Clinical and imaging findings after antiviral treatment in cats with presumptive neurologic feline infectious peritonitis: 9 cases (2020-2024)

BACKGROUND: Limited information is available regarding comparison between clinical outcomes and post-treatment imaging findings in cats with the neurologic form of feline infectious peritonitis (FIP). HYPOTHESIS/OBJECTIVE: Describe post-treatment imaging findings and clinical outcomes in cats with neurologic FIP treated with antiviral drugs. ANIMALS: Nine cats with a presumptive diagnosis of neurologic FIP undergoing advanced imaging before and after antiviral treatment (GS-441524, remdesivir, or both). METHODS: Medical records from 8 institutions were reviewed retrospectively (January 1, 2020 to December 31, 2024). Included cats had pre- and post-treatment magnetic resonance imaging (MRI) or computed tomography (CT) studies. Treatments consisted of GS-441524 (6/9), remdesivir (2/9), or both (1/9). All cats received corticosteroids. Follow-up imaging was performed at variable time points after completion of antiviral treatment. RESULTS: Central vestibular disease was the most common neuroanatomical localization (6/9). Obstructive hydrocephalus was present in all cases on initial and follow-up imaging. Cerebrospinal fluid analysis, available in 4 cats, included pleocytosis (neutrophilic in 3/4; lymphocytic in 1/4). Complete clinical recovery occurred in 5/9 cats. Four of 9 had residual neurologic deficits and 2/9 experienced recurrence of neurologic signs. None showed complete resolution of imaging abnormalities. Imaging changes did not consistently reflect clinical outcomes. In 4/9 cats, progression of obstructive hydrocephalus was observed, including cases with clinical improvement. CONCLUSIONS AND CLINICAL IMPORTANCE: Clinical improvement is not always consistent with radiologic resolution in cats with neurologic FIP treated with antivirals. Post-treatment MRI findings, particularly persistent or worsening obstructive hydrocephalus, should be interpreted alongside clinical status rather than as a sole indicator of therapeutic response.