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

Truscott J, Hollingsworth TD, Anderson R - PLoS Negl Trop Dis (2014)

Bottom Line: For a given rate of drug use, sexual reproduction dictates that less frequent, higher coverage treatment is more effective.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.

View Article: PubMed Central - PubMed

Affiliation: London Centre for Neglected Tropical Disease Research, Department of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, St Marys Campus, Norfolk Place, London, United Kingdom.

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.

Show MeSH

Related in: MedlinePlus

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.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4256169&req=5

pntd-0003323-g003: 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.

Mentions: We now examine the effect of including the dynamics of sexual reproduction in the host into the model. A commonly made assumption is that the sexual reproduction mechanism has a negligible impact on parasite dynamics except at the lowest worm loads. This situation is illustrated by Figure 1A, which shows equilibrium worm burden as a function of R0 with and without sexual reproduction. Significant discrepancies arise only for R0 values around 1.5 and lower and result from the assumption implicit in standard R0 calculations that female worms still generate fertile eggs at very low population levels. Figure 3A contrasts the critical treatment efficacies for models with (labelled SR) and without (labelled non-SR) sexual reproduction as a function of R0. It is clear that, in general, the presence of the sexual reproduction mechanism in the model makes interrupting transmission much easier, placing it now at the low end of measured R0 values (1.5–2.5) for an annual treatment regime. Even for 2-yearly intervention, elimination is possible for R0<2.


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

Truscott J, Hollingsworth TD, Anderson R - PLoS Negl Trop Dis (2014)

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.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4256169&req=5

pntd-0003323-g003: 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.
Mentions: We now examine the effect of including the dynamics of sexual reproduction in the host into the model. A commonly made assumption is that the sexual reproduction mechanism has a negligible impact on parasite dynamics except at the lowest worm loads. This situation is illustrated by Figure 1A, which shows equilibrium worm burden as a function of R0 with and without sexual reproduction. Significant discrepancies arise only for R0 values around 1.5 and lower and result from the assumption implicit in standard R0 calculations that female worms still generate fertile eggs at very low population levels. Figure 3A contrasts the critical treatment efficacies for models with (labelled SR) and without (labelled non-SR) sexual reproduction as a function of R0. It is clear that, in general, the presence of the sexual reproduction mechanism in the model makes interrupting transmission much easier, placing it now at the low end of measured R0 values (1.5–2.5) for an annual treatment regime. Even for 2-yearly intervention, elimination is possible for R0<2.

Bottom Line: For a given rate of drug use, sexual reproduction dictates that less frequent, higher coverage treatment is more effective.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.

View Article: PubMed Central - PubMed

Affiliation: London Centre for Neglected Tropical Disease Research, Department of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, St Marys Campus, Norfolk Place, London, United Kingdom.

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.

Show MeSH
Related in: MedlinePlus