The effect of assortative mixing on stability of low helminth transmission levels and on the impact of mass drug administration: Model explorations for onchocerciasis

Authored by Vlas Sake J de, Vos Anneke S de, Wilma A Stolk, Luc E Coffeng

Date Published: 2018

DOI: 10.1371/journal.pntd.0006624

Sponsors: Bill and Melinda Gates Foundation

Platforms: R

Model Documentation: Other Narrative

Model Code URLs: https://doi.org/10.1371/journal.pntd.0006624.s006

Abstract

Background 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 preyalences <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.

Tags

Simulation Dynamics Validation Program Mathematical-models Hookworm infection