Global assessment of schistosomiasis control over the past century shows targeting the snail intermediate host works best
Sokolow SH, et al. PLoS Negl Trop Dis
2016, 10(7): e0004794. doi:10.1371/journal.pntd.0004794
Despite control efforts over the past century, human schistosomiasis (SCH) remains prevalent throughout Africa, Asia and South America, affecting more than 250 million people worldwide. SCH infections occur where aquatic intermediate snail hosts shed the infective parasite, which penetrates human skin upon contact. Infected individuals suffer from anemia, stunted growth, cognitive impairment, fatigue, infertility and sometimes, liver fibrosis or bladder cancer. Comprehensive SCH control strategies aim to disrupt the parasite’s life cycle: sanitation prevents parasite eggs in urine or feces from entering aquatic snail habitats; snail control reduces intermediate host density; education informs people to avoid high-risk water contacts and seek treatment; and mass drug administration (MDA) with praziquantel kills adult worms in the human host, providing immediate and long-term health benefits for infected individuals.
When praziquantel was discovered roughly 40 years ago, the drug provided the biggest hope for SCH elimination, rendering snail control an “old fashioned” strategy. In 2001, the World Health Assembly (WHA) endorsed preventive chemotherapy as the primary strategy to control SCH. Yet, despite years of MDA with praziquantel, the global disease burden has changed little, calling for more research and implementation of non-pharmaceutical SCH control strategies. This article reviews SCH control program outcomes over the past century to determine the most effective control strategy.
The study evaluated the success of SCH control programs in 77 countries and six semi-autonomous territories using peer-reviewed published sources and non-peer-reviewed published reports. Countries were categorized as “minimal control
” with little to no control effort, “fortuitous elimination
” where there was loss of schistosomiasis with minimal control, “not (yet) successful
” where there has been control but the disease transmission continues and not yet eliminated, and “successful
” where transmission has been stopped (Figure 1). Information was collected on country- or territory-level SCH prevalence, infected and at-risk population size, control strategies implemented and their time-course, environmental factors and economics, as well as the snail and schistosome species present. For comparison, the authors categorized different control strategies as either MDA, snail control, or engineering interventions (e.g., sanitation infrastructure), further ranking each control category as extensive/complete
(>70% at-risk population receiving treatment), intermediate
(>30%) or focal to none
(<30%). A quantitative generalized linear mixed model was used to assess what factors best predicted relative change in SCH prevalence over time.
Out of the 83 countries/territories evaluated, 72 countries and 5 territories were endemic for SCH and only 51 (66%) of endemic countries/territories had coordinated national- or territory-level SCH control programs during the past century. Nine endemic countries/territories (Iran, Japan, Jordan, Lebanon, Martinique, Mauritius, Morocco, Puerto Rico, and Tunisia) applied coordinated control programs and achieved “success” as defined in the study. Nine countries/territories that have not yet achieved schistosomiasis elimination achieved a 90% or greater reduction in their country-level prevalence since baseline (before control): China, Egypt, Guadeloupe, Indonesia, Iraq, Philippines, Saudi Arabia, St. Lucia, and Venezuela. Elimination or non-endemicity was not found to be associated with having a control program, confounded due to several countries achieving “fortuitous elimination
” without documented control efforts and several other countries failing to eliminate schistosomiasis, despite substantial prevalence reductions. The prevalence reduction rate depended strongly on strategy type and coverage. Snail control programs showed the strongest prevalence reductions with/without MDA (Figure 2). Widespread snail control reduced prevalence by 92 ± 5% (N = 19) vs. 37 ± 7% (N = 29) for programs using little or no snail control.
Discussion and Comments
This study supports recent hypotheses that snail control is key to SCH reduction. The most common strategy for snail control was the use of expensive and toxic molluscicides, which is not feasible for many poor countries. Although chemotherapy has undeniable health benefits for infected individuals, MDA alone has done little to curb re-infection. In addition, the fortuitous elimination of SCH in six countries suggests that additional factors affect elimination, such as species invasions, sanitation and other health care improvements, and ecosystem changes. Programs have been most effective when snail control is coordinated with chemotherapy via a progression from widespread MDA to a focus on high-risk groups and, finally, surveillance and passive treatment distribution within health centers.
In the past decade, many countries in Africa have established national control programs and implemented SCH control through MDA with praziquantel. While significant progress has been made in reducing SCH prevalence in many countries, major challenges remain: high re-infection persists in high transmission areas and low level of prevalence persists in low transmission areas, even with many rounds of MDA. The success of snail control strategies should not be disregarded. Introduction of snail control may accelerate SCH elimination in Africa. Further research on innovative, safe and cost-effective tools to target the snail intermediate host is urgently needed to propel countries toward elimination.