Malignant histiocytosis in three cats.

Introduction: Malignant histiocytosis (MH) or disseminated histiocytic sarcoma is a systemic neoplastic proliferation of macrophages that is well characterized in dogs, but rare in cats, as only eight cases have been published in the international literature. In this report the clinical course and diagnostic findings of three feline patients with MH are presented, along with differential diagnosis considerations and a comparison with other cases described in the literature. Materials and Methods: Samples for cytology were taken by fine needle aspiration and stained with May Grünwald-Giemsa. Tissue for light microscopy was fixed in buffered formalin. Sections were stained with haematoxylin and eosin, PAS, Masson trichrome, toluidine blue, Perls for haemosiderin, methyl green-pyronin and Buffalo black for haemoglobin. Immunohistochemistry was performed using an avidin-biotin-peroxidase technique for vimentin, CD3, CD79α, cytokeratin AE1/AE3, MAC387, MHC-II, and lysozyme. Small pieces of tissue were also post-fixed in osmium tetroxide, epon embedded, sectioned and stained for transmission electron microscopy (TEM). Results: Case 1 was a 13 years old european male neutered cat presented with a laryngeal mass and moderate lymphadenopathy; case 2 was a 10 years old European male neutered cat with marked jaundice and hepatomegaly; case 3 had lumbar lymphadenopathy and hepato-splenomegaly. The FIV-FeLV status was negative in case 1, FIV+ in case 2 and unknown in case 3. All patients were markedly anaemic and the disease rapidly progresses to death. The main post mortem findings common to all cases were marked hepatomegaly and splenomegaly and moderate lymphadenopathy. Histology and cytology showed the infiltrative proliferation of large mononuclear and multinucleate round-to-polygonal atypical discrete cells, with huge and pleomorphic nuclei and nucleoli. These cells had abundant vacuolated cytoplasm and occasionally displayed erythrophagocytosis. Immunohistochemistry (vimentin and lysozyme expression) and TEM supported the morphologic hypothesis of histiocytic malignancies, and MH diagnosis was then formulated. Conclusion: Diagnostic alternatives in our cases included systemic histiocytosis, histiocytic (anaplastic large cell) lymphoma, anaplastic systemic plasmacytoma and visceral histiocytic mast cell tumour, but histological, histochemical and immunohistochemical figures ruled them out. MH has been extensively described in dogs, and a genetic predilection has been recorded in Bernese mountain dogs, Rottweilers and Golden retrievers. Although no predisposing factors seem to be evident for feline MH, this disease should be considered in the differential diagnosis for cats with anaemia and hepato-splenomegaly.