Rare agouti thoracic tumor reclassified as histiocytic sarcoma

A new brief report in the Journal of Veterinary Diagnostic Investigation describes what the authors say is the first reported case of histiocytic sarcoma in a Brazilian red-rumped agouti (Dasyprocta leporina). The thoracic mass, published online May 11, 2026, initially looked most consistent with a chemodectoma based on its cranial mediastinal location and gross appearance. But histology and immunohistochemistry shifted the diagnosis: the tumor showed variable labeling for IBA1, CD204, MHC II, and CD163, and was negative for neuroendocrine and endothelial markers, supporting pulmonary histiocytic sarcoma with cranial mediastinal involvement. (journals.sagepub.com)

Why it matters: For veterinary professionals, especially pathologists and clinicians working with exotics, this is a reminder that thoracic masses in uncommon species may not follow the usual script. The case underscores how gross appearance alone can mislead, and why integrating histopathology with immunohistochemistry is essential when working through cranial mediastinal differentials in species with limited published oncology data. The paper also fits a broader pattern in veterinary literature: histiocytic sarcoma is aggressive, can involve the lung, and is already recognized as a morphologic mimic across species, even if reports in exotic mammals remain sparse. (journals.sagepub.com)

What to watch: Watch for follow-on case reports that clarify how often histiocytic sarcoma is underrecognized in exotic mammals, and which immunohistochemical panels prove most reliable across species. (journals.sagepub.com)