The Most Effective Molnupiravir Dosage for Eliminating Feline Calicivirus from Cats with and Without Chronic Gingivostomatitis

Feline chronic gingivostomatitis (FCGS) and gingival hyperplasia are extremely painful conditions for which there is no universally recognised cause or cure. Feline calicivirus (FCV) was detected by RT-qPCR in two pet Domestic Shorthair cats with FCGS and isolated from mouth swabs of 14 cats following a gingivitis and fading kitten outbreak in a British Shorthair and Maine Coon breeding cattery. Oral inflammation was graded 0 (healthy) to 4 (FCGS). Nine of 16 gingivitis cases presented with FCGS. Mean inflammation grade significantly improved (p < 0.001) from 3.1 ± 1.1 to 0.3 ± 0.5 when cats became FCV negative. Eleven of 16 (69%) cats gained weight. Cats became FCV negative following antiviral treatment with molnupiravir (n = 15) and nitazoxanide (n = 1). (Nitazoxanide (31–48 mg/kg q24h for 70 days) failed to stop FCV shedding in three other cats.) Across 39 treatment courses, FCV shedding ceased with once daily oral molnupiravir doses of 22–39 mg/kg in 5/10 (50%); 40–55 mg/kg in 15/16 (94%); and >55 mg/kg in 10/13 (77%). Subclinically infected cats became FCV negative after 14 days molnupiravir, but gingivitis cases required longer (p = 0.005). Three cattery cats were reinfected and relapsed. Conclusion: Molnupiravir at a dose of 40–55 mg/kg q24h stopped FCV shedding, enabling recovery of some gingivitis cases.