Background - Limited information is available regarding the treatment course with molnupiravir treatment for feline infectious peritonitis (FIP). Hypothesis/Objectives - To clarify the clinical course and relapse rate of molnupiravir treatment for FIP. Animals - Cats diagnosed with FIP and treated with molnupiravir. Methods - Retrospective study at Hokkaido University Veterinary Teaching Hospital between January 2023 and January 2024. Results - Eleven cats were eligible for inclusion. Six cats had pleural or peritoneal effusion. Two cats showed neurological abnormalities at the diagnosis. The median initial dosage of molnupiravir was 13.0 mg/kg q12h (range, 10.0-15.0 mg/kg q12h). While one cat died in 11 days, ten cats completed an 84-day course of treatment. The dosage was increased in two cats, and the median final dosage of molnupiravir was 13.6 mg/kg q12h (range, 10.0-17.2 mg/kg q12h). In cats that did not require an increase in dosage, improvement of hyporexia, lethargy, pyrexia, high serum amyloid A (SAA), effusion, and lymph node enlargement were seen within 15 days after treatment initiation. Also, the owners noticed an improvement in clinical signs within a week (2-6 days) in five cases. Albumin: globulin ratio remained <0.6 in four cats. After 84 days of treatment, follow-up periods exceeded 84 days (median 113 days; range, 85-271 days), and no relapse was reported. Conclusions and clinical importance - In the treatment of FIP with Molnupiravir, clinical signs, pyrexia, high SAA, effusion, and lymph node swelling were improved within 15 days. No relapse was observed during the study.
