Spontaneous multiple cutaneous mixed tumors in Japanese giant salamander Andrias japonicus

We examined 7 cutaneous mixed tumors in 2 wild-captured Japanese giant salamanders Andrias japonicus. The tumors were either already present and/or increased in size, or newly occurred during capativity. We sampled the 7 tumors from these animals and 3 verrucose protrusions from 3 unaffected animals, as controls, and examined them pathologically and virologically. The tumors (5 mm to 4 cm in size) were papillary protrusions or pendulated on the skin surface. The cut surface of the tumors was white, lobulated, partially hard, and contained mucus. All tumors presented similar histological characteristics of a hyaline structure and exhibited biphasic proliferation, with neoplastic epithelial cells partially composing the pseudo-ductal structure and staining positive for cytokeratin AE1/AE3. Vimentin 3B4-positive blast-like mesenchymal cells proliferated to fill the gaps in the epithelial components. Transition from unique mucous gland to neoplastic tissue was observed. The hyaline structure was stained blue by AZAN stain, Alcian blue-periodic acid-Schiff (PAS) double stain, and toluidine blue (TB) stain of pH 7.0, but was unstained by TB with pH values of 4.1 and 2.5. The mucus in the neoplastic tissue and in the mucous gland in verrucose protrusions was stained blue by Alcian blue-PAS double stain; TB staining at pH 7.0, 4.1, and 2.5 revealed metachromasy. No virus was detected in the tumors. The 7 tumors were diagnosed as cutaneous mixed tumors, and it was confirmed that the neoplastic cells originated from the mucous gland in the dermis. The biological behavior and pathological development of tumors should be elucidated because the tumors have the potential to negatively affect A. japonicus.