Anti-influenza activity of Euglena extract augmented by zinc ionophore quercetin

[Display omitted] •Euglena hydrothermal extract displays notable antiviral activity.•The main active ingredient is zinc which takes up to 16 h to penetrate cells.•Quercetin, a zinc ionophore, reduces zinc penetration time in infected cells.•Synergism of Euglena extract and quercetin shows enhanced anti-influenza effects. Euglena extract has anti-influenza virus activity, which can be enhanced using quercetin. This zinc ionophore reduces the time needed for zinc to penetrate infected cells and exerts strong anti-influenza virus activity. We investigated this phenomenon by exposing influenza A virus H1N1, cultured in Madin-Darby canine kidney cells, to a Euglena extract and quercetin solution. The viral titer was measured using the focus-forming reduction assay, and zinc levels in the cells were determined using Zinquin ethyl ester. Colby's equation was used to determine whether viral inhibition was due to a synergistic effect. Quercetin synergistically enhanced the anti-influenza virus effect of Euglena extract (inhibition rate: 89.3 % after 24 h) by rapidly inducing cellular zinc production and increasing intracellular zinc content (1.25 times higher than that of the control). This medicinal combination may provide new possibilities for preventing and treating influenza.