Metastatic Leiomyosarcoma Causing Ataxia in a Horse

•Horse with progressive ataxia suspected to have infectious neurologic disease.•Necropsy: large mass in hind limb musculature with dissemination, including to CNS.•Histopathology: leiomyosarcoma with disseminated metastases.•This case is a reminder of noninfectious causes of ataxia in horses.

A 24-year-old Thoroughbred mare had a history of weight loss, progressive bilateral hind limb weakness, and ataxia. An infectious neurologic disease was suspected, and the mare was euthanized due to poor prognosis. At necropsy, a yellow firm mass replaced most of the skeletal muscles of the left thigh. Numerous similar nodules were in several other muscles and in multiple organs, including brain and spinal cord. Microscopic examination revealed a leiomyosarcoma with disseminated metastases. Although of rare occurrence, this case is a reminder of noninfectious causes of ataxia in horses.