Sustained clinical remission and long-term safety of molnupiravir as first-line therapy for effusive feline infectious peritonitis: a 40-week retrospective study

Feline infectious peritonitis (FIP) is a highly fatal disease caused by feline coronavirus (FCoV). Although the acute clinical efficacy of molnupiravir as first-line therapy has been recognized, there are limited data regarding the long-term durability of clinical outcomes and safety profile posttreatment. This retrospective study aimed to evaluate the acute response (weeks 0–12) and long-term clinicopathological outcomes (up to week 40) in cats with naturally occurring effusive FIP treated with molnupiravir. Forty-two cats were included. They received a median molnupiravir dose of 14.3 mg/kg twice daily for at least 84 days (12 weeks). Molnupiravir treatment rapidly resolved clinical signs and effusions (median 7 days). Moreover, significant improvements in white blood cell and neutrophil counts were observed within the first week ( p < 0.05). A transient elevation in alanine aminotransferase levels occurred initially but completely normalized by week 7 without requiring treatment discontinuation. Importantly, paired analysis between posttreatment (week 12) and long-term follow-up (week 40) data revealed no significant deviations in hematological parameters (e.g., hematocrit and lymphocytes) or renal function (creatinine) ( p > 0.05), indicating sustained physiological stability. In summary, molnupiravir is highly effective and safe as first-line treatment for effusive FIP. The transient nature of hepatic enzyme elevation and long-term hematological and renal stability provide strong clinical assurance for its long-term safety profile and the potential for a sustained clinical cure.