Teratoma Associated With Testicular Tissue in a Female-Like Horse With 64,XY (SRY-Positive) Disorder of Sex Development.

The study focuses on a four-year-old female-like horse who presented unusual aggressive behavior and absence of typical female horse reproductive behavior. After comprehensive physical, gynecological and genetic examinations, the horse was diagnosed with a disorder of sex development. It was designated as an XY female due to the presence of a Y chromosome and a specific gene called SRY, typically associated with male sexual development. The horse also developed a benign teratoma (a type of tumor) linked with male reproductive cells, which led to elevated testosterone levels and resulted in the horse’s atypical behavior.

Case Establishment and Clinical Evaluation

• The horse, which resembled a female physically, exhibited masculinized-aggressive behavior unusual for its gender and showed no signs of estrous, a regular part of a female horse’s reproductive cycle.
• Visual and gynecological examinations revealed normal female characteristics such as well-formed mammary glands, vulva, vagina, and cervix. The only other irregularity found during physical exam–besides the masculinized behavior–was the underdeveloped uterus and a discrepancy in the firmness and size of the horse’s gonads.

Diagnostic Procedures and Findings

• Due to the irregularities observed in the gonads combined with the suspicion of excessive testosterone production, the vet team decided to perform a bilateral gonadectomy, a surgery to remove both gonads.
• Testosterone levels were measured before the operation and, as suspected, they were higher than a typical female’s levels. Genetic investigation confirmed the presence of male sex chromosomes (XY) and the SRY gene, which initiates the pathway towards forming male genitalia. Therefore, despite the horse’s female appearance, it was genotypically male.
• The excised gonads were subjected to histopathological examination. This revealed the presence of a benign adult teratoma consisting of male-specific tissues like seminiferous tubules and Leydig cells.

Final Conclusions

• The horse’s aggressive behavior and reported lack of estrous signs were likely influenced by the excessive testosterone levels produced by the teratoma-associated testicular tissues.
• This case presents a rare occurrence of an adult female-like horse with a disorder of sex development. While the horse appeared female, it was genotypically male (64,XY) and SRY-positive, and had a teratoma associated with male reproductive tissues.
• After the removal of the gonads and the associated teratoma, the horse’s testosterone levels fell to within normal female ranges.