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Micropterus salmoides rhabdovirus (MSRV) is an significant pathogen that causes high mortality and related economic losses in bass aquaculture. There is no effective or approved therapy to date. In this study, we evaluated the anti‐MSRV effects of 22 quinoline derivatives in grass carp ovary (GCO) cells. Among these compounds, 8‐hydroxyquinoline exhibited valid inhibition in decreasing MSRV nucleoprotein gene expression levels of 99.3% with a half‐maximal inhibitory concentrations (IC50) value of 4.66 μM at 48 h. Moreover, 8‐hydroxyquinoline significantly enhanced a protective effect in GCO cells by reducing the cytopathic effect (CPE). By comparing the anti‐MSRV activity of 22 quinoline derivatives, we found that 8‐hydroxyquinoline possessed the efficient active site of 8‐hydroxyl and inhibited MSRV infection in vitro. For in vivo studies, 8‐hydroxyquinoline via intraperitoneal injection exhibited an antiviral effect in MSRV‐infected largemouth bass by substantially enhancing the survival rate by 15.0%. Importantly, the viral loads in the infected largemouth bass notably reduced in the spleen on the third days post‐infection. Overall, 8‐hydroxyquinoline was considered to be an efficient agent against MSRV in aquaculture. [ABSTRACT FROM AUTHOR]

Grass carp (Ctenopharyngodon idella) is subject to a hemorrhagic disease caused by grass carp reovirus (GCRV), which can lead to mass mortality in grass carp culture, causing significant economic loss. Vaccination is the most promising strategy for the prevention of infectious diseases. Immersion vaccination is considered the most effective disease prevention method for juvenile fish because it can be implemented on many fish at once and administered without causing stress. However, immune responses by immersion vaccination are markedly less robust due to the skin barrier and insufficient antigen uptake. The display of heterologous proteins on the cell surface has been explored as a delivery system for viral antigens in veterinary and human vaccine studies. To improve the efficacy of the immersion vaccine, the major capsid protein (VP7) of GCRV was co-displayed with Aeromonas hydrophila outer membrane protein a (OmpA) and major adhesion protein (Mah) on the outer membrane surface of nonpathogenic Escherichia coli BL21 using the anchoring motif of ice-nucleation protein (Inp). The immune responses and protection efficiency against GCRV infection via both the injection and immersion routes were evaluated. The results indicated that the activities of anti-oxidant enzymes (ACP, AKP, SOD and T-AOC), as well as the expression of immune-related genes (TNF-α , IL-1β , MHCI and IgM) and specific VP7 antibody levels, were strongly increased in the grass carp from 7 to 21 days post-injection inoculation in a dose dependent manner. The cumulative mortality rates of injection-vaccinated groups were much lower than those of the control group after the GCRV challenge, and the relative percent survival (RPS) was greater than 80 %. Vitally, the surface co-display of vp7-Mah protein conferred marked protection to grass carp against GCRV infection after immersion administration (RPS >50 %); this was consistent with the production of high level of specific serum antibodies, non-specific immune responses, and the expression of immune-related genes. Moreover, the invasion analysis further showed that surface co-display of the vp7-Mah protein indeed significantly improved the invasion of E. coli BL21 (DE3) in vitro. Altogether, this study demonstrated that surface display GCRV core antigen vaccine system accompanied by invasion component from aquatic pathogenic microorganism is an effective prophylactic against GCRV viral diseases via the immersion administration approach. • A recombinant surface display vaccine prevented grass carp hemorrhagic disease. • Grass carp immersed in a vaccine with surface display of vp7-Mah elicited robust immunity against GCRV. • Surface display of the vp7-Mah improved the invasion of immersed vaccine. • Surface display-based immersion vaccines have prospects for application in aquaculture. [ABSTRACT FROM AUTHOR]

• UA effectively inhibits MSRV proliferation in vitro. • UA has a protective effect on MSRV-infected fish. • UA is a valuable agent for MSRV in the industry. Micropterus salmoides rhabdovirus (MSRV) has a high mortality rate and causes huge economic losses to the aquaculture industry. In this study, we identified that ursolic acid (UA) had antiviral efficacy against MSRV in vitro and in vivo. The results showed that UA inhibited MSRV replication in grass carp ovary (GCO) cells with a half-maximal inhibitory concentration (IC 50) of 5.55 μM, reduced viral titers and decreased cytopathic effects (CPE). Mechanistically, UA does not directly damage viral particles. On the other hand, UA inhibits MSRV replication by altering viral binding and release. Furthermore, pre- and post-treatment assays revealed that UA had preventive and therapeutic effects. For in vivo studies, UA could enhance the survival rate of MSRV-infected largemouth bass. Similarly, UA reduced the viral load of MSRV in the heart, spleen and brain at 3, 5 and 7 d post-infection. In conclusion, UA is an effective inhibitor of rhabdovirus in aquaculture. [ABSTRACT FROM AUTHOR]