Organs that are often infiltrated by the neoplastic cells include the lungs, liver, kidneys, spleen, bone marrow, and lymph nodes, with the spleen most commonly affected.
Clinical symptoms are often vague, including anorexia, weight loss, fever, and lethargy. Additional clinical signs, such as dyspnea, lameness, episcleritis and neurological deficits, depending on the organ system involved.
A tentative diagnosis can be established by ultrasonography and radiography. Common radiographic abnormalities include the presence of a mediastinal mass, pulmonary nodules or consolidation, hepatomegaly, pulmonary edema and pleural effusion and splenomegaly.
A definitive diagnosis requires tissue biopsy of tumors. Histologically, malignant histiocytosis cells are atypical round cells that no longer resemble normal macrophages and display multiple criteria of malignancy. Marked anisokaryosis is usually present. Phagocytosis of erythrocytes, neutrophils, and other tumor cells by both mononuclear and multinucleated tumor cells is also observed.
Immunohistochemical staining is useful to determine the lineage of the highly anaplastic round cells seen with canine MH/DHS.
Clinical outcome in dogs with MH/DHS reflects the aggressive behavior and poor prognosis of the neoplasm.
Chemotherapy with human major histocompatibility complex, or zoledronate in conjunction with cyclophosphamide, vincristine, doxorubicin and prednisolone may result in survival times ranging from several months to 6 years.
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