Department of Public Health, Erasmus MC, University Medical Center Rotterdam, Rotterdam, The Netherlands.
PLoS Negl Trop Dis. 2018 Oct 8;12(10):e0006624. doi: 10.1371/journal.pntd.0006624. eCollection 2018 Oct.
Stable low pre-control prevalences of helminth infection are not uncommon in field settings, yet it is poorly understood how such low levels can be sustained, thereby challenging efforts to model them. Disentangling possible facilitating mechanisms is important, since these may differently affect intervention impact. Here we explore the role of assortative (i.e. non-homogenous) mixing and exposure heterogeneity in helminth transmission, using onchocerciasis as an example.
METHODOLOGY/PRINCIPAL FINDINGS: We extended the established individual-based model ONCHOSIM to allow for assortative mixing, assuming that individuals who are relatively more exposed to fly bites are more connected to each other than other individuals in the population as a result of differential exposure to a sub-population of blackflies. We used the model to investigate how transmission stability, equilibrium microfilarial (mf) prevalence and intensity, and impact of mass drug administration depend on the assumed degree of assortative mixing and exposure heterogeneity, for a typical rural population of about 400 individuals. The model clearly demonstrated that with homogeneous mixing and moderate levels of exposure heterogeneity, onchocerciasis could not be sustained below 35% mf prevalence. In contrast, assortative mixing stabilised onchocerciasis prevalence at levels as low as 8% mf prevalence. Increasing levels of assortative mixing significantly reduced the probability of interrupting transmission, given the same duration and coverage of mass drug administration.
CONCLUSIONS/SIGNIFICANCE: Assortative mixing patterns are an important factor to explain stable low prevalence situations and are highly relevant for prospects of elimination. Their effect on the pre-control distribution of mf intensities in human populations is only detectable in settings with mf prevalences <30%, where high skin mf density in mf-positive people may be an indication of assortative mixing. Local spatial variation in larval infection intensity in the blackfly intermediate host may also be an indicator of assortative mixing.
在实地环境中，稳定的低预控流行率的寄生虫感染并不罕见，但人们对如何维持如此低的水平知之甚少，这对寄生虫感染建模工作提出了挑战。因此，理清可能存在的促进机制很重要，因为这些机制可能会以不同的方式影响干预效果。在这里，我们以盘尾丝虫病为例，探讨了寄生虫感染的聚集（即不均匀）混合和暴露异质性在寄生虫传播中的作用。
方法/主要发现：我们扩展了现有的基于个体的 ONCHOSIM 模型，以允许聚集混合，假设由于对黑蝇亚群的不同暴露，与人群中的其他个体相比，相对更多地接触到蚊虫叮咬的个体彼此之间的联系更为紧密。我们使用该模型来研究在假定的聚集混合和暴露异质性程度下，传输稳定性、平衡微丝蚴（mf）流行率和强度以及大规模药物管理的影响，针对约 400 人的典型农村人口。模型清楚地表明，在均匀混合和适度的暴露异质性水平下，盘尾丝虫病的流行率不能低于 35% mf 流行率。相比之下，聚集混合可将盘尾丝虫病的流行率稳定在低至 8% mf 流行率的水平。给定相同的大规模药物管理持续时间和覆盖率，聚集混合程度的增加显著降低了中断传播的可能性。
结论/意义：聚集混合模式是解释稳定低流行率情况的一个重要因素，对于消除寄生虫感染的前景具有重要意义。它们对人类种群中 mf 强度的预控制分布的影响只有在 mf 流行率<30%的情况下才能检测到，因为 mf 阳性人群中的高皮肤 mf 密度可能表明存在聚集混合。黑蝇中间宿主幼虫感染强度的局部空间变化也可能是聚集混合的一个指标。
