Although Humphrey (1930) suggested the suitability of tea tree oil for treatment of vaginal infections, the first report in the literature of its effectiveness in treating such infections is that of Pena (1962). He reported clinical cure following treatment of 130 cases of vaginal infections, due mostly to Trichomonas (116 cases) and Candida albicans (4 cases). The treatment regime involved douching with a 0.4% solution of Melaleuca alternifolia oil and insertion of tampons saturated with a 20-40% solution of the oil. This treatment was reported to be as effective as use of standard antitrichomonal suppositories, and without side-effects of irritation and burning.
Twenty-eight chronic cases of vaginal infection caused by Candida albicans were treated by nightly insertion of a pessary containing 20mg of tea tree oil for a period of three months (Belaiche 1985b). Infection and symptoms were alleviated in 21 cases, with an improvement in symptoms, but persistence of the yeast, in a further three cases. The author also comments that the preparation was well tolerated by all but one of the participants who withdrew from the study early in the first week. Barnes (1990) also reports the alleviation of symptoms of vaginal irritation and burning in a number of women following treatment with a tea tree cream and/or douche. The women in this report all had chronic conditions which had not responded to conventional treatments. Once again, the evidence which suggests that tea tree oil is useful both for its antimicrobial activity and its soothing and pain-relieving effects, is anecdotal, and appropriately designed and controlled clinical trials are needed to establish a sound basis for the marketing of therapeutically useful vaginal products.
The role of essential oils within the plants producing them, is primarily one of defence of the plant from attack by other organisms. It is possible that tea tree oil will find many marketable applications in agriculture through exploitation and expansion of this natural function. Essential oils would have the advantage over synthetic chemicals, such as those currently used to control post-harvest pathogens, of being more acceptable both environmentally and to the consumer. Potential applications include control agents of plant pathogens, insect repellants and antifeedants and insecticides. Terpinen-4-ol, the major ingredient of tea tree oil, has been shown to be very active as a repellant of the yellow-fever mosquito, Aedes aegyptii (Hwang et al. 1985). Bishop and Thornton (1997) demonstrated the ability of tea tree oil to inhibit hyphal growth of fifteen common fungal post-harvest pathogens. Whilst direct contact was more effective, the oil also demonstrated significant antifungal activity in the vapour phase, a characteristic which suggests the possibility of its use as a fumigant for stored crops.
There are very few reports of field trials testing the efficacy of tea tree oil in the control of fungal or viral pathogens of economically important crops. A 1% aqueous solution of tea tree oil was reported to control powdery mildew of greenhouse-grown cucurbits caused by the fungus Sphaerotheca fuliginea (Olsen et al. 1988). In another study, it was reported that Nicotiana glutinosa plants sprayed with 100, 250 and 500 ppm of tea tree oil in distilled water prior to inoculation with the Tobacco Mosaic Virus showed significantly fewer lesions than control plants for 10 days following inoculation (Bishop 1995). Neither of these studies distinguish between microbial inactivation and inhibition of infection through changes to the host plant, and, hence, conclusions cannot be drawn about the mechanism of action. However, these studies indicate the potential of tea tree oil in the treatment of a variety of plant diseases.
The range of possibilities for products containing tea tree oil as an active ingredient is vast. The uses reported in the literature are as diverse as its use as an additive in an aerosol system used for cleansing of air conditioning systems (Ryan 1990), its potential for addition to laundry detergents as an acaricidal agent to destroy mites in bedding and clothing (McDonald and Tovey 1993) and its use in burn preparations for its properties of soothing the damaged tissue, rapid healing, prevention of infection and pain relief (Price 1989).
The literature indicates an interest in the role of plant volatile oils as antioxidants (Deans and Waterman 1993). Essential oils have been shown to act as hepatoprotective agents in aging mammals and to have a positive effect upon docohexanoic acid levels in aging rodent retinas. In a recent study, essential oils of geranium, monarda, nutmeg, oregano and thyme, which contain a number of monoterpenes also present in tea tree oil, demonstrated extensive antioxidant capacities at final concentrations of 0.75 ppm to 100ppm (Dorman et al. 1995). Once the active ingredients in the oils have been elucidated, the potential of tea tree oil as an antioxidant can be assessed.
Tea tree oil has a well-established reputation, supported by laboratory data, as an effective, well-tolerated, broad spectrum antimicrobial which possesses a number of advantages over its synthetic counterparts. Results of a limited number of clinical trials have been promising, but further clinical testing, both of standard oils of known chemical composition and of formulated products, is required to enable tea tree oil to broaden its acceptance in the marketplace. Although its antimicrobial activity is well established, little is understood about the way in which it acts on microbial cells. A more rational approach to breeding programs and to the incorporation of tea tree oil in formulated products will be possible once the mode of action of tea tree oil against a range of microbial cell types is elucidated.
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