Meningoencephalitis of unknown origin with predominant cerebellar involvement in a cat: clinical, MRI and pathological findings.

Case summary: Meningoencephalitis of unknown origin (MUO) is a term encompassing non-infectious inflammatory brain diseases with no identifiable underlying cause, widely studied in dogs but rarely in cats. This case describes a 9-year-old male castrated domestic shorthair cat diagnosed with feline MUO (FMUO). The cat presented with progressive vestibulocerebellar signs refractory to immunosuppressive treatment. MRI revealed multifocal, symmetrical intra-axial cerebellar lesions that were isointense to mildly hyperintense on T2-weighted and fluid-attenuated inversion recovery images, mildly hyperintense on T1-weighted images and enhanced with contrast. Cerebrospinal fluid analysis showed elevated protein concentration and lymphocytic pleocytosis. A post-mortem examination revealed widespread inflammatory infiltration affecting the frontal cortex, thalamus, parietal cortex and cerebellum in a bilateral and random distribution. Immunohistochemistry for common viral and protozoal pathogens was negative. Relevance and novel information: This case illustrates the heterogeneity of FMUO, highlighting MRI and histo-pathological features that deviate from previously reported patterns. The findings highlight the need to include FMUO in the differential diagnosis for progressive vestibulocerebellar signs in cats, despite its rarity. Further research is necessary to better characterise FMUO and refine diagnostic and therapeutic approaches.