Testicular seminoma: clinical, imaging, and histologic features in nine horses

BACKGROUND: Testicular seminomas in horses are infrequently reported in the veterinary literature and long-term survival and metastatic potential are not well documented. HYPOTHESIS/OBJECTIVES: Describe the clinical, imaging, and histologic features in 9 horses with testicular seminoma. ANIMALS: Medical records of 9 horses diagnosed with testicular seminoma at a referral hospital between 2000 and 2024 were reviewed. METHODS: Horses were included if seminoma was confirmed on histopathology from biopsy or necropsy. Information retrieved included signalment, history, clinical pathology, ultrasonographic findings, treatment, and outcome. RESULTS: Median age was 18 years (range, 8-26). Three of 9 horses presented for systemic disease (colic, 2; ventral edema, 1). All 3 had large abdominal masses palpable on rectal examination and were euthanized after diagnostic evaluation. Six horses presented for unilateral testicular enlargement (5) or cryptorchid castration (1). These horses underwent castration and the removed testes had features of malignancy with lymphatic neoplastic emboli. Five of 6 horses were euthanized between 4 months and 3 years after castration because of acute weight loss and abdominal masses identified on rectal examination or ultrasonographic examination, attributable to metastatic spread. One additional horse was euthanized for unrelated reasons. CONCLUSIONS AND CLINICAL IMPORTANCE: Horses presented with testicular enlargement and diagnosis of testicular seminoma should undergo diagnostic screening, including transabdominal or transrectal ultrasonographic examination to determine the presence of possible metastasis. Negative screening results at the time of diagnosis do not rule out the possibility of metastasis, and frequent screening at regular intervals is prudent in such cases. Horses with testicular seminoma of retained or descended testicles have a poor long term prognosis because of high potential for metastatic spread.