Global Assessment of Schistosomiasis Control Over the Past Century Shows Targeting the Snail Intermediate Host Works Best

Abstract

Background: Despite control efforts, human schistosomiasis remains prevalent throughout Africa, Asia, and South America. The global schistosomiasis burden has changed little since the new anthelmintic drug, praziquantel, promised widespread control.

Methodology: We evaluated large-scale schistosomiasis control attempts over the past century and across the globe by identifying factors that predict control program success: snail control (e.g., molluscicides or biological control), mass drug administrations (MDA) with praziquantel, or a combined strategy using both. For data, we compiled historical information on control tactics and their quantitative outcomes for all 83 countries and territories in which: (i) schistosomiasis was allegedly endemic during the 20th century, and (ii) schistosomiasis remains endemic, or (iii) schistosomiasis has been "eliminated," or is "no longer endemic," or transmission has been interrupted.

Principal findings: Widespread snail control reduced prevalence by 92 ± 5% (N = 19) vs. 37 ± 7% (N = 29) for programs using little or no snail control. In addition, ecological, economic, and political factors contributed to schistosomiasis elimination. For instance, snail control was most common and widespread in wealthier countries and when control began earlier in the 20th century.

Conclusions/significance: Snail control has been the most effective way to reduce schistosomiasis prevalence. Despite evidence that snail control leads to long-term disease reduction and elimination, most current schistosomiasis control efforts emphasize MDA using praziquantel over snail control. Combining drug-based control programs with affordable snail control seems the best strategy for eliminating schistosomiasis.

Fig 1. Control strategies used against schistosomiasis during the past century.

Fig 2. Strategy details for schistosomiasis control: the current paradigm and an alternative based on past successes.

(A) The current paradigm for global schistosomiasis control, adapted from [30], and (B) an alternative strategy based on historical successes (this paper). MDA = mass drug administration, TAT = targeted (“test-and-treat”) chemotherapy.

Fig 3. Countries and territories evaluated and their outcomes for schistosomiasis control or elimination.

“Successful” = there was a control program that preceded elimination or non-endemic status; “Fortuitous elimination” = elimination or non-endemic status with no control program; “Minimal control” = endemic disease in the face of minimal to no control, even if there were some pilot or small-scale programs; “Not (yet) successful” = endemic disease with a past or present control program.

Fig 4. Schistosomiasis control programs over time.

See Fig 5 for more details on control strategies and outcomes.

Fig 5. Schistosomiasis prevalence change over time.

Prevalence change by control program strategy (time 0 on the x-axis is set when control began; negative values for the normalized year show data n years before control started and positive values n years after).

Fig 6. Prevalence change by extent of snail control, or MDA, or both—see text for category definitions.

Fig 7. Sensitivity of schistosome prevalence reduction to the infected population size before the control program began.

Fig 8. Per capita gross domestic product (GDP) as it relates to control strategy and control start date in each country.

Points represent mean inflation-adjusted, per-capita GDP throughout the relevant time for which disease data were available, and bars represent the range.