Case Report: Testicular teratoma with malignant transformation to melanoma and concurrent metastatic carcinoma of undetermined primary origin in a red deer (Cervus elaphus)

Miguel Criado^1,2*Paloma Prieto³Juan José Palomares³Desiderio Lopezosa³David Zapico^1,2Pedro Mendívil^1,2Julio Benavides²María del Carmen Ferreras^1,2José Espinosa^1,2

• ¹Departamento de Sanidad Animal, Universidad de León, León, Spain
• ²Instituto de Ganaderia de Montana, Vega de Infanzones, Spain
• ³Parque Natural Sierras de Cazorla, Segura y Las Villas, Segura de La Sierra, Jaén, Spain

Testicular teratomas are rare germ cell neoplasms composed of two or more embryonic layers (ectoderm, mesoderm, endoderm). They are uncommon in domestic species, with only a few cases reported in wild mammals, and malignant transformation is rare. We report the case of a European red deer (Cervus elaphus) approximately four years of age, which presented with marked scrotal enlargement. Necropsy revealed an enlarged left testis weighing 3.29 kg. On sectioning, it showed complete loss of its normal architecture, with multiple multinodular solid areas and cystic regions containing dark fluid. An inguinal lymph node was diffusely darkly pigmented, and an iliac lymph node was enlarged and showed complete architectural effacement. Histopathological examination confirmed the presence of a teratoma with differentiation into tissues derived from all three germ layers. Additionally, two distinct cell populations with clear malignant features were identified in the testis and the iliac lymph node. Immunohistochemical and immunofluorescence studies were performed for their characterization. Antibodies used included pan-cytokeratin (PCK), Melan-A, vimentin (Vim), c-KIT receptor tyrosine kinase (CD117), S100 protein, and ionized calcium-binding adaptor molecule 1 (IBA1), some of which had not previously been used in this species. The results revealed a malignant transformation of the teratoma, with the presence of a poorly differentiated, invasive melanoma (Vim+, Melan-A+, S100–, CD117–), accompanied by numerous melanophages (IBA1+) in the surrounding tissue and inguinal lymph nodes; and a concomitant metastatic carcinoma (PCK+) identified in the iliac lymph node, of undetermined origin. To our knowledge, there are no documented cases of testicular teratomas with these characteristics in wild species of the Cervidae family or in other animal species.