Localized histiocytic sarcoma in a captive capybara (Hydrochoerus hydrochaeris)

An irregular, subcutaneous mass (2x3x2 cm) located between the first and second digits of the left hindlimb of a nine-year-old, male, captive capybara (Hydrochoerus hydrochaeris) was biopsied. Microscopically, the mass scattered throughout the dermis. The neoplastic cells were arranged in solid-sheet patterns and comprised of pleomorphic histiocytic cells with pleomorphic nuclei and abundant eosinophillic cytoplasm. Mitotic figures were 2-3 cells/HPF. Numerous, multinucleated, giant cells, up to 10 nuclei/cells, were also prominently observed and found scattered throughout the mass. Immunohistochemically, the neoplastic cells were strongly positive for vimentin and ionized calcium-binding adaptor molecule 1 (Iba-1), while immunoreactivity for cytokeratin was negative. Based on histopathological and immunohistochemical characterizations, localized histiocytic sarcoma was diagnosed.