Abstract
BACKGROUND AND AIM: Newcastle disease (ND), caused by velogenic viscerotropic ND virus (VVNDV), remains a major threat to global poultry production, with outbreaks persisting even in vaccinated flocks. No approved antiviral therapy exists for ND, highlighting the urgent need for effective interventions. Favipiravir, a broad-spectrum RNA polymerase inhibitor, has shown promise against several RNA viruses. This study evaluated the toxicity, antiviral efficacy, and molecular mechanisms of favipiravir against VVNDV in an in ovo model. MATERIALS AND METHODS: Specific pathogen-free embryonated chicken eggs (9-10 days old) were inoculated with VVNDV and treated with graded doses of favipiravir (75-2280 mg/kg/egg biomass). Toxicity was assessed through embryo survival, relative weight, morphological scoring, biochemical markers, and histopathology of liver tissues. Antiviral efficacy was evaluated through embryo survival, growth, hemagglutination (HA) titers, and 50% egg infectious dose (EID(50)). Molecular docking was performed to characterize favipiravir's interaction with viral RNA-dependent RNA polymerase (RdRp). Statistical analyses included Kruskal-Wallis, analysis of variance, and correlation tests. RESULTS: Favipiravir displayed dose-dependent toxicity, with the highest dose (2,280 mg/kg) significantly reducing embryo survival (p = 0.027) and inducing hepatic necrosis and elevated alkaline phosphatase and urea levels. In contrast, therapeutic doses of 300 and 600 mg/kg achieved 100% embryo survival, significant weight gains, and complete viral suppression, with undetectable HA activity and EID(50) values. Favipiravir demonstrated antiviral efficacy by suppressing viral replication and conferring protection against VVNDV. Docking analysis revealed a strong binding affinity of favipiravir to RdRp, primarily mediated by electrostatic interactions and hydrogen bonding with residues Arg1189, Tyr1192, and Ser1288, suggesting inhibition of viral RNA synthesis. CONCLUSION: This study provides the first in ovo evidence of favipiravir's efficacy against VVNDV, demonstrating complete viral inhibition at optimized doses while emphasizing the importance of dose-dependent toxicity monitoring. These findings establish favipiravir as a promising antiviral candidate for ND virus control and potentially other RNA viruses of veterinary and One Health importance. Further in vivo and field-based studies are warranted to validate its safety, optimize dosing regimens, and evaluate large-scale applicability in poultry production.