Herpes simplex virus type 1 (HSV-1) is a widespread human pathogen that causes epidermal lesions in and around the oral cavity. The symptoms caused by herpes infections are usually self-limiting but can be extensive and prolonged in immunocompromised patients. All members of the family Herpesviridae are able to establish a latent infection (e.g. HSV in the nervous system that can be reactivated quite frequently). Several drugs are currently available for the management of HSV infections, such as aciclovir. Aciclovir, a guanine nucleoside analogue, has been widely used for the therapy of herpesvirus infections; its preferential phosphorylation by the HSV-encoded thymidine kinase (TK) makes it a selective antiviral drug (Cassady & Whitley, 1997; De Clercq 2004). The HSV-1 TK is a multisubstrate enzyme that phosphorylates a broad spectrum of purine and pyrimidine nucleoside substrate analogues. In the case of aciclovir-resistant herpetic infections, antiviral therapy with foscarnet or cidofovir is indicated. In immunocompromised patients, a prolonged antiviral therapy is required, resulting in the emergence of drug-resistant mutants in approximately 4–7% (Christophers et al 1998; Chakrabarti et al 2000; Chen et al 2000). This trend has led to the search for alternative anti-herpetic agents that have a wide range of efficacy without serious adverse effects, and which are effective against viral strains resistant to current antiviral agents.
Essential oils of many plants have been widely used in traditional medicine (Reichling 1999). Recently, the anti-herpes activity of several essential oils from different plant sources, as well as of various constituents of essential oils, was demonstrated (Sivropoulou et al 1997; Benencia & Courre`ges 1999; Bourne et al 1999; De Logu et al 2000).