Department of Infectious Disease Epidemiology, Faculty of Medicine (St. Mary's Campus), Imperial College London, Norfolk Place, London W2 1PG, UK.
Int J Parasitol. 2010 Oct;40(12):1381-8. doi: 10.1016/j.ijpara.2010.07.001. Epub 2010 Jul 8.
The importance of the mode of acquisition of infectious stages of directly-transmitted parasitic helminths has been acknowledged in population dynamics models; hosts may acquire eggs/larvae singly in a "trickle" type manner or in "clumps". Such models have shown that the mode of acquisition influences the distribution and dynamics of parasite loads, the stability of host-parasite systems and the rate of emergence of anthelmintic resistance, yet very few field studies have allowed these questions to be explored with empirical data. We have analysed individual worm weight data for the parasitic roundworm of humans, Ascaris lumbricoides, collected from a three-round chemo-expulsion study in Dhaka, Bangladesh, with the aim of discerning whether a trickle or a clumped infection process predominates. We found that hosts tend to harbour female worms of a similar weight, indicative of a clumped infection process, but acknowledged that unmeasured host heterogeneities (random effects) could not be completely excluded as a cause. Here, we complement our previous statistical analyses using a stochastic infection model to simulate sizes of individual A. lumbricoides infecting a population of humans. We use the intraclass correlation coefficient (ICC) as a quantitative measure of similarity among simulated worm sizes and explore the behaviour of this statistic under assumptions corresponding to trickle or clumped infections and unmeasured host heterogeneities. We confirm that both mechanisms are capable of generating aggregates of similar-sized worms, but that the particular pattern of ICCs described pre- and post-anthelmintic treatment in the data is more consistent with aggregation generated by clumped infections than by host heterogeneities alone. This provides support to the notion that worms may be acquired in clumps. We discuss our results in terms of the population biology of A. lumbricoides and highlight the significance of our modelling approach for the study of the population dynamics of helminth parasites.
直接传播寄生虫蠕虫的感染阶段的获得方式在种群动态模型中得到了认可；宿主可能以“涓滴”方式或“成簇”方式单独获得卵/幼虫。这些模型表明，获得方式会影响寄生虫负荷的分布和动态、宿主-寄生虫系统的稳定性和驱虫剂耐药性的出现速度，但很少有实地研究允许用经验数据来探讨这些问题。我们分析了从孟加拉国达卡进行的三轮化学驱虫研究中收集的人体寄生蛔虫 Ascaris lumbricoides 的个体蠕虫重量数据，目的是确定是涓滴感染过程还是成簇感染过程占主导地位。我们发现，宿主往往寄生着相似体重的雌性蠕虫，这表明存在成簇感染过程，但承认未测量的宿主异质性（随机效应）不能完全排除作为原因。在这里，我们使用随机感染模型补充了我们之前的统计分析，以模拟感染人群的个体 A. lumbricoides 的大小。我们使用组内相关系数（ICC）作为模拟蠕虫大小之间相似性的定量度量，并根据涓滴或成簇感染和未测量的宿主异质性的假设来探索该统计量的行为。我们确认这两种机制都能够产生相似大小的蠕虫聚集，但数据中驱虫前后描述的 ICC 特定模式更符合成簇感染产生的聚集，而不是仅由宿主异质性产生的聚集。这为蠕虫可能成簇获得的观点提供了支持。我们根据 A. lumbricoides 的群体生物学讨论我们的结果，并强调我们的建模方法在研究寄生虫蠕虫种群动态方面的重要性。
