The Tea tree, Melaleuca alternifolia, is a tree of the Myrtaceae family native to Australia. Its medicinal properties were already known to the aborigines who used it in the manufacture of poultices to treat infected wounds, several centuries ago. Its antiseptic and antibacterial properties have been confirmed since the 1920s. Tea tree essential oil was part of the Australian Army and Navy first aid kit until the advent of antibiotics.
It is traditionally used for oral and upper respiratory ailments, dermatological diseases such as herpes, shingles, acne, skin abscesses and the treatment of wounds. Its most important component is terpinene-4-ol, which makes up between 30 and 40% of its composition.
Tea tree essential oil decreases the bacterial concentration in otitis externa as well as inflammation and pruritus (NEVES & al., 2018). Like many other essential oils, it exerts part of its anti-bacterial effect by interacting with the cell membrane of bacteria (PAZYAR & al., 2012 HAMMER K.A, 2014 ZHANG et al., 2017). It causes potassium ions to leak, disrupting the cellular respiration process in Staphylococcus aureus. It is effective against MRSA. It also causes cell lysis of E. Coli and inhibits the growth of Pseudomonas Aeruginosa. It also reduces the formation of biofilms in Gram positive and Gram negative bacteria (ZHANG & al., 2017 BRUN & al., 2018 OLIVA et al., 2018). The terpenes, alcohols and ethers present in tea tree essential oil interfere with the chemical signals involved in quorum sensing (BRUN & al., 2018).
Terpinen-4-ol decreases the production of pro-inflammatory cytokines such as interferon ϒ, IL-1β, IL-6, IL-8, IL-10, IL-17 and PGE 2, among others. It also decreases histamine-induced edema in type I hypersensitivity reactions (PAZYAR & al., 2012 NEVES & al., 2020).
Terpinene-4-ol and α-terpineol inhibit the production of superoxide by monocytes. Tea tree essential oil is able to reduce DPPH, inhibit lipid peroxidation and eliminate hydroxyl radicals (PAZYAR & al., 2012 ZHANG & al., 2017).
Tea tree essential oil has a fungicidal effect in vitro on fungi of the Candida. It has an efficacy comparable to nystatin against Malassezia. It also improves symptoms during onychomycosis (GUPTA & al., 2004 PAZYAR & al., 2012). It synergizes with ketoconazole and itraconazole against Trichophyton rubrum, reducing their minimum inhibitory concentration to 8 times and vice versa (ROANA & al., 2021).
According to a 2014 review, tea tree essential oil reduces inflammatory lesions associated with acne by 49% after 3 months of treatment. For non-inflammatory lesions, the decrease is comparable to that caused by benzoyl peroxide (a standard treatment for acne). When compared to a gel containing erythromycin, it allows a 55% reduction in lesions in 87.5% of patients compared to 40% in 53.8% of patients for erythromycin gel, after treatment of 6 weeks. Its beneficial effect on acne is explained by the inhibition of the production of pro-inflammatory cytokines and the growth of P. acnes (HAMMER K.A, 2014).
In 2020, NEVES & al. demonstrated the in vitro lethal effect of tea tree on Demodex canis. Demodex exposed to tea tree essential oil will die after 4 to 214 min. Those exposed to amitraz succumb after 268 to 434 min and those exposed to placebo 436 to 540 min. This lethal effect has also been demonstrated on human demodex (PAZYAR & al., 2012 NEVES & al., 2020). It is also effective against Sarcoptes scabei (NEVES & al., 2020). Sarcoptes scabei var. homini is completely eradicated in vitro within 3 hours with a solution containing 5% tea tree essential oil (PAZYAR & al., 2012).
Terpinen-4-ol also exerts a lethal effect on Otodectes cynotis in vitro : 85% of mites die after 1 h of exposure to a solution containing 2.1% of terpinen-4-ol. Whereas only 10% of mites die after 1 hour exposure to ivermectin or a solution containing 5% permethrin. In the placebo control, 100% of otodects survive (WALTON & al., 2004).
Tea tree essential oil rich in 1,8-cineole has an adulticidal effect on stomox flies (S. calcitrans) (DILLMAN & al., 2020). Its insecticidal effect is explained by the blockade of acetylcholinesterase in parasites (MILLS & al., 2003 PAZYAR & al., 2012 NEVES & al., 2020).
Tea tree essential oil is effective against HSV-1 and HSV-2. It affects the virus before or during its absorption by host cells (PAZYAR & al., 2012 BRUN & al., 2018).
