Case Study: Tumor-Laden Testicles Found in XY ‘Mare’
“Mares” showing stallionlike behavior could actually be chromosomally male. | Photo: iStock
Although the owners thought they had a mare, veterinarians recently revealed that a Quarter Horse with masculine behavior had testicles in the place of the ovaries, according to a case report. The testicles had partially developed into benign tumors, requiring surgical removal.

“On palpation and ultrasound examination, we could not tell if the gonads were ovaries or testicles, or if there was a tumor present,” said Elisa Sant’Anna Monteiro da Silva, DVM, PhD, of the School of Veterinary Medicine in the Federal University of Uberlandia in Minas Gerais, Brazil.

Because the left gonad was softer than expected when they palpated it during a transrectal exam, however, the veterinarians “suspected a testicle,” which isn’t as firm as an ovary. Because the “mare” had never shown estrus behavior and, on the contrary, tended to display stallion behavior toward other mares, the clinicians decided to surgically remove the gonads.

Uneven Gonads Filled With Tumor Tissue

After surgery, Monteiro da Silva said they found two unequal gonads resembling neither ovaries nor normal testicles. The left gonad had testicular tissue embedded with a small benign tumor, known as a teratoma. And the right gonad, which was larger and harder, appeared to be made entirely of tumor (teratoma) tissue. DNA analysis of the horse’s blood revealed that it had XY chromosomes, consistent with being genetically a male

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