Grenfell B T, Wilson K, Isham V S, Boyd H E, Dietz K
Department of Zoology, University of Cambridge, UK.
Parasitology. 1995;111 Suppl:S135-51. doi: 10.1017/s0031182000075867.
The characteristically aggregated frequency distribution of macroparasites in their hosts is a key feature of host-parasite population biology. We begin with a brief review of the theoretical literature concerning parasite aggregation. Though this work has illustrated much about both the sources and impact of parasite aggregation, there is still no definite analysis of both these aspects. We then go on to illustrate the use of one approach to this problem--the construction of Moment Closure Equations (MCEs), which can be used to represent both the mean and second moments (variances and covariances) of the distribution of different parasite stages and phenomenological measures of host immunity. We apply these models to one of the best documented interactions involving free-living animal hosts--the interaction between trichostrongylid nematodes and ruminants. The analysis compares patterns of variability in experimental infections of Teladorsagia circumcincta in sheep with the equivalent wildlife situation--the epidemiology of T. circumcincta in a feral population of Soay sheep on St Kilda, Outer Hebrides. We focus on the relationship between mean parasite load and aggregation (inversely measured by the negative binomial parameter, k) for cohorts of hosts. The analysis and empirical data indicate that k tracks the increase and subsequent decline in the mean burden with host age. We discuss this result in terms of the degree of heterogeneity in the impact of host immunity or parasite-induced mortality required to shorten the tail of the parasite distribution (and therefore increase k) in older animals. The model is also used to analyse the relationship between estimated worm and egg counts (since only the latter are often available for wildlife hosts). Finally, we use these results to review directions for future work on the nature and impact of parasite aggregation.
宿主体内大型寄生虫典型的聚集频率分布是宿主 - 寄生虫种群生物学的一个关键特征。我们首先简要回顾一下有关寄生虫聚集的理论文献。尽管这项工作已经阐明了很多关于寄生虫聚集的来源和影响，但对于这两个方面仍没有确切的分析。然后我们继续说明解决这个问题的一种方法——构建矩封闭方程（MCEs），它可用于表示不同寄生虫阶段分布的均值和二阶矩（方差和协方差）以及宿主免疫的现象学指标。我们将这些模型应用于有充分记录的涉及自由生活动物宿主的相互作用之一——毛圆线虫与反刍动物之间的相互作用。该分析将绵羊体内环形泰勒虫实验感染的变异性模式与类似的野生动物情况——外赫布里底群岛圣基尔达岛索艾羊野生种群中环形泰勒虫的流行病学进行了比较。我们关注宿主群体中平均寄生虫负荷与聚集之间的关系（通过负二项分布参数k的倒数来衡量）。分析和实证数据表明，k随着宿主年龄的增长跟踪平均负担的增加以及随后的下降。我们根据宿主免疫或寄生虫诱导的死亡率在缩短老年动物体内寄生虫分布的尾部（从而增加k）方面的异质性程度来讨论这一结果。该模型还用于分析估计的蠕虫数量和虫卵数量之间的关系（因为对于野生动物宿主通常只能获得后者）。最后，我们利用这些结果来审视未来关于寄生虫聚集的性质和影响的研究方向。
