How important is the spatial movement of people in attempts to eliminate the transmission of human helminth infections by mass drug administration?

Abstract

Human mobility contributes to the spatial dynamics of many infectious diseases, and understanding these dynamics helps us to determine the most effective ways to intervene and plan surveillance. In this paper, we describe a novel transmission model for the spatial dynamics of hookworm, a parasitic worm which is a common infection across sub-Saharan Africa, East Asia and the Pacific islands. We fit our model, with and without mobility, to data obtained from a sub-county in Kenya, and validate the model's predictions against the decline in prevalence observed over the course of a clustered randomized control trial evaluating methods of administering mass chemotherapy. We find that our model which incorporates human mobility is able to reproduce the observed patterns in decline of prevalence during the TUMIKIA trial, and additionally, that the widespread bounce-back of infection may be possible over many years, depending on the rates of people movement between villages. The results have important implications for the design of mass chemotherapy programmes for the elimination of human helminth transmission. This article is part of the theme issue ‘Challenges and opportunities in the fight against neglected tropical diseases: a decade from the London Declaration on NTDs’.

Spatial heterogeneity in mass drug administration from a longitudinal epidemiological study assessing transmission interruption of soil transmitted helminths in the Wolaita zone of southern Ethiopia (Geshiyaro Project)

“…Recent stochastic mathematical modelling studies of STH transmission and control within a spatially heterogeneous environment in Kenya [ 27 ] show that the correct spatial scale at which to evaluate intervention progress does indeed depend on the movement patterns of people as linked to the spatial dispersal of infective stages in defined social settings, as does the likelihood and degree to which transmission will persist. Our understanding of how STH transmission, as well other NTDs, is affected by spatial scale is very poor at present but molecular epidemiological studies would act to remedy this situation.…”

“…Identifying variable genome regions creates the opportunity to ascertain 'who infects whom' over different spatial scales, for example, within households, within and between households in a village, and between villages Add citation. Sampling the genetic material of expelled adult STH worms post-MDA via worm expulsion studies and associated phylogenetic analyses, offers the ability to describe the STH genomic landscape in a geographical region [25,26] Recent stochastic mathematical modelling studies of STH transmission and control within a spatially heterogeneous environment in Kenya [27] show that the correct spatial scale at which to evaluate intervention progress does indeed depend on the movement patterns of people as linked to the spatial dispersal of infective stages in defined social settings, as does the likelihood and degree to which transmission will persist. Our understanding of how STH transmission, as well other NTDs, is affected by spatial scale is very poor at present but molecular epidemiological studies would act to remedy this situation.…”

Introduction to the special issue: challenges and opportunities in the fight against neglected tropical diseases: a decade from the London Declaration on NTDs

“…As such, even in high average coverage regions, pockets of high infection may thus persist with the implications that they can re-seed infection once PC ceases. The contribution of Collyer and collaborators to this collection highlights the importance of considering human movement in STH models to explain declines in prevalence owing to MDA and bounce-back of hookworm infection using data from the TUMIKIA trial in Kenya [ 27 ].…”

“…For this issue, NTD modellers have contributed seven papers (50% of the contributions), including the potential of alternative treatment strategies for accelerating programmatic action towards attaining interruption of transmission [ 9 ]; the design of NTD impact surveys [ 12 ]; the usefulness of current and novel tools for diagnosis of helminthiases in areas ranging from low to high prevalence [ 17 ]; the feasibility and challenges posed by NTDs with long incubation periods and diagnostic delays [ 20 ]; the impact of human movement on reaching and sustaining elimination efforts [ 27 ]; the relationship between cumulative exposure to infection and development of severe morbidity [ 28 ], and the use of serological surveys and force-of-infection models linked to frameworks of disease progression for quantifying spatio-temporal patterns of disease burden accounting for uncertainty at all steps of the proposed pipeline [ 29 ]. The linkage between NTD models and data have certainly improved considerably over the past decade, but lessons learnt from model construction on what should be measured to better understand the impact of given control interventions on infection, transmission and morbidity have yet to filter through to the practical design of most monitoring and evaluation programmes.…”

“…cross-sectional data across multiple time-points), and used to analyse the significance of spatial heterogeneity in both infection prevalence and intervention coverage. The model described by Collyer et al [ 27 ] is the first, fully spatially dynamic NTD individual-based stochastic model of helminth transmission and control by MDA, with parameters estimated by fitting to a large spatially structured database recording infection prevalence and intensity at village level. However, much further work will be required in this area given a growing volume of data revealing much heterogeneity in MDA coverage at fine, village-level, spatial scales.…”