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How effective is school-based deworming for the community-wide control of soil-transmitted helminths?

Anderson RM, Truscott JE, Pullan RL, Brooker SJ, Hollingsworth TD - PLoS Negl Trop Dis (2013)

Bottom Line: Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective.Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of Ascaris lumbricoides worms (the highest proportion of parasites for our examples).These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates.

View Article: PubMed Central - PubMed

Affiliation: London Centre for Neglected Tropical Diseases, Department of Infectious Disease Epidemiology, School of Public Health, Imperial College London, London, United Kingdom. roy.anderson@imperial.ac.uk

ABSTRACT

Background: The London Declaration on neglected tropical diseases was based in part on a new World Health Organization roadmap to "sustain, expand and extend drug access programmes to ensure the necessary supply of drugs and other interventions to help control by 2020". Large drug donations from the pharmaceutical industry form the backbone to this aim, especially for soil-transmitted helminths (STHs) raising the question of how best to use these resources. Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective. However, the impact of school-based deworming on transmission in the wider community remains unclear.

Methods: We first estimate the proportion of parasites targeted by school-based deworming using demography, school enrolment, and data from a small number of example settings where age-specific intensity of infection (either worms or eggs) has been measured for all ages. We also use transmission models to investigate the potential impact of this coverage on transmission for different mixing scenarios.

Principal findings: In the example settings <30% of the population are 5 to <15 years old. Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of Ascaris lumbricoides worms (the highest proportion of parasites for our examples). In addition, it is estimated that from 40-70% of these children are enrolled at school.

Conclusions: These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates. We discuss the consequences for transmission for a range of scenarios, including when infective stages deposited by children are more likely to contribute to transmission than those from adults.

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Related in: MedlinePlus

Critical fraction of the population to be treated.The predicted relationship between the critical fraction of the human population to be treated, , per annum with efficacy, , 0.9, and the basic reproductive number, , and parasite life expectancy,  in years (from equation 11 in the main text).
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pntd-0002027-g005: Critical fraction of the population to be treated.The predicted relationship between the critical fraction of the human population to be treated, , per annum with efficacy, , 0.9, and the basic reproductive number, , and parasite life expectancy, in years (from equation 11 in the main text).

Mentions: The impact of the parasite coverage levels presented in Table 5 on transmission under the assumption of homogeneous mixing can be seen using the approximations to the critical treatment coverage, (derived above in relation to the parasite life expectancy, , and the basic reproduction number, , as in Figure 5 (equation 11)) and the impact of increasing on the mean worm load, , and the prevalence of infection, (Figure 6, equation 6). Figure 6 reveals that the mean worm load decays approximately linearly as the fraction treated rises, while prevalence only begins to fall steeply as the fraction treated approaches the critical value and effective transmission ceases. Note that from Figure 5 the critical fraction treated reaches 1 for high /low parasite life expectancy. This indicates the parasite may be very difficult to eradicate by treatment with the given efficacy ( = 0.9) when is large (∼>4, estimated values for particular parasites in Table 3) and is short (∼<1.2 years, values in Table 4) e.g.for A. lumbricoides and T. trichiura in high transmission settings.


How effective is school-based deworming for the community-wide control of soil-transmitted helminths?

Anderson RM, Truscott JE, Pullan RL, Brooker SJ, Hollingsworth TD - PLoS Negl Trop Dis (2013)

Critical fraction of the population to be treated.The predicted relationship between the critical fraction of the human population to be treated, , per annum with efficacy, , 0.9, and the basic reproductive number, , and parasite life expectancy,  in years (from equation 11 in the main text).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3585037&req=5

pntd-0002027-g005: Critical fraction of the population to be treated.The predicted relationship between the critical fraction of the human population to be treated, , per annum with efficacy, , 0.9, and the basic reproductive number, , and parasite life expectancy, in years (from equation 11 in the main text).
Mentions: The impact of the parasite coverage levels presented in Table 5 on transmission under the assumption of homogeneous mixing can be seen using the approximations to the critical treatment coverage, (derived above in relation to the parasite life expectancy, , and the basic reproduction number, , as in Figure 5 (equation 11)) and the impact of increasing on the mean worm load, , and the prevalence of infection, (Figure 6, equation 6). Figure 6 reveals that the mean worm load decays approximately linearly as the fraction treated rises, while prevalence only begins to fall steeply as the fraction treated approaches the critical value and effective transmission ceases. Note that from Figure 5 the critical fraction treated reaches 1 for high /low parasite life expectancy. This indicates the parasite may be very difficult to eradicate by treatment with the given efficacy ( = 0.9) when is large (∼>4, estimated values for particular parasites in Table 3) and is short (∼<1.2 years, values in Table 4) e.g.for A. lumbricoides and T. trichiura in high transmission settings.

Bottom Line: Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective.Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of Ascaris lumbricoides worms (the highest proportion of parasites for our examples).These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates.

View Article: PubMed Central - PubMed

Affiliation: London Centre for Neglected Tropical Diseases, Department of Infectious Disease Epidemiology, School of Public Health, Imperial College London, London, United Kingdom. roy.anderson@imperial.ac.uk

ABSTRACT

Background: The London Declaration on neglected tropical diseases was based in part on a new World Health Organization roadmap to "sustain, expand and extend drug access programmes to ensure the necessary supply of drugs and other interventions to help control by 2020". Large drug donations from the pharmaceutical industry form the backbone to this aim, especially for soil-transmitted helminths (STHs) raising the question of how best to use these resources. Deworming for STHs is often targeted at school children because they are at greatest risk of morbidity and because it is remarkably cost-effective. However, the impact of school-based deworming on transmission in the wider community remains unclear.

Methods: We first estimate the proportion of parasites targeted by school-based deworming using demography, school enrolment, and data from a small number of example settings where age-specific intensity of infection (either worms or eggs) has been measured for all ages. We also use transmission models to investigate the potential impact of this coverage on transmission for different mixing scenarios.

Principal findings: In the example settings <30% of the population are 5 to <15 years old. Combining this demography with the infection age-intensity profile we estimate that in one setting school children output as little as 15% of hookworm eggs, whereas in another setting they harbour up to 50% of Ascaris lumbricoides worms (the highest proportion of parasites for our examples). In addition, it is estimated that from 40-70% of these children are enrolled at school.

Conclusions: These estimates suggest that, whilst school-based programmes have many important benefits, the proportion of infective stages targeted by school-based deworming may be limited, particularly where hookworm predominates. We discuss the consequences for transmission for a range of scenarios, including when infective stages deposited by children are more likely to contribute to transmission than those from adults.

Show MeSH
Related in: MedlinePlus