DISSEMINATED MALIGNANT HISTIOCYTOSIS IN A GOLDEN RETRIEVER - CLINICOPATHOLOGICAL, ULTRASTRUCTURAL, AND IMMUNOHISTOCHEMICAL FINDINGS

Diagnosis of malignant histiocytosis (MH), a disorder characterized by systemic proliferation of morphologically atypical histiocytes and th eir precursors, in an 8-year-old neutered female Golden Retriever was based on light and electron microscopic and immunohistochemical findin gs. Clinically, the dog presented with unilateral forelimb lameness. E ight days after. surgical exploration of a swollen brachium, the dog d eveloped sudden onset of posterior paresis, fecal and urinary incontin ence, and a flaccid tail. Necropsy revealed infiltrative and nodular l esions in the right forelimb and regional lymph nodes, thoracic and ab dominal cavities, and lumbar epidural space. Gross lesions were not fo und in the lungs or integument. Histopathologic examination showed inf iltrates of atypical histiocytes in skeletal muscle, joint. and region al lymph nodes of the right forelimb; intercostal muscle; lung; liver; spleen; pancreas; kidneys; and spinal dura. Most tumor infiltrates we re nodular and composed of loosely aggregated cells that were 10-30 mu m in diameter with abundant eosinophilic to foamy cytoplasm, had centr al or eccentric nuclei, and were periodic acid-Schiff negative. Many b inucleated cells, multinucleated giant cells, and mitotic figures were seen. Tumor cells contained phagocytosed erythrocytes, mononuclear ce lls, and some leukocytes. Ultrastructural features of tumor cells incl uded cytoplasmic lipid droplets, lysosomes, and phagolysosomes. Immuno histochemical studies on paraffin-embedded sections showed positive re activity to human T-cell Ag (clone UCHL-1) and for lysozyme, alpha-1-a ntitrypsin, and cathespin B. Polyclonal intracellular immunoglobulin r eactivity and lectin binding (peanut, soybean, and wheat germ agglutin ins and concanavalin A) were also demonstrated. Criteria for diagnosis of malignant histiocytic tumors and differential diagnosis are discus sed.