Spontaneous histiocytic sarcoma with peritoneal dissemination in a Damaraland mole-rat ( <i>Fukomys damarensis</i> ): The first spontaneous tumor in this species

An 11-year-old, Damaraland mole-rat queen died after several months of abdominal distension with no other clinical signs, and a necropsy was performed. Grossly, numerous red-white, smooth masses, ranging from a few millimeters to 1 cm in diameter, were widely scattered across the surfaces of the diaphragm, mesentery, and peritoneum. The pulmonary hilar, mediastinal, mesenteric, and renal lymph nodes were enlarged. Histologically, the masses and the enlarged lymph nodes consisted of atypical cells with abundant eosinophilic cytoplasm and large, eccentrically positioned nuclei. Multinucleated giant cells were often intermixed with these cells. Based on the positive immunolabeling of macrophage markers IBA1 and CD163, the lack of detectable pathogens, and the prominent dissemination of these cells, these lesions were diagnosed as histiocytic sarcoma. This is the first case of histiocytic sarcoma and the first case of a spontaneous tumor in a Damaraland mole-rat, a burgeoning non-traditional animal model of longevity and purported cancer resistance.