Spontaneous cutaneous mast cell tumor with lymph node metastasis in a Richardson's ground squirrel (Spermophilus richardsonii).

A 4-year-old female Richardson's ground squirrel (Spermophilus richardsonii) presented with multicentric nodules arising from the skin of the middle of the tail and lumbosacral regions. Histologically, the nodules were composed of a proliferation of spindloid to pleomorphic cells that sometimes formed sheets and fascicular to storiform patterns. Diffuse infiltration of eosinophils was also noted. The results of immunohistochemistry indicated positive labeling for vimentin, mast cell tryptase, c-kit, and Ki-67. Toluidine blue stain revealed fine, metachromatic, cytoplasmic granules. The histologic diagnosis was mast cell tumor. The neoplasm recurred and metastasized to the right lumbar lymph node 1 month later.