Purpose：To determine the active ingredient in tea tree oil (TTO) responsible for its reported killing effect on Demodex mites, the most common ectoparasite found in the human skin extending to the eye.
Methods：Using a reported in vitro killing assay to measure the survival time of adult Demodex folliculorum up to 150 minutes, we have screened serial concentrations of 13 of the 15 known ingredients of TTO that were soluble in mineral oil and examined their synergistic relationships in killing mites. The most potent ingredient was then tested for its efficacy in killing Demodex in vivo.
Results：All ingredients exhibited a dose-dependent killing effect. Besides Terpinen-4-ol, the order of relative potency did not correlate with the order of relative abundance in TTO for the remaining 12 ingredients. Terpinen-4-ol was the most potent ingredient followed by α-Terpineol, 1,8-Cineole and Sabinene. Terpinen-4-ol, the most abundant ingredient in TTO, was more potent than TTO at equivalent concentrations and its killing effect was even observable at a mere concentration of 1%. Terpinen-4-ol exhibited a significant synergistic effect with Terpinolene, but an antagonistic effect with α-Terpineol in killing mites (both P < 0.05). In vivo, Terpinen-4-ol was shown to eradicate mites.
Conclusions：The above finding suggests that deployment of Terpinen-4-ol alone should enhance its potency in killing Demodex mites by reducing the adverse and antagonistic effects from other ingredients in TTO.
Translational Relevance：Terpinen-4-ol can be adopted in future formulations of acaricides to treat a number of ocular and cutaneous diseases caused by demodicosis.
Keywords: terpinen-4-ol, tea tree oil, demodex, acaricide, ocular drug delivery