We tested the hypothesis that Haemonchus contortus and Trichostrongylus colubriformis would adapt to long-term exposure to sheep that were either genetically resistant or susceptible to H. contortus. Sheep genotypes were from lines with 10 years prior selection for low (resistant, R) or high (susceptible, S) faecal worm egg count (WEC) following H. contortus infection. Long-term exposure of H. contortus and T.colubriformis to R or S genotypes was achieved using serial passage for up to 30 nematode generations. Thus, we generated four nematode strains; one strain of each species solely exposed to R sheep and one strain of each species solely exposed to S sheep. Considerable host genotype differences in mean WEC during serial passage confirmed adequate nematode selection pressure for both H. contortus (R 4900 eggs per gram (epg), S 19,900 epg) and T. colubriformis (R 5300 epg, S 13,500 epg). Adaptation of nematode strain to host genotype was tested using seven cross-classified tests for H. contortus, and two cross-classified and one outbred genotype test for T. colubriformis. In the cross-classified design, where each strain infects groups of R, S or randomly bred control sheep, parasite adaptation would be indicated by a significant host genotype by nematode strain interaction for traits indicating parasite reproductive success; specifically WEC and, for H. contortus strains, packed cell volume. We found no significant evidence of parasite adaptation to host genotype (P>0.05) for either the H. contortus or T. colubriformis strains. Therefore, we argue that nematodes will not adapt quickly to sheep bred for nematode resistance, where selection is based on low WEC, although selecting sheep using a subset of immune functions may increase adaptation risk. Our results support the hypothesis that nematode resistance is determined by many genes each with relatively small effect. In conclusion, selection of sheep for nematode resistance using WEC should be sustainable in the medium to long-term.

中文翻译：

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捻转血矛线虫和蛇形毛圆线虫不适应长期接触对线虫感染具有遗传抵抗力或易感性的绵羊。

我们测试了以下假设：捻转血矛线虫和蛇形毛圆线虫会适应长期暴露于对捻转血矛线虫具有遗传抗性或易感性的绵羊。绵羊基因型来自 10 年前选择的品系，其感染捻转螺旋体后，粪便虫卵计数低（耐药，R）或高（易感，S）。通过连续传代多达 30 代线虫，实现了 H. contortus 和 T.colubriformis 长期暴露于 R 或 S 基因型。因此，我们产生了四种线虫菌株；每个物种的一株仅暴露于 R 羊，每个物种的一株仅暴露于 S 羊。连续传代过程中平均 WEC 存在相当大的宿主基因型差异，证实了捻转线虫（R 4900 卵每克 (epg)，S 19,900 epg）和蛇形线虫（R 5300 epg，S 13,500 epg）有足够的线虫选择压力。线虫菌株对宿主基因型的适应性通过针对H. contortus的七次交叉分类测试和针对T. colubriformis的两次交叉分类和一种远交基因型测试来测试。在交叉分类设计中，每个菌株感染 R、S 组或随机饲养的对照羊，寄生虫适应将通过线虫菌株相互作用的显着宿主基因型来指示，以表明寄生虫繁殖成功的性状；特别是 WEC，对于 H. contortus 菌株，是细胞堆积体积。我们没有发现旋扭线虫或蛇形线虫菌株对寄生虫适应宿主基因型的显着证据（P>0.05）。因此，我们认为，线虫不会很快适应为抗线虫而培育的绵羊，其中选择是基于低 WEC，尽管使用免疫功能子集选择绵羊可能会增加适应风险。我们的结果支持这样的假设：线虫抗性是由许多基因决定的，每个基因的影响相对较小。总之，利用 WEC 选育绵羊以抵抗线虫，从中长期来看应该是可持续的。