Interpreting in vitro bioassays used to determine resistance against acaricides in Rhipicephalus microplus can be challenging without parallel in vivo studies that assess for lost efficacy. The larval immersion test (LIT) is currently the most widely used bioassay to detect ivermectin resistance. The objective of this study was to compare results of the LIT and a field trial using ivermectin in naturally infested cattle. Criteria to consider ticks as resistant with the LIT were based on discriminating doses (DD) and the ratio of lethal concentration (LC) in test populations over the LC of the susceptible Deutch strain, known as the resistance ratio (RR). Ticks were collected from 4 farms, two where ivermectin provided good control of tick infestations and two that claimed lack of efficacy. In two farms where administration of a long-acting ivermectin formulation reduced body tick counts to 45 and 25% of the initial counts at 10-days post-treatment, the RR₅₀ and RR₉₉ were approximately 6 and 20, respectively. The LC50 value approximated the DD for the two farms with claimed resistance, suggesting that about half of the population in each farm was resistant. These LIT values are equal to those reported for the most resistant ticks, which supports the use of the LIT to predict control failure in field situations. The two farms where ivermectin provided good control of tick infestations had LC₅₀s similar to the susceptible strain, although for one farm the LC₉₉ and RR₉₉ suggested incipient resistance.

如果没有并行的体内实验，解释体外生物测定法来确定细头鼻hip对杀螨剂的抗药性可能是具有挑战性的评估疗效损失的研究。幼虫浸没试验（LIT）是目前用于检测伊维菌素耐药性的最广泛的生物测定法。这项研究的目的是比较LIT的结果和使用伊维菌素在自然感染牛中进行的田间试验。认为tick对LIT有抗性的标准是基于区分剂量（DD）和测试种群中致死浓度（LC）对易感Deutch菌株LC的致死浓度比（LC），称为抗性比（RR）。从4个农场收集虫，其中两个伊维菌素可很好地控制tick虫侵袭，另外两个声称缺乏功效。在两个使用长效伊维菌素制剂的农场中，在治疗后10天，其tick虫计数降低至初始计数的45％和25％，RR ₅₀和RR ₉₉分别约为6和20。LC50值近似于两个声称具有抗性的农场的DD，这表明每个农场中约有一半的人口具有抗性。这些LIT值等于报告的最耐resistant虫的值，这支持使用LIT预测现场情况下的控制故障。伊维菌素能够很好地控制tick虫侵袭的两个农场的LC ₅₀类似于易感菌株，尽管其中一个农场的LC ₉₉和RR _99表现出初期耐药性。