Antiviral activity of Australian tea tree oil and eucalyptus oil against herpes simplex virus in cell culture

The antiviral effect of Australian tea tree oil (TTO) and eucalyptus oil (E UO) against herpes simplex virus was examined. Cytotoxicity of TTO and EUO was evaluated in a standard neutral red dye uptake assay. Toxicity of TTO a nd EUO was moderate for RC-37 cells and approached 50% (TC50) at concentrat ions of 0.006% and 0.03%, respectively, Antiviral activity of TTO and EUO a gainst herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) was tested in vitro on RC-37 cells using a plaque reduction assay. The 50% inhibitory concentration (IC50) of TTO for herpes simplex virus pla que formation was 0.0009% and 0.0008% and the IC50 of EUO was determined at 0.009% and 0.008% for HSV-1 and HSV-2, respectively. Australian tea tree o il exhibited high levels of virucidal activity against HSV-1 and HSV-2 in v iral suspension tests. At noncytotoxic concentrations of TTO plaque formati on was reduced by 98.2% and 93.0% for HSV-1 and HSV-2, respectively. Noncyt otoxic concentrations of EUO reduced virus titers by 57.9% for HSV-1 and 75 .4% for HSV-2. Virus titers were reduced significantly with TTO, whereas EU O exhibited distinct but less antiviral activity. In order to determine the mode of antiviral action of both essential oils, either cells were pretrea ted before viral infection or viruses were incubated with TTO or EUO before infection, during adsorption or after penetration into the host cells. Pla que formation was clearly reduced, when herpes simplex virus was pretreated with the essential oils prior to adsorption. These results indicate that T TO and EUO affect the virus before or during adsorption, hut not after pene tration into the host cell. Thus TTO and EUO are capable to exert a direct antiviral effect on HSV. Although the active anti herpes components of Aust ralian tea tree and eucalyptus oil are not yet known. their possible applic ation as antiviral agents in recurrent herpes infection is promising.