Histopathological and immunohistochemical diagnostic of carcinosarcoma of mandible in a rainbowtrout (Onchorhyincus mykiss)

Background: The most common health problems described in fishes are noninfectious, infectious, and metabolic disorders. Fishes have neoplasias that are similar to those observed in mammals. Though these neoplasias do not cause economic losses to aquaculture, they have aroused a crescent interest from the comparative pathology’s point of view. Neoplasias of the oral cavity in fishes are usually reported as individual cases, but mandible tumors are not common, and most of them are squamous cells carcinomas. The aim of this study is to report a case of carcinosarcoma of the mandible in a rainbow trout (Oncorhynchus mykiss), obtained from a trout farm in Campos do Jordão, São Paulo, Brazil. Case: The fish underwent necropsy and tissue samples were collected and fixed with a buffer-neutralized formalin solution, which were then embedded in paraffin, sectioned in 5 µm sections and stained with hematoxylin-eosin. The histological sections were stained for immunohistochemical procedures, according to a modified avidin-biotin-peroxidase complex technique. Slides containing tissue were deparaffinized with xylol and rehydrated with alcohol of different grades. The endogenous peroxidase activity was blocked by incubating the slides for 20 min at 0.3% H2 O2 in a solution of 5% methanol. After rinsing the slides in water and in a PBS/0.05%-Tween 20 solution, they were incubated in normal serum diluted 1:100 containing 10% bovine albumin in PBS (BSA), at room temperature for 30 min in humid chamber. After incubation, the primary anti-cytokeratin and anti-vimentin antibody was added to a 1:200 dilution in 10% PBS-BSA, and the slides were incubated until the next day at 41 o C in humid chamber. The slides were then rinsed in PBS and incubated for 7 min in a 50 mL 30.3-diaminobenzidine solution containing 1% PBS-BSA in 50 mL H2 O2. Hematoxylin was used for counterstaining. The immunohistochemical analysis showed a tumor with epithelial and mesenchymatous neoplastic components. The epithelial component presented an epidermoid carcinoma with well differentiated and keratinizing areas, in addition to other little differentiated, non-keratinizing areas. The mesenchymatous component presented a fibrosarcoma. The immunohistochemical analysis demonstrated signs of cytokeratin in carcinomatous elements and of vimentin in sarcomatous elements. Discussion: Fishes are valuable resources in human nutrition, medicine, science and leisure. Similarly to other poikilothermic vertebrates, fishes also develop benign and malign neoplasias. However, it is their infectious diseases that are more known and studied. Teleost fishes constitute the widest and most diverse class of vertebrates, with over 20,000 known species. The diversity and placement in the phylogenetic tree make teleost fishes the ideal objects for comparative carcinogenesis studies, which could allow a broader knowledge about basic neoplastic development mechanisms, in comparison to studies limited to mammal models only. According to our knowledge and considering the literature available on fish tumors, this is the first description of a carcinosarcoma of the mandible affecting a rainbow trout