Date Published: December 28, 2015
Publisher: Public Library of Science
Author(s): Charles H. King, Laura J. Sutherland, David Bertsch, Eric S Loker. http://doi.org/10.1371/journal.pntd.0004290
Abstract: BackgroundPrograms for schistosomiasis control are advancing worldwide, with many benefits noted in terms of disease reduction. Yet risk of reinfection and recurrent disease remain, even in areas with high treatment coverage. In the search for means to better prevent new Schistosoma infections, attention has returned to an older strategy for transmission control, i.e., chemical mollusciciding, to suppress intermediate host snail species responsible for S. mansoni and S. haematobium transmission. The objective of this systematic review and meta-analysis was to summarize prior experience in molluscicide-based control of Bulinus and Biomphalaria spp. snails, and estimate its impact on local human Schistosoma infection.Methodology/Principal FindingsThe review was registered at inception with PROSPERO (CRD42013006869). Studies were identified by online database searches and hand searches of private archives. Eligible studies included published or unpublished mollusciciding field trials performed before January 2014 involving host snails for S. mansoni or S. haematobium, with a primary focus on the use of niclosamide. Among 63 included papers, there was large variability in terms of molluscicide dosing, and treatment intervals varied from 3–52 weeks depending on location, water source, and type of application. Among 35 studies reporting on prevalence, random effects meta-analysis indicated that, on average, odds of infection were reduced 77% (OR 0.23, CI95% 0.17, 0.31) during the course of mollusciciding, with increased impact if combined with drug therapy, and progressively greater impact over time. In 17 studies reporting local incidence, risk of new infection was reduced 64% (RR 0.36 CI95% 0.25, 0.5), but additional drug treatment did not appear to influence incidence effects.Conclusion/SignificanceWhile there are hurdles to implementing molluscicide control, its impact on local transmission is typically strong, albeit incomplete. Based on past experience, regular focal mollusciciding is likely to contribute significantly to the move toward elimination of schistosomiasis in high risk areas.
Partial Text: Schistosomiasis, the chronic human disease caused by Schistosoma spp. parasite infections, is a preventable illness that, if left untreated, is associated with long-term undernutrition, anemia, organ scarring and fibrosis, resulting in disabling patient symptoms [1, 2]. Current anti-schistosomiasis chemotherapy programs focus on controlling or preventing morbidity by treating school-age children who typically have the highest levels of Schistosoma infection . However, because pre-school infection [4–6] and recurrent infection during childhood [7, 8] are associated with significant risk for disease, optimal disease prevention can occur only when parasite infection or reinfection can be effectively blocked . By themselves, Preventive Chemotherapy (PCT) campaigns  using mass drug administration have not been very successful in limiting transmission in high-risk areas [4, 10–13]. The WHO roadmap’s new focus on ‘transmission control, wherever possible’  means it is appropriate to re-examine the efficacy of intermediate-host snail control for prevention of human-to-snail-to-human parasite transmission.
This systematic review and meta-analysis summarizes what has been a broad and lengthy experience with the use of mollusciciding for control of S. mansoni and S. haematobium transmission. Results of our analysis suggest that chemical-based snail control, particularly with the compound niclosamide, can effectively reduce local transmission of Schistosoma parasites when delivered at regular interval and under skilled supervision . Direct treatment effects on snails were difficult to summarize, because of the many differences in sampling and reporting used in the included studies. However, where snail reductions were quantified, most programs saw very significant reductions or complete disappearance of local Schistosoma host snails during program implementation. Earlier studies tended to favor broad, intensive mollusciciding in an attempt to eliminate intermediate host Biomphalaria or Bulinus spp. snails. With such approaches, and particularly where water flow could be controlled in irrigation systems and transmission was more seasonal, treatment intervals could be extended to 6–12 months [8, 79, 83]. Later studies, often dealing with natural water bodies and more focal human water contact, tended to favor more frequent focal administration of mollusciciding [13, 41, 43, 62, 66, 84], allowing snails to persist elsewhere outside the main human water contact zones. While not fully explored, several preliminary studies suggested that slow-release strips or pellet formulations [85, 86] or delayed-release molluscicide capsules  might better focus the impact of chemical molluscicide and extend its duration of impact, with concomitant cost-savings due to a reduced need for frequent delivery.