Acute neurologic presentation of a 2-year-old Standardbred colt with multicentric diffuse large B cell lymphoma

Lymphoma in horses is uncommon and rarely diagnosed as the cause of ataxia in horses. This case report describes a two-year-old Standardbred cryptorchid colt who presented with acute onset of grade 3.5/5 ataxia. Due to the severity of the ataxia, a full neurologic examination could not be completed, and the colt was humanely euthanized. Post-mortem examination revealed multiple osteochondritis dissecans lesions in the cervical and lumbar spine. Initially this was thought to be the cause of the ataxia, however histopathology revealed multifocal areas where the spinal nerve roots and epidural surface of the dural mater of the meninges were surrounded or covered by plaques of neoplastic round cells which infiltrated the adipose tissue surrounding the spinal tissues. Atypical round cells similar to those observed in the spinal meningeal tissues were also found in the kidneys, lungs, liver, testes and one lymph node. Immunohistochemical staining of meningeal tissue samples revealed moderate to intense cytoplasmic/membranous immunoreactivity for CD20 and a diagnosis of multicentric diffuse large B cell lymphoma was made. Lymphoma should be included in the differential diagnoses for a horse presenting with ataxia and neurologic deficits