Evaluation on the antiviral activity of aloe emodin against Micropterus salmoides rhabdovirus in vitro and in vivo

Micropterus salmoides rhabdovirus (MSRV) is a highly pathogenic rhabdovirus that has caused significant economic losses in the aquaculture industry of largemouth bass. To date, there is no approved treatment for MSRV infection. In order to address this urgent need, virtual screening of the natural product monomer compound library targeting MSRV glycoprotein was conducted and the antiviral activity of 13 natural compounds identified in the screening was studied. Among these compounds, Aloe emodin (AE) exhibited high binding affinity and antiviral activity, inhibiting MSRV replication in grass carp ovary (GCO) cells with a half-maximal inhibitory concentration (IC₅₀) of 2.55 μM. Additionally, viral titers and cytopathic effects (CPE) were significantly reduced in the presence of AE. AE was demonstrated to directly destroy MSRV virions and inhibit their adsorption to GCO cells according to ultracentrifugation assay and viral binding assay. In vivo studies demonstrated that AE injection increased the survival rate of MSRV-infected largemouth bass by 63.3 %, which was 46.7 % higher than that of the virus group. RT-qPCR and pathological tissue section results demonstrated that AE treatment could significantly reduce viral loads in the liver, spleen, and kidney on days 1, 4, and 7 post-infection, and AE effectively mitigated tissue damage caused by MSRV infection. Circular dichroism (CD) spectroscopy and Isothermal titration calorimetry (ITC) showed that AE induced changes in the secondary structure of the MSRV glycoprotein and exhibited high affinity for the glycoprotein (Kd = 2.803 × 10⁻⁵ M, ΔG = −25.99 kJ/mol). These results suggested that AE was a natural product with potent anti-MSRV activity through direct targeting of the glycoprotein.