Animal Health Department, Agrifood Research and Technology Centre of Aragon (CITA), Ctra, Montañana, 930, 50059 Zaragoza, Spain.
Vet Parasitol. 2012 Mar 23;184(2-4):193-203. doi: 10.1016/j.vetpar.2011.08.020. Epub 2011 Aug 19.
A survey to determine the level of parasite resistance to benzimidazoles (BZ) under field conditions was performed on 107 commercial sheep farms located in the Aragon region of northeast Spain. Resistance was measured using the discriminant dose, a simplified form of the in vitro egg hatch assay (EHA). Taking into account the spatial structure of the data, a multivariate approach was applied to management and environmental variables as well as to their relationships with BZ resistance levels compiled from each flock. Results estimated that 11% of flocks had resistant parasite populations, although we suspected the presence of BZ-resistant parasite strains in 98% of the sample. Resistance levels were more similar among the nearest flocks, suggesting a contagious spatial distribution of resistance (i.e., resistance at neighbouring farms was not independent from one another). Management variables such as frequency of deworming, grazing in private pastures and underdosing were positively related to resistance levels, whereas only the use of BZ was negatively related to resistance levels, likely because BZ were replaced by other anthelmintics in flocks where reduced BZ efficacy was suspected. In addition to climatic conditions and seasonality, land use was an environmental variable associated with observed BZ resistance levels. Generally, resistance was highest in cooler and wetter areas but was lower in flocks sampled during January-March compared to flocks sampled in April-June or October-December. Variation partitioning procedures showed that the variation of resistance explained by the effect of environmental variables was higher than management variables. The effects of both variable groups, however, highly overlapped with the spatial structure of resistant levels, which suggests that a considerable amount of the effects attributable to both variable groups may be actually due to the spatial distribution of resistance. The resistance variation explained by the spatial component suggested that other uncontrolled factors acting at short spatial scale (e.g., common management and environmental variables; the importation of resistant strains and their posterior spread across neighbouring flocks; the selection history of the worms carried out by historical management events previous to this survey; and genetic, physiological or both types of parasite population variation) could yield this contagious spatial structure of BZ resistance. Although further research is needed, both seasonal variation and the dependence of resistance levels among neighbouring flocks should be taken into account in the design of future research or observational resistance programmes to minimise spatial and temporal pseudo-replication. Thus, research would avoid biased estimations of resistance prevalence or of its relationship with putative factors.
在西班牙东北部的阿拉贡地区，对 107 家商业绵羊养殖场进行了一项实地条件下抗苯并咪唑类药物（BZ）寄生虫水平的调查。使用判别剂量（体外卵孵化试验（EHA）的简化形式）来测量抗性。考虑到数据的空间结构，采用多变量方法来处理管理和环境变量及其与从每个羊群中收集的 BZ 抗性水平的关系。结果估计，有 11%的羊群存在具有抗性的寄生虫种群，但我们怀疑在 98%的样本中存在 BZ 抗性寄生虫株。最近的羊群之间的抗性水平更为相似，这表明存在传染性的抗性空间分布（即，相邻农场之间的抗性不相互独立）。驱虫频率、在私人牧场放牧和剂量不足等管理变量与抗性水平呈正相关，而仅使用 BZ 与抗性水平呈负相关，这可能是因为在怀疑 BZ 疗效降低的羊群中，BZ 被其他驱虫药所取代。除了气候条件和季节性外，土地利用是与观察到的 BZ 抗性水平相关的环境变量。一般来说，在较凉爽和潮湿的地区，抗性最高，但与 4 月至 6 月或 10 月至 12 月采集的羊群相比，1 月至 3 月采集的羊群的抗性较低。变分分配程序表明，由环境变量效应解释的抗性变异高于管理变量。然而，这两个变量组的影响都与抗性水平的空间结构高度重叠，这表明归因于这两个变量组的大量影响实际上可能是由于抗性的空间分布。由空间组成部分解释的抗性变化表明，在短空间尺度上作用的其他不受控制的因素（例如，共同的管理和环境变量；抗性菌株的引入及其随后在相邻羊群中的传播；本调查之前历史管理事件对蠕虫的选择历史；以及寄生虫种群的遗传、生理或两者的变异）可能导致 BZ 抗性的这种传染性空间结构。尽管需要进一步研究，但在设计未来的研究或观察性抗性计划时，应考虑季节性变化和相邻羊群之间的抗性水平的依赖性，以最大限度地减少空间和时间上的伪重复。因此，研究将避免对抗性流行率或其与假定因素的关系的有偏差估计。
