The efficacy of essential oil components with ivermectin against Rhipicephalus annulatus: An in-vitro study

Ivermectin (Iv) is widely used to control Rhipicephalus annulatus infestations in cattle in Egypt. However, increasing resistance to ivermectin necessitates alternative approaches. This study aimed to address this resistance by investigating the interactions between ivermectin and essential oil components (EOCs)-carvacrol, cinnamaldehyde, D-limonene, trans-anethole, and thymol-against four R. annulatus tick isolates collected from Beni-Suef, El-Minia, and El-Fayoum. The larval immersion test was conducted using various ivermectin concentrations (3, 1.5, 0.75, 0.38, 0.19, 0.09, 0.045, 0.025, and 0.0125 mg/mL) prepared in EthTX1 %. Additionally, EOCs were tested at different concentrations (20, 10, 5, and 2.5 mg/mL) prepared by dilution in 5 % ethyl alcohol and EthTX 1 %. Ivermectin and each EOC were tested individually against the isolates. Subsequently, the LC50 of ivermectin was combined with the LC50 of each EOC in a 1:1 ratio to evaluate potential synergistic effects. All EOCs demonstrated significant toxicity against the isolates. The combination of carvacrol and thymol with ivermectin increased mortality, but the combination index (CI) exceeded one for all isolates, indicating no synergism. Similarly, while D-limonene, cinnamaldehyde, and trans-anethole also resulted in high mortality rates, including up to 100 %, no synergism was observed (CI > 1). However, these three EOCs exhibited CIs close to 1, suggesting an additive effect when combined with ivermectin. In conclusion, combining ivermectin with certain EOCs, particularly D-limonene, cinnamaldehyde, and trans-anethole, may offer an effective tool for controlling cattle ticks. Developing topical or pour-on formulations of ivermectin-EOC combinations could be an integral part of integrated management strategies to combat resistance to macrocyclic lactones like ivermectin. Further studies, including tests on adult ticks and field trials, are needed to evaluate the efficacy of these combinations.