Umbelliferone immersion therapy controls viral hemorrhagic septicemia virus in olive flounder (Paralichthys olivaceus) via direct virucidal and immunomodulatory effects.

Viral hemorrhagic septicemia virus (VHSV) is a major threat to global aquaculture, but no effective therapeutics currently exist. In this study, we identified umbelliferone (7-hydroxycoumarin) as a potent immersion-based antiviral agent against VHSV. Umbelliferone showed complete direct virucidal effects in vitro concentration- and time-dependently. Molecular docking predicted umbelliferone may bind to the pleckstrin homology domain of VHSV glycoprotein, a possible mechanism for viral inhibition. Immersion treatment of olive flounder (Paralichthys olivaceus) decreased the VHSV replication in naïve recipients and infected donors after cohabitation for 72 h. Efficacy of umbelliferone on the challenged fish was tested, with the efficacy of immersion treatment increasing with shorter immersion periods. Immersion for 2 h × 3 days at 10 mg/L had the highest efficacy, improving survival up to 90 % (P < 0.0001) by suppressing viral gene expression and the viral titer. Umbelliferone was found to modulate key innate immune responses, including the downregulation of ifn-α and the upregulation of ifn-γ, isg15 in the kidneys, and proinflammatory cytokines in the skin. Subsequent pharmacokinetic analysis based on immersion treatment at 10 mg/L revealed rapid distribution, and elimination in olive flounder. Cmax being lower than in vitro IC50 of 13.86 μg/mL suggested pivotal role of immunomodulation than direct virucidal activity in conferring efficacy. Umbelliferone exhibited a half-life of 8.1 days at 15 °C in seawater, highlighting its fast degradation in environment and warranting environmental safety assessment. Overall, umbelliferone immersion treatment is safe and practical, positioning it as a promising candidate for the control of VHS in aquaculture.