What is your diagnosis?: Multifocal subcutaneous tumors in a young male baboon

A juvenile male baboon (Papio cynocephalus anubis), while being held in quarantine prior to research assignment at the Washington National Primate Research Center (WaNPRC), presented with three soft tissue masses on the right elbow, wrist, and knee on routine physical examination. The masses were subcutaneous and did not appear to cause any discomfort to the animal. An excisional biopsy of the knee mass was submitted for histopathologic analysis and found to be poorly defined, rapidly growing, and of undetermined histogenesis. Euthanasia was elected, and a full necropsy done with samples collected for histopathologic analysis. By immunohistochemistry, the "large" cells in the masses were suggestive of a mesenchymal, nonmacrophage origin because of their being positive for vimentin and negative for cytokeratin and the macrophage maturation marker CD68. Eosinophilic intracytoplasmic inclusions were seen on HE-stained sections and corresponded to aggregates of ovoid to brick-shaped viral particles on transmission electron microscopy (TEM), which proved to be the key diagnostic tool for this case. The masses were determined to be Yaba monkey tumor virus (YMTV)-induced "benign histiocytomas" by TEM.