Antiviral effects of rhein on largemouth bass ranavirus (LMBRaV)

Largemouth bass ranavirus (LMBRaV), also known as largemouth bass virus (LMBV), is a highly damaging viral pathogen that causes widespread mortality in cultured largemouth bass and poses a significant threat to the farming industry in China. However, there is a paucity of effective preventive methods for this disease by LMBRaV. Research has established that herbal extracts possess antiviral activity, potentially exerting a controlling effect on viruses in aquaculture. Therefore, the identification of simple and structurally safe herbal extract immunomodulators is critical for largemouth bass health and development. Rhein, an herbal compound, exhibits a broad spectrum of pharmacological effects in herbal medicine. This study aimed to evaluate the properties and effects of rhein against LMBRaV in epithelioma papilloma cyprinid (EPC) and largemouth bass. In vitro CCK-8 assays were performed to determine the highest safe concentrations. Significantly, 50 mg/L rhein effectively inhibited cytopathic effects (CPE) in EPC induced by LMBRaV, resulting in a 97 % reduction in viral replication rates. Furthermore, a six-point dose-response experiment revealed that rhein doses as low as 6.4 mg/L inhibited LMBRaV and suppressed LMBRaV infection with a dose-dependent manner. Subsequent experiments demonstrated that sustained rhein activity effectively alleviated CPE and nuclear damage caused by LMBRaV infection. Additionally, rhein exerted protective effects against declines in LMBRaV-induced mitochondrial membrane potential. In vivo experiments validated that largemouth bass diets supplemented with 0.075 % rhein exerted a protective effect against LMBRaV infection. Moreover, LMBRaV MCP gene expression levels were lower in several organs of fish in the rhein supplementation group. Immunogene analysis determined that rhein upregulated relevant gene expression levels in multiple organs of largemouth bass, with higher activities observed for the Mx, INF-γ, and IRF3 genes. Histopathological results displayed that a supplemental dosage of 0.075 % rhein significantly mitigated LMBRaV infection-induced pathological damage in the liver, spleen, and kidney. Taken together, these in vivo and in vitro results support the effective use of rhein as a promising candidate for the development of antiviral treatment against LMBRaV infection, highlighting its role in aquatic animal husbandry as a wide-spectrum antiviral and immune response booster.