Leathwick D M, Vlassoff A, Barlow N D
AgResearch, Flock House Agricultural Centre, Bulls, New Zealand.
Int J Parasitol. 1995 Dec;25(12):1479-90. doi: 10.1016/0020-7519(95)00059-3.
A model for nematodiasis in lambs was expanded to incorporate both the contribution of ewes to nematode epidemiology and the genetic parameters required to simulate the development of anthelmintic resistance in the nematode population. The expanded model was used to assess the impact of various drench and grazing management strategies for ewes and lambs on the rate of development of anthelmintic resistance. Three grazing management options, under a range of drenching schedules, were compared: one in which lambs and ewes were rotationally grazed as separate flocks over the same area after weaning (common grazing); a second in which lambs were grazed, after weaning, on areas from which ewes were excluded (separate grazing); and a third in which lambs were moved to "safe" pasture at weaning and again in early autumn (integrated control). Drenching strategies examined under the first 2 grazing options included a 5 lamb-drench "preventive" programme with 0, 1, 2, 3 or 4 additional lamb drenches, and 0 or 1 ewe drench treatment at either tall-docking or mating. Under the third grazing option, lambs were given either 1 or 2 drench treatments at or following each move to safe pasture and ewes 0 or 1 drench treatment at either tail-docking (i.e., 3-4 weeks after lambing) or mating. Model output suggests that drenching ewes prior to any lamb drenching programme is likely to significantly increase selection for drench resistance by pre-selecting the larval challenge to the lambs and, under some grazing systems, by reducing the diluting effect of eggs of susceptible genotypes passed by undrenched ewes. The results highlight the potential importance of undrenched ewes as a refuge for susceptible worm genotypes and indicate that on its own, drenching frequency is likely to be a poor indicator of selection pressure for resistance and thus of limited value in selecting strategies for the management of anthelmintic resistance.
一个针对羔羊线虫病的模型得到扩展，纳入了母羊在线虫流行病学中的作用以及模拟线虫种群抗驱虫药耐药性发展所需的遗传参数。扩展后的模型用于评估针对母羊和羔羊的各种驱虫及放牧管理策略对驱虫药耐药性发展速度的影响。在一系列驱虫计划下，比较了三种放牧管理方案：一种是断奶后羔羊和母羊作为单独的畜群在同一区域轮牧（共同放牧）；第二种是断奶后羔羊在母羊被排除的区域放牧（分开放牧）；第三种是断奶时以及初秋时将羔羊转移到“安全”牧场（综合控制）。在前两种放牧方案下研究的驱虫策略包括一个给5只羔羊驱虫的“预防性”方案，再额外进行0、1、2、3或4次羔羊驱虫，以及在剪尾或配种时对母羊进行0次或1次驱虫处理。在第三种放牧方案下，每次将羔羊转移到安全牧场时或之后给羔羊进行1次或2次驱虫处理，在剪尾（即产羔后3 - 4周）或配种时给母羊进行0次或1次驱虫处理。模型输出结果表明，在任何羔羊驱虫计划之前先给母羊驱虫，可能会通过预先选择对羔羊的幼虫感染，以及在某些放牧系统中通过减少未驱虫母羊排出的易感基因型虫卵的稀释作用，显著增加对驱虫药耐药性的选择。结果突出了未驱虫母羊作为易感蠕虫基因型避难所的潜在重要性，并表明仅就驱虫频率而言，它可能是耐药性选择压力的一个较差指标，因此在选择驱虫药耐药性管理策略方面价值有限。
