Parasite Control Laboratory, Pathology Department, Federal University of Maranhão, Brazil.
Laboratorio de Farmacología Centro de Investigación Veterinaria de Tandil (CIVETAN)(CONICET-CICPBA-UNCPBA), Facultad de Ciencias Veterinarias, Universidad Nacional del Centro, Tandil, Argentina.
Ticks Tick Borne Dis. 2022 Jan;13(1):101874. doi: 10.1016/j.ttbdis.2021.101874. Epub 2021 Nov 23.
The cattle tick Rhipicephalus microplus is one of the most important ectoparasites in the tropical and subtropical regions of the world. Synthetic pyrethroids are widely used to control this tick, and the selection of resistant populations is a huge problem worldwide. The activity of thymol, a natural monoterpene, free or in combination with other compounds, has been demonstrated against different species of ticks. However, the mode of action is not fully understood. The present study aimed to evaluate the efficacy and the potential mode of action of the combination of cypermethrin and thymol on ticks from two populations with different levels of susceptibility to cypermethrin (low and high susceptibility). The isolated acaricidal activity of cypermethrin and thymol on larvae was carried out in different concentrations. The combination with different concentrations of cypermethrin and fixed concentrations of thymol (1300 µg/mL for the low susceptibility population; 690 µg/mL for the high susceptibility population) were performed. Adult engorged females were divided into five experimental groups (n = 20): 1) Control group untreated; 2) Control group: 2.0% (v/v) DMSO; 3) Thymol group: 1300 µg/mL thymol; 4) Cypermethrin group: 3700 µg/mL cypermethrin; 5) Association of cypermethrin (3700 µg/mL) + thymol (1300 µg/mL). A subgroup was used to study the efficacy of the reproductive parameters and another subgroup, with ten adults from each treatment, was used to quantify thymol and cypermethrin by HPLC chromatographic analysis. All compounds tested were effective on larvae from both populations, and the combination with thymol decreased the LC of cypermethrin (232.4 to 52.7 µg/mL) on the low-susceptibility population. The combination of thymol and cypermethrin was effective in both populations of R. microplus (reproductive performance of engorged females) when compared to the untreated control group, even with higher percent control values (pop. 1: 93.5 ± 5.6% and pop. 2: 92.7 ± 1.1%) than the group treated only with cypermethrin (pop. 1: 87.3 ± 7.3% and pop. 2: 83.5 ± 1.2%). From the HPLC analyzes, a higher concentration of cypermethrin (pop. 1: 30.3 ± 6.9 and pop. 2: 45.4 ± 17.7 ng/mg) was detected in the tissues of engorged females treated with the combination compared to analyte concentrations in groups treated with cypermethrin only (pop. 1: 12.4 ± 4.4 pop. 2: 25.5 ± 9.4 ng/mg). This was the first study to investigate the acaricidal efficacy of the combination of thymol + cypermethrin on R. microplus and demonstrate that the presence of thymol increases the concentration of cypermethrin in the internal tissues of engorged females through a possible mechanism for increasing the penetration of cypermethrin at the cuticular level.
牛蜱 Rhipicephalus microplus 是世界上热带和亚热带地区最重要的外寄生虫之一。拟除虫菊酯类化合物被广泛用于控制这种蜱，而抗性种群的选择是全世界的一个巨大问题。百里香酚，一种天然单萜烯，无论是游离状态还是与其他化合物结合，对不同种类的蜱都表现出了活性。然而，其作用模式尚不完全清楚。本研究旨在评估氯菊酯和百里香酚组合对两种对氯菊酯敏感性不同（低敏感性和高敏感性）的种群的蜱的功效和潜在作用模式。在不同浓度下进行了氯菊酯和百里香酚对幼虫的单独杀蜱活性的测定。用不同浓度的氯菊酯和固定浓度的百里香酚（低敏感性种群为 1300 µg/mL；高敏感性种群为 690 µg/mL）进行了组合。将成年饱血雌蜱分为五个实验组（n=20）：1）对照组未处理；2）对照组：2.0%（v/v）DMSO；3）百里香酚组：1300 µg/mL 百里香酚；4）氯菊酯组：3700 µg/mL 氯菊酯；5）氯菊酯（3700 µg/mL）+百里香酚（1300 µg/mL）联合组。使用一个亚组来研究生殖参数的功效，另一个亚组由每个处理的 10 只成虫组成，用于通过 HPLC 色谱分析定量测定百里香酚和氯菊酯。所有测试的化合物对两种种群的幼虫都有效，并且百里香酚的组合降低了氯菊酯的 LC（低敏感性种群从 232.4 降至 52.7 µg/mL）。氯菊酯和百里香酚的组合对 R. microplus 的两种种群（饱血雌蜱的生殖性能）都有效，与未处理的对照组相比，即使氯菊酯单独处理组的控制百分比更高（种群 1：93.5±5.6%；种群 2：92.7±1.1%）。与仅用氯菊酯处理的组（种群 1：87.3±7.3%；种群 2：83.5±1.2%）相比，从 HPLC 分析中检测到联合处理组的饱血雌蜱组织中氯菊酯的浓度更高（种群 1：30.3±6.9；种群 2：45.4±17.7 ng/mg）。这是首次研究百里香酚+氯菊酯组合对 R. microplus 的杀蜱功效，并证明百里香酚的存在通过增加氯菊酯在表皮水平的穿透性，增加了饱血雌蜱内部组织中氯菊酯的浓度。
