Acaricidal bioactivity and molecular target analysis of Origanum onites and Ocimum gratissimum essential oils against Haemaphysalis doenitzi ticks.

Background: Haemaphysalis doenitzi is a parasite mainly found on the body surface of birds that is capable of transmitting rickettsiae and borrelia, which can cause serious zoonotic diseases. Chemical acaricides are controversial because they pollute the environment and predispose ticks to resistance. In contrast, plant essential oils (EOs) are favored for their effective acaricide properties and environmental friendliness.

Methods: The constituents of Origanum onites and Ocimum gratissimum EOs were profiled using gas chromatography-mass spectrometry (GC-MS). Acaricidal activities were evaluated by immersing unfed nymphs and adults of H. doenitzi in serial solutions for 5 min and monitoring mortality after 24 h. Enzyme activities (Na⁺/K⁺-ATPase, GST, CarE, AChE) and transcript levels of HDABCE1, HDCYP450a and HD-GSTa were quantified in homogenates of treated survivors. Homology models of the three target proteins were generated and docked with carvacrol and eugenol to predict binding sites and affinities.

Results: GC-MS analysis showed 93.3% of carvacrol in O. onites EO and 66.68% of eugenol in O. gratissimum EO. Immersion test showed that O. onites EO had significant acaricidal activity against nymphs and adults, with median lethal concentration (LC₅₀) values of 14.694 mg/ml and 24.357 mg/ml, respectively. Ocimum gratissimum EO showed significant acaricidal activity against nymphs and adults, with LC₅₀ values of 9.379 mg/ml and 18.299 mg/ml, respectively. Carvacrol also showed significant acaricidal activity against unfed nymphs and adults. Repellency tests showed that O. onites EO had more significant repellent activity against nymphs than DEET, with median effective concentration (EC₅₀) values of 2.162 mg/ml and 7.039 mg/ml, respectively. To explore the molecular mechanisms of O. onites EO and carvacrol on ticks, we investigated the enzyme activity and gene expression of ATP-binding cassette transporter, cytochrome P450, and glutathione S-transferase. Finally, molecular docking was used to verify the enzyme effects.

Conclusions: The results of this study provide important insight into the toxicity mechanisms of ticks, and indicate that carvacrol and O. onites EO can be used as alternatives to chemically synthesized acaricides.