Review 2: "How does Treatment Coverage and Proportion Never Treated Influence the Success of Schistosoma Mansoni Elimination as a Public Health Problem by 2030?"

David Guararie

doi:doi:10.1162/2e3983f5.55a1ab76

How does treatment coverage and proportion never treated influence the success of Schistosoma mansoni elimination as a public health problem by 2030?

by Klodeta Kura, Nyamai Mutono, Maria-Gloria Basáñez, Luc E. Coffeng, S.M. Thumbi, and Roy M. Anderson

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Published on Oct 21, 2023
www.medrxiv.org

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Abstract Background The 2030 target for schistosomiasis is elimination as a public health problem (EPHP), achieved when the prevalence of heavy intensity infection among school-aged children (SAC) reduces to <1%. To achieve this, the new World Health Organization (WHO) guidelines recommend a broader target of population to include pre-school (pre-SAC) and adults. However, the probability of achieving EPHP should be expected to depend on patterns in repeated uptake of mass drug administration (MDA) by individuals.Methods We employed two individual-based stochastic models to evaluate the impact of school-based and community-wide treatment and calculated the number of rounds required to achieve EPHP for Schistosoma. mansoni by considering various levels of the population never treated (NT). We also considered two age intensity profiles, corresponding to a low and high burden of infection in adults.Results The number of rounds needed to achieve this target depends on the baseline prevalence and the coverage used. For low and moderate transmission areas, EPHP can be achieved within seven years if NT ≤10% and NT <5%, respectively. In high transmission areas, community wide treatment with NT<1% is required to achieve EPHP.Conclusions The higher the intensity of transmission, and the lower the treatment coverage, the lower the acceptable value of NT becomes. Using more efficacious treatment regimens would permit NT values to be marginally higher. A balance between target treatment coverage and NT values may be an adequate treatment strategy depending on the epidemiological setting, but striving to increase coverage and/or minimise NT can shorten programme duration.

RR:C19 Evidence Scale rating by reviewer:

Potentially informative. The main claims made are not strongly justified by the methods and data, but may yield some insight. The results and conclusions of the study may resemble those from the hypothetical ideal study, but there is substantial room for doubt. Decision-makers should consider this evidence only with a thorough understanding of its weaknesses, alongside other evidence and theory. Decision-makers should not consider this actionable, unless the weaknesses are clearly understood and there is other theory and evidence to further support it.

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Review: The paper aims to assess quantitatively the effect of partial drug coverage in long-term MDA control programs targeted at burden reduction or elimination of Schistosomiasis on community level.

They employ agent-based stochastic transmission models developed by Imperial College London (ICL) and the University of Oxford (SCHISTOX). Two models have similar structure in terms of population demographics and within-host biology (worm mating, aggregation egg-release), the only difference being density dependence of egg-release in ICL.

To assess long-term outcomes of annual MDA on long term outcomes (community burden), the authors explore a range of transmission environments represented by basic reproduction number R₀, and associate baseline prevalence.

The main result (Table 2) shows expected program duration as function of child-adult coverage levels, relative adult burden (low/high) and baseline prevalence (low, moderate, high).

One drawback of models and analysis is its reliance on a single parameter (R₀ and a related baseline prevalence) as a predictor of long term MDA outcome. The missing component is putative contribution of snail environment on control outcomes. Snails are active hosts for Schistosome transmission that could amplify the force of human infection, independent of human input. So identical host communities could exhibit markedly different MDA-response patterns in different environments. Such snail effect can be demonstrated with different human transmission models, from basic MacDonald-type to AMB.

So the results and conclusions of the paper are provisional on the underlying ‘passive snail’ assumption, and may have limited scope in realistic transmission settings.

Review 2: "How does Treatment Coverage and Proportion Never Treated Influence the Success of Schistosoma Mansoni Elimination as a Public Health Problem by 2030?"

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