Impact and cost-effectiveness of snail control to achieve disease control targets for schistosomiasis

Abstract

Schistosomiasis is a parasitic disease that affects over 240 million people globally. To improve population-level disease control, there is growing interest in adding chemical-based snail control interventions to interrupt the lifecycle of Schistosoma in its snail host to reduce parasite transmission. However, this approach is not widely implemented, and given environmental concerns, the optimal conditions for when snail control is appropriate are unclear. We assessed the potential impact and cost-effectiveness of various snail control strategies. We extended previously published dynamic, age-structured transmission and cost-effectiveness models to simulate mass drug administration (MDA) and focal snail control interventions against Schistosoma haematobium across a range of low-prevalence (5-20%) and high-prevalence (25-50%) rural Kenyan communities. We simulated strategies over a 10-year period of MDA targeting school children or entire communities, snail control, and combined strategies. We measured incremental cost-effectiveness in 2016 US dollars per disability-adjusted life year and defined a strategy as optimally cost-effective when maximizing health gains (averted disability-adjusted life years) with an incremental cost-effectiveness below a Kenya-specific economic threshold. In both low- and high-prevalence settings, community-wide MDA with additional snail control reduced total disability by an additional 40% compared with school-based MDA alone. The optimally cost-effective scenario included the addition of snail control to MDA in over 95% of simulations. These results support inclusion of snail control in global guidelines and national schistosomiasis control strategies for optimal disease control, especially in settings with high prevalence, "hot spots" of transmission, and noncompliance to MDA.

Keywords: cost-effectiveness; environmental control; epidemiology; mathematical modeling; parasitology.

Fig. 1.

Effectiveness of selected MDA, snail control, and combined interventions for schistosomiasis in low- and high-burden Kenyan communities. We simulated interventions of MDA, snail control, and combined approaches in an age-stratified population of preschool-aged children, school-aged children, and adults in (A) low-prevalence Kenyan communities and (B) high-prevalence Kenyan communities with 75% coverage for MDA. The figure displays selected interventions for visualization purposes; plots for all tested interventions are available in SI Appendix.

Fig. 2.

Cost-effectiveness efficiency frontier for selected MDA, snail control, and combined interventions for schistosomiasis in low- and high-burden Kenyan communities. We computed the costs (US dollars) and averted DALYs for nondominated interventions of MDA, snail control, and combined approaches in the (A) low-prevalence Kenyan communities and (B) high-prevalence Kenyan communities. Dominated strategies are not shown, and full results are available in SI Appendix, Table S1. The cost-effectiveness of each strategy is measured with the ICER. The ICER is computed in reference with the next best strategy in terms of averted DALYs (corresponding to the strategy directly to the left on the frontier). The ICER is computed as the difference in cost divided by the difference in DALYs, which is shown as the slope between strategies. A steeper slope indicates a lower ICER (more cost-effective), while a flatter slope suggests a higher ICER (less cost-effective). Notably, there is strong nonlinearity in effectiveness (averted DALYs), whereby additional intervention yields smaller gains and high ICER. CWT, community-wide treatment with MDA; SBT, school-based treatment with MDA; SC, snail control.

Fig. 3.

One-way sensitivity analysis of key model parameters. This analysis tested the effect of changing a single model input on the ICER of the highly cost-effective interventions from the primary analysis: (A) semiannual school-based MDA with semiannual snail control in low-prevalence settings, (B) annual community-wide MDA with semiannual snail control in low-prevalence settings, (C) annual community-wide MDA with semiannual snail control in high-prevalence settings, and (D) semiannual community-wide MDA with semiannual snail control in high-prevalence settings. We varied values for model inputs related to transmission dynamics, costs, and intervention effectiveness, including sampling from the posterior distribution generated during model calibration, which affects transmission projections and snail control effectiveness. The horizontal axis represents the ICER values (US dollars per DALY averted), while the vertical axis includes tested parameters with respective ranges of values. A lower ICER can be interpreted as a more cost-effective intervention, and we considered all strategies left of the $1,377 US per DALY averted to be highly cost-effective, although the full axis is provided to relax reliance on a single threshold. *The 95% credible interval of the transmission projection incorporates the full range of values for the effectiveness snail control and in some cases, was dominated by extension in the lower ranges. **Snail control effectiveness on schistosomiasis was calibrated based on empirical data and is a function of multiple parameters (including snail control efficacy); the lower MDA coverage range still simulated 75% coverage for school-based MDA.