Primary carcinoma of the rete testis in a Nellore bull.

Carcinoma of the rete testis is an extremely rare neoplasm, which has not been previously reported in cattle. Therefore, the aim of this report is to describe a case of carcinoma of the rete testis in a bull, including clinical, ultrasonographic, histopathological, and immunohistochemical findings. An eight-year-old Nellore bull was referred for evaluation following a history of hemospermia. In the ultrasonographic evaluation, dilation of the left mediastinum testis was observed, as well as the presence of anechoic cystic structures. Unilateral orchiectomy was performed on the affected testis, which was subsequently submitted for histopathological and immunohistochemical examination. Microscopically, there was a neoplasm in the rete testis, unencapsulated, poorly demarcated, and infiltrative, but restricted to the mediastinum testis. Neoplastic cells had marked and diffuse immunostaining for vimentin and cytokeratin but did not stain for CD31. Based on the anatomic location of the lesion restricted to the mediastinum testis and adjacent to the rete testis epithelium, the histopathologic features, and the immunohistochemical expression of vimentin and cytokeratin by the neoplastic cells, a diagnosis of carcinoma of the rete testis was established. Therefore, it is important to include this neoplasm as a rare differential diagnosis in bulls presenting with hemospermia or intratesticular cystic alterations.