Case Report: Transitional meningioma in a free ranging mantled howler monkey (Alouatta palliata).

Neoplasia in free ranging wildlife remains poorly documented, particularly in non-human primates. An adult free ranging female mantled howler monkey (Alouatta palliata) was found conscious but non-ambulatory on a country road in northwestern Costa Rica. Following admission to a wildlife rescue center, euthanasia was elected due to poor prognosis and body condition. At necropsy, three firm, gritty, white masses were identified on the inner surface of the parietal bones. Histopathologic examination revealed a well-circumscribed, moderately cellular, encapsulated neoplastic proliferation arising from the dura mater and compressing the cerebral cortex while sparing the leptomeninges. Neoplastic cells were arranged in whorls and occasional interlacing streams, with sporadic psammoma bodies. A diagnosis of transitional meningioma was made. Immunohistochemically, neoplastic cells were positive for S100 and negative for pan-cytokeratin, vimentin, and E-cadherin. Reports of meningiomas in non-human primates, particularly in free ranging individuals, are scarce. This case contributes to the limited knowledge of neoplasia in wildlife and underscores the value of documenting neoplasia in free ranging primates.