Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort, 0110, Republic of South Africa.
BioScience Center, TBD International BV, Wageningen University & Research, Runderweg 6, 8219 PK, Lelystad, The Netherlands.
Parasit Vectors. 2024 Nov 20;17(1):480. doi: 10.1186/s13071-024-06561-6.
One bioassay for detecting acaricide resistance in livestock ticks is the adult immersion test (AIT), wherein engorged ticks are briefly immersed into a solution of a particular acaricidal compound and examined for mortality, their egg-laying capacity and offspring hatchability in vitro. Usually, the recommended label dose or an established discriminating dose of an acaricide is used to determine high mortality (≥ 95%) of susceptible tick strains. Such a test intends to detect the presence of resistance in a tick population. However, the adult immersion test does not directly translate the bioassay results to the predicted efficacy in the field. In this paper, we used the AIT as an initial screening bioassay supplemented with the resistance intensity test (RIT), a novel larval-based bioassay, wherein the resistance level can be determined and translated to the expected field efficacy. This was done by adopting World Health Organisation (WHO) guidelines for resistance detection in mosquitoes, which combines a 1 × recommended dose with 5 × and 10 × concentrated doses to reveal low, moderate and high resistance intensity, respectively.
Engorged Rhipicephalus microplus ticks were collected from cattle at six different ranches across Rio Grande do Sul, Brazil, as part of the state's acaricide resistance surveillance program. Groups of adult ticks from each field collection were subjected to the AIT from each field sample. Additionally, engorged female ticks from each ranch were allowed to lay eggs, and their larval progeny aged 14 to 28 days were then used in the RIT. Deltamethrin and a combination of cypermethrin, chlorpyrifos and piperonyl butoxide were used in both tests, and the results were statistically analysed.
The in vitro efficacy of deltamethrin against adult ticks in the AIT ranged between 8.74% and 25.38%. The corresponding RIT results on their larval progeny indicated a high resistance level. In the immersion test, the in vitro efficacy of the combination of cypermethrin, chlorpyrifos, and piperonyl butoxide against adult ticks ranged between 49.31% and 100%. The corresponding RIT results on their larval progeny indicated a similar response ranging from fully susceptible to low or moderate resistance. The Pearson correlation coefficient (r = 0.883) showed a high correlation between tick mortality at the 1 × recommended concentrations of acaricides in both tests.
The resistance intensity test is a valuable addition to the range of bioassays currently available for detecting acaricide resistance by determining the level of acaricide resistance. This is relevant to whether or not to continue using a particular acaricidal class for controlling cattle ticks.
检测家畜蜱抗药性的生物测定法之一是成虫浸浴试验（AIT），在此过程中，饱血的蜱会短暂浸入特定杀蜱化合物的溶液中，并检查其死亡率、产卵能力和体外孵化的后代活力。通常，使用推荐标签剂量或已建立的杀蜱剂鉴别剂量来确定敏感蜱株的高死亡率（≥95%）。这种测试旨在检测蜱种群中是否存在抗药性。然而，成虫浸浴试验并不能直接将生物测定结果转化为田间的预期功效。在本文中，我们使用 AIT 作为初始筛选生物测定法，并补充了抗药性强度测试（RIT），这是一种新的基于幼虫的生物测定法，可以确定抗药性水平并将其转化为预期的田间功效。这是通过采用世界卫生组织（WHO）针对蚊子抗药性检测的指南来实现的，该指南将推荐剂量的 1 倍与 5 倍和 10 倍浓缩剂量相结合，分别揭示低、中和高抗药性强度。
作为该州杀蜱剂抗药性监测计划的一部分，从巴西南里奥格兰德州的六个不同牧场的牛身上采集了饱血的 Rhipicephalus microplus 蜱。从每个野外样本中采集的成虫蜱的各个群体都接受了来自每个野外样本的 AIT。此外，来自每个牧场的饱血雌性蜱被允许产卵，其 14 至 28 天龄的幼虫后代随后用于 RIT。在这两个测试中都使用了溴氰菊酯和氯菊酯、毒死蜱和增效醚的混合物，并对结果进行了统计学分析。
AIT 中溴氰菊酯对成虫蜱的体外功效在 8.74%至 25.38%之间。对其幼虫后代进行的相应 RIT 结果表明存在高抗药性水平。在浸浴试验中，氯菊酯、毒死蜱和增效醚混合物对成虫蜱的体外功效在 49.31%至 100%之间。对其幼虫后代进行的相应 RIT 结果表明，其反应从完全敏感到低或中度抗药性相似。皮尔逊相关系数（r=0.883）表明，在这两个测试中，在推荐浓度的杀蜱剂对蜱的死亡率之间存在高度相关性。
抗药性强度测试是一种有价值的补充生物测定法，可通过确定杀蜱剂的抗药性水平来检测杀蜱剂的抗药性。这对于是否继续使用特定的杀蜱剂类别来控制牛蜱具有重要意义。
