A subcutaneous tumor was identified in the lateral neck of a wild adult serotine bat (Eptesicus serotinus). Histologic examination revealed a non-encapsulated, poorly demarcated, densely cellular and infiltrative round-cell neoplasm, with cytonuclear atypia and a high mitotic count. Neoplastic cells had numerous Giemsa-positive metachromatic intracytoplasmic granules leading to a diagnosis of subcutaneous mast cell tumor (MCT). Immunohistochemical analysis demonstrated positive staining for KIT with a diffuse cytoplasmic pattern, confirming the mastocytic origin. The tumor had histologic features suggestive of malignant behavior, which was confirmed by the metastatic mast cells in the regional lymph node adjacent to the tumor. MCT has not been reported previously in a chiropteran species, to our knowledge. Based on this case, MCTs in this species have malignant potential, and their diagnosis may require additional stains and immunohistochemistry (IHC). Our results support further investigation regarding the utility and prognostic value of KIT IHC in bats in this context.
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