Unilateral Hormonally Active Testicular Sertoli Cell Tumour and Feminization Syndrome in a Bernese Mountain Dog With Normally Descended Testes

To describe the clinical presentation, diagnostic workup and successful outcome of a hormonally active testicular Sertoli cell tumour (SCT) in a non-cryptorchid Bernese Mountain Dog. A 10-year-old intact male dog was evaluated with a history of progressive alopecia, gynaecomastia and haematuria. Diagnostic investigations included complete blood count, serum biochemistry, oestradiol assay, abdominal and testicular ultrasonography, preputial cytology and histopathological examination following bilateral orchiectomy. Clinical signs included bilaterally symmetrical alopecia, hyperpigmentation and a pendulous prepuce. Laboratory findings revealed mild non-regenerative anaemia and severe hyperestrogenism (oestradiol: 119 pg/mL). Preputial cytology showed a predominance of superficial epithelial cells (> 80%), supporting the state of hyperestrogenism. Ultrasonography identified a large, vascularized left testicular mass and cystic prostatic changes. Histopathology confirmed an SCT. Following surgery, clinical signs and laboratory abnormalities resolved completely within 9 months. This case emphasizes that SCTs should be a differential diagnosis for feminization syndrome in older dogs even without cryptorchidism. It also highlights preputial cytology as a rapid and effective minimally invasive tool for the preliminary identification of hyperestrogenism.