Anthelmintic resistance in animal parasitic nematodes: Mechanisms, genetics, diagnosis, extent, fitness cost, prevention and management

加拿大的畜牧业很重要；仅牛肉一项，每年就达300亿美元。线虫寄生虫使回报率降低了7%。寄生虫的控制依赖于驱虫剂。在美国，50%的牛群被大环内酯(ML)抗性线虫寄生，加拿大的情况也类似。线虫寄生虫一旦失控，将严重影响动物生产和福利。我们需要了解抗药性的机制和遗传学(I)开发敏感的抗药性测试，(Ii)防止高水平抗药性，(Iii)管理现有的抗药性，和(Iv)克服抗药性。 目的：1.比较秀丽线虫和扭曲血吸虫敏感和抗性模式种群的遗传和表型特征。 2.评估旋转毛虫的田间种群，以确定是否存在类似的作用机制。 调查其他重要寄生虫是否存在类似的机制，例如牛的铜绿假单胞菌。3.利用分子标记评价加拿大牛、羊的抗性水平。 4.评价抗性的适合度成本。 5.在了解抗性的基础上，制定减少抗性选择的寄生虫防治策略。 科学方法：了解具有良好特征的ML模式线虫、线虫和螺旋线虫敏感和抗性种群的遗传变化。我们有通过暴露在MLS中而被选择抗药性的线虫种群。我们发现两栖动物的感觉神经元有缺陷。我们在ML耐药螺旋杆菌的神经元中发现了类似的缺陷。这被追溯到这两个物种的dyf-7基因。然而，对更多对ML耐药的线虫和扭曲线虫的检测表明，其他影响双突神经元形态的基因也可能参与其中。我们的数据表明，ML抗性可能发生在两个步骤，最初驱虫剂在神经元中的浓度可能通过ABC外排泵的过度表达而降低，然后是改变两面体形态的变化。我们假设联合作用降低了神经元离子通道受体上的ML水平。这将为抗病田间种群的后续研究提供信息。这是发展抗药性分子诊断的关键一步。我们将重点关注少数经济上重要的寄生虫和加拿大最重要的驱虫药MLS。将评估寄主和自由生活阶段的一般健康状况。了解抗性和适应成本的机制和遗传学对于制定控制线虫寄生虫的管理策略而不选择抗性或失去控制至关重要。 影响：在加拿大，耐药性是一个问题，需要通过了解机制、开发新的诊断、管理策略和出色的HQP培训来采取新的方法。

Canada's livestock industries are important; beef alone >$30 b/year. Nematode parasites reduce returns by 7%. Parasite control depends on anthelmintics. In USA, 50% of the cattle herds are parasitized with macrocyclic lactone (ML) resistant nematodes and the situation appears similar in Canada. A loss of control of nematode parasites, will seriously impact animal production and welfare. We need to understand mechanisms and genetics of resistance (i) to develop sensitive tests for resistance, (ii) to prevent high level resistance, (iii) manage existing resistance, and (iv) overcome resistance. Objectives: 1. To compare the genetic and phenotypic profiles of susceptible and anthelmintic resistant model populations of Caenorhabditis elegans and Haemonchus contortus. 2. To evaluate field populations of H. contortus to determine whether similar mechanisms operate. To investigate whether similar mechanisms occur in other important parasites, e.g., Cooperia oncophora in cattle. 3. To evaluate the level of resistance in Canadian cattle and sheep, using molecular markers. 4. To evaluate fitness cost of resistance. 5. Based on understanding resistance, to develop parasite control strategies which reduce selection for resistance. Scientific approach: To understand the genetic changes in well characterized ML susceptible and resistant populations of model nematodes, C. elegans and H. contortus. We have populations of C. elegans selected for resistance by exposure to MLs. We find a defect in sensory neurons in the amphids. We see a similar defect in neurons in ML resistant H. contortus. This was traced to the dyf-7 gene in both species. However, examination of more isolates of ML resistant C. elegans and H. contortus has revealed that other genes which affect amphid neuron morphology may also be involved. Our data suggests ML resistance may arise in 2 steps, initially the concentration of the anthelmintic in the neurons may be lowered by overexpression of ABC efflux pumps and this is followed by changes which alter amphid morphology. We hypothesize the combined effects reduce the ML level at ion channel receptors in neurons. This will provide us with information for subsequent studies in resistant field populations. This is an essential step in developing molecular diagnostics for resistance. We will focus on a small number of economically important parasites and the most important anthelmintics in Canada, the MLs. Assessments of general fitness in the host and in free-living stages will be assessed. Understanding the mechanisms and genetics of resistance and fitness cost are critical for developing management strategies for controlling nematode parasites without selection for resistance or loss of control. Impact: In Canada resistance is a problem requiring novel approaches by understanding the mechanisms and developing novel diagnostics, management strategies and excellent HQP training.