An aqueous extract of root/stem bark of Rhus aromatica AIT. reveals antiviral activity against herpes simplex virus in cell culture

The sweet or fragrant sumach (Rhus aromatica) is a small shrub of Eastern United States. Amtimicrobial properties of Rhus extracts have been described and we explored the antiviral potency of an aqueous extract of root/stem bark of Rhus aromatica against herpes simplex virus type 1 (HSV-1) in cell culture (RC-37 cells) using a plaque reduction assay. The cytotoxic concentration of the bark extract which reduced viable cell number by 50% (CC₅₀) and the inhibitory concentration of the bark extract which inhibited plaque numbers by 50% (IC₅₀) were determined from dose-response curves. The bark extract exhibited a high level of anti-HSV-1 activity with an IC₅₀-value of 0.02% (20µg/ml) and a selectivity index (CC₅₀/IC₅₀) of 125. In order to determine the mode of antiviral action, the bark extract was added at different times to the cells or viruses during the infection cycle. At maximum noncytotoxic concentration (0.25%; 250µg/ml), plaque formation was significantly reduced by more than 99.9% when HSV-1 was pretreated with the bark extract or when the extract was added during the adsoprtion phase. Surprisingly, when the host cells were treated with the bark extract for one hour prior to virus infection the infectivity of HSV was reduced by 70%. The aqueous bark extract seems to interact not only with the viral envelope but also with the surface of the host cells impairing the ability of HSV-1 to adsorb to and penetrate into the host cells. In conclusion, a strong virucidal activity of Rhus extract against herpesvirus was demonstrated.