Case Study: Tumor-Laden Testicles Found in XY ‘Mare’
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.

Although the horse appeared to have female sex characteristics, including a vulva and a vagina and no penis, it had an enlarged clitoris, said Monteiro da Silva. Despite its bulk, the clitoris did not appear to cause discomfort or require surgical reconstruction, she added.

This isn’t the first time scientists have discovered “hermaphrodite” horses. In fact, Spanish researchers have been developing a genetic test for more easily detecting XY chromosomal abnormalities in horses. However, the Brazilian report is the first to reveal the development of a teratoma in the testicles of a female-appearing horse—which might be due to congenital malformation or possibly exposure to body temperature. Ovaries nestle within the mare’s body, staying at body temperature. But testicles reside inside a scrotum that hangs under the horse’s body, usually maintaining a temperature of a few degrees lower than body temperature. When testicles are exposed to the additional heat of being trapped in the body itself—whether it’s from cryptorchidism (a retained testicle) or XY chromosomal abnormalities—it’s possible that tumors can form, said Monteiro da Silva. However, the scientists can’t determine whether the horse in this case study developed tumors after birth, due to the body heat, or was born with them (as other scientists have found).

The tumors probably didn’t cause any behavior changes in the horse, she said. But the presence of testicular tissue (which was still partially healthy) might have.

“As far as we know, the benign teratoma itself did not cause significant changes at that point on the femalelike horse health or behavior, although the testosterone secreted by the testis did,” said Monteiro da Silva.

Managing the XY Mare

Blood tests three weeks after surgery showed a decline in blood levels of testosterone, indicating that the surgical intervention helped resolve this horse’s “masculinity,” Monteiro da Silva said.

Owners and breeders who find behavioral or reproductive disorders with their mares should consider a sex chromosome abnormality as part of the work-up, said Monteiro da Silva. “At any signs of contradiction between body (phenotypic) and behavioral characteristics, consult your vet,” she said.