Lymphoma with Mott cell differentiation and validation of immunohistochemical lymphoid markers in an African pygmy hedgehog (Atelerix albiventris)

A 2 year old, female entire African pygmy hedgehog was presented for diagnostic investigation of a 2-month reduction in appetite, with weight loss, and recent vomiting. Clinical examination revealed a large, firm mass originating from the left cranial abdomen. Ultrasound guided fine needle aspirates of the mass, the liver and the mesenteric lymph nodes revealed a population of pleomorphic round cells, some of which contained variable numbers of round, clear vacuoles, consistent with a diagnosis of lymphoma with Mott cell differentiation. At post-mortem examination, there was marked diffuse splenic enlargement, with infiltration by a soft tissue mass. There were multiple coalescing masses in the liver, pallor of the kidneys, and enlargement of the mesenteric lymph nodes. On histological examination, the spleen, lymph nodes, and masses in the liver were extensively infiltrated by proliferating lymphoid cells with plasmacytoid and Mott cell differentiation. Cells with Mott cell morphology had accumulation of periodic acid-Schiff positive material in cytoplasmic inclusions, and were positive for cytoplasmic nucleic acids when stained with methyl green pyronin. In the population of neoplastic lymphoid cells, a majority of cells expressed the transcription factor Pax5, which drives B cell differentiation, and a minority expressed transcription factor IRF4/MUM-1, which drives plasma cell differentiation, consistent with a B cell lymphoma with plasmacytoid differentiation.