition periodin normal mares. Consequently, not everybehavioral change is indicative of thepresence of an ovarian tumor. If an unusualbehavior persists or is unexplained, pleaseconsult with your equine veterinarian.The presence of a granulosa cell tumor canusually be diagnosed by an ultrasoundexamination. An affected ovary will beenlarged and will often have a classic‘honeycombed’ internal structure. However,the tumor may also present as a solid massor as a single large cyst. The opposite ovarywill typically be small and inactive. Thepresence of one enlarged ovary and onesmall inactive ovary in the presence ofclassic behavioral changes is usuallysufficient for a diagnosis. If additional‘proof’ is needed, a blood sample can becollected for hormone analysis. A samplecan be submitted to a diagnostic laboratoryfor assessment of the hormones inhibin,testosterone and progesterone. In the UnitedStates the only laboratory currentlymeasuring inhibin levels in the blood ofhorses is the University of California, Davis.Inhibin levels are higher than normal inapproximately 90% of mares with knowngranulosa cell tumors. Testosterone levelsare elevated in only 50 to 60% of affectedmares and usually only in mares exhibitingstallion-like behavior. Testosterone willonly be elevated if a large number of asecond cell type (theca cells) is present inthe tumor, making it a granulosa-theca celltumor. Progesterone levels are not normallyelevated in mares with granulosa cell tumors,since the tumor does not produceprogesterone and mares with granulosa celltumors do not usually continue to cycle. Ifblood progesterone levels are high, itsuggests that the ovarian enlargement iscaused by something other than a tumor.Mares with granulosa cell tumors do notcommonly continue to develop follicles andovulate from either the affected ovary or theopposite ovary. In fact, the opposite ovarybecomes inactive due to the production ofhormones from the tumor.1


Granulosa cell tumors are usually surgicallyremoved since the tumor may be a source ofcolic, may cause behavioral abnormalitiesand will certainly affect folliculardevelopment on the opposite ovary. Theentire ovary containing the tumor may beremoved by a ventral midline surgery, flanksurgery or, with small tumors, through avaginal incision. At some veterinaryfacilities, the tumor may be removed by useof a laparoscope.It may take 6 to 8 months or more forsignificant follicular development andovulation to occur on the opposite ovaryfollowing removal of the tumor. The goodnews is that following surgery the vastmajority of mares will resume normalreproductive function, cycle every 21 days,become pregnant and carry foals to termwith just one ovary.Photo of a mare exhibiting stallion-likebehaviorPhoto of an ovarian tumor2

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