Leptomeningeal dissemination of a poorly differentiated carcinoma involving the trigeminal ganglion in a horse: a case report.

Neoplastic conditions involving cranial nerves are uncommon in horses and represent a significant diagnostic challenge. Leptomeningeal dissemination is a recognized mechanism of tumor progression in aggressive neoplasms but is rarely reported in this species. A 15-year-old mixed-breed horse with a history of periocular squamous cell carcinoma treated by enucleation three years earlier developed progressive neurological signs. Initial differential diagnoses included metastatic neoplasia and equine protozoal myeloencephalitis. Due to rapid neurological deterioration, euthanasia and necropsy were performed. A large infiltrative mass involving the right trigeminal ganglion was identified. Histologically, the neoplasm was a poorly differentiated carcinoma with marked pleomorphism, a high mitotic index, and invasion of adjacent neural structures. Free neoplastic cells were observed within the subarachnoid space at multiple sites without parenchymal invasion, consistent with leptomeningeal dissemination. Immunohistochemistry confirmed epithelial origin. This case expands the recognized biological behavior of equine carcinomas and documents leptomeningeal dissemination associated with trigeminal ganglion involvement.