Abstract
Background:
Feline infectious peritonitis (FIP) is a fatal disease caused by feline coronavirus infection. Even when antiviral therapy is available, standard treatment regimens often require a prolonged therapy duration and may rely on unlicensed drugs. Molnupiravir, an orally administered nucleoside analogue, is increasingly being used off-label for the treatment of FIP.

Case Description:
This retrospective case series included eight client-owned cats with FIP, including effusive (n = 6) and non-effusive (ocular n = 1; neurologic n = 1) types. Molnupiravir (10–15 mg/kg, orally q12 h) was administered for 14–69 days based on therapeutic response, together with case-specific supportive therapy, including systemic prednisolone with individualized tapering regimens. Seven of eight cats showed prompt improvement in clinical symptoms after molnupiravir administration. Ascites resolved within 14–18 days in five of the six cats with effusive FIP. One cat with advanced disease did not respond to treatment and died. No adverse effects attributable to molnupiravir administration were observed. At the final follow-up (214–1118 days after treatment initiation), all seven surviving cats were alive, and no further recurrence was observed.

Conclusion:
These findings suggest that individualized, response-based short-course molnupiravir therapy may be a potential treatment option for inducing remission in selected cats with FIP that show an early response to treatment. However, careful patient selection, post-treatment monitoring, and consideration of the risks associated with adjunctive glucocorticoid administration are required. Prospective controlled studies are warranted to optimize dosing strategies, discontinuation criteria, and long-term safety.

Key words: Duration of therapy; Feline coronavirus; Off-label antiviral use; Nucleoside analogue; Response-based treatment.