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.

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Related in: MedlinePlus

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.
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pntd-0003323-g001: 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.

Mentions: We will refer to the resulting model as the sexual reproduction or SR model. The factor is effectively the fraction of total egg production from the host population that is fertilized. For large values of M, φ is effectively equal to 1 and sexual reproduction has no influence. For low values of M, φ approaches zero, indicating the decreasing probability of a female worm co-infecting with a male. As can be seen from Figure 1A (inset), there is no clear boundary for the effect of sexual reproduction, but it has a strong impact on fertile egg production for mean worm burdens of less than about 2.5. We define this approximate cut-off point as MSR. For worm burdens below MSR, the decline in fertile egg production reaches a point at which it balances the ability of the worms and infectious material to persist in the environment, defining a ‘breakpoint’ [9], [20], [21]). Below the breakpoint is a stable parasite-free state. The breakpoint is generally at very low values of mean worm burden and has a minimal effect on the normal endemic state of the parasite population, except at low values of R0 at which the endemic solution disappears [9] (See Figure 1A, main panel).


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) 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.
© Copyright Policy
Related In: Results  -  Collection

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

pntd-0003323-g001: 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.
Mentions: We will refer to the resulting model as the sexual reproduction or SR model. The factor is effectively the fraction of total egg production from the host population that is fertilized. For large values of M, φ is effectively equal to 1 and sexual reproduction has no influence. For low values of M, φ approaches zero, indicating the decreasing probability of a female worm co-infecting with a male. As can be seen from Figure 1A (inset), there is no clear boundary for the effect of sexual reproduction, but it has a strong impact on fertile egg production for mean worm burdens of less than about 2.5. We define this approximate cut-off point as MSR. For worm burdens below MSR, the decline in fertile egg production reaches a point at which it balances the ability of the worms and infectious material to persist in the environment, defining a ‘breakpoint’ [9], [20], [21]). Below the breakpoint is a stable parasite-free state. The breakpoint is generally at very low values of mean worm burden and has a minimal effect on the normal endemic state of the parasite population, except at low values of R0 at which the endemic solution disappears [9] (See Figure 1A, main panel).

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