Case Report: Complete MRI-confirmed remission of presumptive meningoencephalitis of unknown origin in a Chihuahua using a steroid-free combination of leflunomide and mycophenolate mofetil

Meningoencephalitis of unknown origin (MUO) is an idiopathic inflammatory disorder of the canine central nervous system predominantly affecting small-breed dogs. Standard treatment typically involves glucocorticoids; however, chronic steroid exposure carries substantial risk for adverse effects and may compromise long-term management. A 9-year-old, 2.5 kg female Chihuahua presented with 1 week of rightward circling, head tilt, and cervical stiffness. MRI revealed multifocal intra-axial hyperintensities involving the cerebellum, brainstem, and medulla with associated meningeal enhancement. CSF cytology demonstrated moderate lymphocytic pleocytosis with elevated protein. A steroid-free immunosuppressive protocol using leflunomide and mycophenolate mofetil was initiated. Neurologic signs progressively improved, and repeat MRI at 12 months documented complete resolution of inflammatory lesions. Both medications were continued for 2 years, during which serial hematologic and biochemical monitoring remained within normal limits. No adverse events occurred. This case provides, to the authors' knowledge, the first documented MRI-confirmed remission of MUO achieved with exclusive steroid-free immunosuppression. Relevant literature is reviewed to contextualize the immunopathogenesis of MUO and considerations for steroid-sparing strategies.