Modeling the interruption of the transmission of soil-transmitted helminths by repeated mass chemotherapy of school-age children

Abstract

Background: The control or elimination of neglected tropical diseases has recently become the focus of increased interest and funding from international agencies through the donation of drugs. Resources are becoming available for the treatment of soil-transmitted helminth (STH) infection through school-based deworming strategies. However, little research has been conducted to assess the impact of STH treatment that could be used to guide the design of efficient elimination programs.

Methodology: We construct and analyse an age-structured model of STH population dynamics under regular treatment. We investigate the potential for elimination with finite rounds of treatment, and how this depends on the value of the basic reproductive number R0 and treatment frequency.

Principal findings: Analysis of the model indicates that its behaviour is determined by key parameter groupings describing the basic reproduction number and the fraction of it attributable to the treated group, the timescale of material in the environment and the frequency and efficacy of treatment. Mechanisms of sexual reproduction and persistence of infectious material in the environment are found to be much more important in the context of elimination than in the undisturbed baseline scenario. For a given rate of drug use, sexual reproduction dictates that less frequent, higher coverage treatment is more effective. For a given treatment coverage level, the lifespan of infectious material in the environment places a limit on the effectiveness of increased treatment frequency.

Conclusions: Our work suggests that for models to capture the dynamics of parasite burdens in populations under regular treatment as elimination is approached, they need to include the effects of sexual reproduction among parasites and the dynamics infectious material in the reservoir. The interaction of these two mechanisms has a strong effect on optimum treatment strategies, both in terms of how frequently to treat and for how long.

Figure 1. A) Solutions of equations (1–2), without age structure, both with and without mating probability factor φ.

Broken lines represent unstable solutions [Inset: Mating probability factor φ as a function of mean worm burden]. B) Time series for mean worm burden in children and adults and reservoir content in response to annual treatment.

Figure 2. Dependence of growth rate, k, on A) R₀ and effective treatment, γ; B) contribution of children to parasite reproduction, rc, and effective treatment; C) relative reservoir timescale, ε, and effective treatment.

D) Extinction point for parasite for different values of ε. [For C, R₀ = 2.5 and D, r_c = 1].

Figure 3. A) Critical treatment efficacy for SR and non-SR dynamics and different treatment intervals.

B) Evolution of worm burden in children under annual treatment with and without sexual reproduction dynamics (default parameter values and R₀ = 2) C) Time series showing effect of different intervention frequencies with same annual treatment rate. D) Minimum number of treatment rounds necessary to achieve elimination (with SR) as a function of R0 and the interval between treatments.