Paramesonephric (Müllerian) tumor in a rabbit initially misidentified as male: clinical, histologic, and molecular findings.

A 5-y-old Holland Lop rabbit, initially presumed male based on a small penis-like structure and pet store information, became anorexic and lethargic. Imaging revealed a large caudal abdominal mass distinct from the urinary bladder and rectum. Exploratory laparotomy revealed the mass was associated with uterine-like structures, which were excised. Histopathology and immunohistochemistry were consistent with a malignant paramesonephric (Müllerian) tumor. PCR analysis of Y-linked genes (SRY, ZFY, RBMY) found none of these genetic markers, suggesting the rabbit was genetically female. No gonadal structures were identified on imaging or at surgery. Despite initial postoperative improvement, the rabbit died 2 wk later as a result of severe rhinitis. This rare case highlights the challenges of identifying reproductive tumors in rabbits, especially when sex is misidentified, and emphasizes the importance of accurate clinical and genetic sex determination for early detection, intervention, and preventive care.