Central nervous system feline infectious peritonitis with a mass lesion in the fourth ventricle revealed by magnetic resonance imaging.

An 11-month-old neutered male mixed-breed cat was presented to the referring hospital with the chief complaint of progressive ataxia. Magnetic resonance imaging (MRI) revealed a well-defined, ovoid, solitary mass on the left side of the fourth ventricle. Immunostaining of surgically excised tissue led to a diagnosis of feline infectious peritonitis (FIP). To our knowledge, this is the first report of a central nervous system (CNS)-FIP presenting as a solitary fourth ventricular mass identified by MRI. CNS-FIP should be considered as a differential diagnosis when encountering cases of young cats with a solitary mass lesion in the fourth ventricle.