Quercetin exhibits antiviral activity against duck Tembusu virus in vitro and in vivo.

Duck Tembusu virus (DTMUV) is an avian pathogenic flavivirus that causes substantial economic losses to the poultry industry in China and Southeast Asia. The current antiviral drugs for the prevention and treatment of DTMUV infection are limited. Quercetin is a natural flavonoid widely found in fruits and vegetables and has been reported to exhibit antiviral activity against many viruses. However, whether quercetin possesses antiviral activity against DTMUV remains unknown. Therefore, the present study aimed to investigate the antiviral efficacy of quercetin against DTMUV. A Cell Counting Kit-8 (CCK-8) assay was performed to evaluate the cytotoxicity of quercetin in BHK-21 cells. The antiviral activity of quercetin was assessed using 50% tissue culture infectious dose (TCID₅₀) assay, real-time quantitative PCR (RT-qPCR), Western blotting and indirect immunofluorescence assay (IFA). Molecular docking analysis was conducted to predict potential viral targets. Moreover, the antiviral efficacy of quercetin in vivo was further evaluated in DTMUV-infected goslings. The results of the CCK-8 assay revealed that quercetin at concentrations up to 40 μM had no significant effect on BHK-21 cells. Quercetin significantly inhibited DTMUV replication in BHK-21 cells in a dose- and time-dependent manner, as demonstrated by reduced viral titres, RNA copy numbers, and E protein expression. Direct virucidal assays revealed that quercetin could directly inactivate DTMUV particles. Adsorption and post-adsorption assays revealed that quercetin inhibited DTMUV infection during the post-adsorption stage. Furthermore, molecular docking predictions indicate that the viral NS5 protein is a potential target for quercetin. In vivo, quercetin treatment significantly improved the survival rate of infected goslings; reduced viral loads in the liver, spleen, and brain; and alleviated gross pathological lesions and histopathological damage caused by DTMUV infection. In conclusion, quercetin exhibits potent anti-DTMUV activity both in vitro and in vivo. These findings provide a theoretical basis for the development of quercetin as a natural antiviral agent and highlight its potential application in the prevention and control of DTMUV infection.