Abstract
The predictions of two mathematical models of the transmission dynamics of Ascaris lumbricoides and hookworm infection and the impact of mass drug administration (MDA) are compared, using data from India. One model has an age structured partial differential equation (PDE) deterministic framework for the distribution of parasite numbers per host and sexual mating. The second model is an individual-based stochastic model. Baseline data acquired prior to treatment are used to estimate key transmission parameters, and forward projections are made, given the known MDA population coverage. Predictions are compared with observed post-treatment epidemiological patterns. The two models could equally well predict the short-term impact of deworming on A. lumbricoides and hookworm infection levels, despite being fitted to different subsets and/or summary statistics of the data. As such, the outcomes give confidence in their use as aids to policy formulation for the use of PCT to control A. lumbricoides and hookworm infection. The models further largely agree in a qualitative sense on the added benefit of semi-annual vs. annual deworming and targeting of the entire population vs. only children, as well as the potential for interruption of transmission. Further, this study also illustrates that long-term predictions are sensitive to modelling assumptions about which age groups contribute most to transmission, which depends on human demography and age-patterns in exposure and contribution to the environmental reservoir of infection, the latter being notoriously difficult to empirically quantify.
Highlights
Over 1 billion people are infected with soil-transmitted helminths (STH) (Hotez et al, 2014)
Transmission parameter estimated based on the pre-control data showed considerable correlation, both for the Erasmus MC model (Supplemental Fig. S2) and the Imperial College London (ICL) model (Supplemental Fig. S3), highlighting the appropriateness of using Bayesian methods to account for multiple possible parameter combinations
Both models were run forward in time assuming that 68% of the overall population were treated with pyrantel pamoate, and model predictions for the average adult female worm burden at the time point 11 months after the first mass drug administration (MDA) round were compared to the observed number of adult female worms expulsed after a second treatment with pyrantel pamoate
Summary
Over 1 billion people are infected with soil-transmitted helminths (STH) (Hotez et al, 2014). The majority of infecting STH species are Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm), and hookworm (Necator americanus and Ancyclostoma duodenale). L.E. Coffeng et al / Epidemics 18 (2017) 38–47 bounce back to their pre-perturbation equilibrium state in a predictable manner. Coffeng et al / Epidemics 18 (2017) 38–47 bounce back to their pre-perturbation equilibrium state in a predictable manner This is the result of the presence of a reservoir of infection in the environment (soil contaminated with faecal matter containing worm eggs or larvae) and is further exacerbated by density-dependent processes operating on worm population growth (Anderson and May, 1985). Because there is no strong immune response in STH infections, after deworming, individuals reacquire new infections at rates that depend on exposure to the environmental reservoir (i.e. ingestion of contaminated soil for ascariasis and trichuriasis, and exposure to free-living hookworm larvae that may burrow through exposed skin). Given the relatively short lifespan of STH of one to three years, bounce back can be rapid, and, in the case of A. lumbricoides in high transmission settings, even within one year (Anderson and May, 1985; Seo et al, 1983; Thein-Hlaing et al, 1990)
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