Effective interruption of the Schistosoma life cycle is essential to blocking the parasite’s transmission, and thus truly preventing human schistosomiasis over the long term. Our current mass treatment campaigns were expected to limit transmission by reducing environmental contamination with parasite eggs. However, the process of transmission has proven to be very focal and highly efficient, such that, in practice, mass drug treatment by itself has not been effective in lowering transmission in many affected areas. Alternative means for interruption of transmission are now being considered, including means to reduce or eliminate intermediate snail hosts from local habitats, and means to prevent water contamination through sanitation.

https://journals.plos.org/plosntds/article/file?id=10.1371/journal.pntd.0003657&type=printable

Historical perspective: snail control to prevent schistosomiasis.

Background 
Programs 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 Findings 
The 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 Eligible studies included published or unpublished mollusciciding field trials performed before January 2014 involving host snails for S. mansoni or S. haematobium, with a primary focus on the use of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/Significance 
While 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.

/pdf/systematic-review-and-meta-analysis-of-the-impact-of-1xc695mymr.pdf

Systematic Review and Meta-analysis of the Impact of Chemical-Based Mollusciciding for Control of Schistosoma mansoni and S. haematobium Transmission

Background 
In 2012 the World Health Assembly adopted resolution WHA65.21 on elimination of schistosomiasis, calling for increased investment in schistosomiasis control and support for countries to initiate elimination programs. This study aims to analyze prevalence and intensity of Schistosoma mansoni infection in children in Latin America and the Caribbean countries and territories (LAC), at the second administrative level or lower.
Methodology 
A systematic review of schistosomiasis prevalence and intensity of infection was conducted by searching at PubMed, LILACS and EMBASE. Experts on the topic were informally consulted and institutional web pages were reviewed (PAHO/WHO, Ministries of Health). Only SCH infection among children was registered because it can be a ‘proxi-indicator’ of recent transmission by the time the study is conducted.
Principal Findings 
One hundred thirty two full-text articles met the inclusion criteria and provided 1,242 prevalence and 199 intensity of infection data points. Most of them were from Brazil (69.7%). Only Brazil published studies after 2001, showing several 'hot spots' with high prevalence. Brazil, Venezuela, Suriname and Saint Lucia need to update the epidemiological status of schistosomiasis to re-design their national programs and target the elimination of Schistosoma mansoni transmission by 2020. In Antigua and Barbuda, Dominican Republic, Guadeloupe, Martinique, Montserrat and Puerto Rico schistosomiasis transmission may be interrupted. However the compilation of an elimination dossier and follow-up surveys, per WHO recommendations, are needed to verify that status. Hence, the burden of subtle SCH chronic infection may be still present and even high in countries that may have eliminated transmission. Heterogeneity in the methodologies used for monitoring and evaluating the progress of the schistosomiasis programs was found, making cross-national and chronological comparisons difficult.
Conclusions 
There is a need for updating the schistosomiasis status in the historically endemic countries and territories in LAC to address the required public health interventions for control and elimination programs or to verify the elimination of transmission of Schistosoma mansoni. Improved reporting and standardization of the monitoring and evaluation methodologies used are recommended, while using available WHO guidelines. Meeting a regional elimination goal will require additional and improved epidemiological data by age group and sex.

/pdf/schistosomiasis-prevalence-and-intensity-of-infection-in-23rt6b62zq.pdf

Schistosomiasis Prevalence and Intensity of Infection in Latin America and the Caribbean Countries, 1942-2014: A Systematic Review in the Context of a Regional Elimination Goal.

In Brazil, Biomphalaria glabrata, B. tenagophila, and B. straminea are naturally infected by the trematode Schistosoma mansoni, the causative agent of schistosomiasis. Despite decades of governmental efforts through official control programs, schistosomiasis remains an important public health problem in the country: thousands of people are infected with the trematode each year and millions live in endemic areas. The World Health Organization recommends using a combination of molluscicide (niclosamide) and mass chemotherapy to control the transmission of schistosomiasis, with this treatment successfully reducing the morbidity of the disease. In the past, niclosamide has been used in official schistosomiasis control programs in Brazil. However, as B. glabrata recolonizes even after molluscicide application, the use of molluscicides has gradually decreased in the country until they were discontinued in 2002, mainly due to the rising global pressure to preserve the environment and the difficulties of obtaining licenses from the Brazilian Ministry of Environment to use toxic substances in aquatic ecosystems. Therefore, the discovery of new molluscicides, which could be more selective to Biomphalaria species and less harmful to the aquatic ecosystem, is necessary. In addition, political efforts to sensitize funders to provide grants for this field of research are required. In this context, this article aims to make a critical analysis of molluscicide application in schistosomiasis control programs in Brazil.

/pdf/critical-analysis-of-molluscicide-application-in-msi68r7jmi.pdf

Critical analysis of molluscicide application in schistosomiasis control programs in Brazil.

This study assesses the evolution of schistosomiasis in the endemic area of Pernambuco, using data from five campaigns of chemotherapy control carried out by national health programmes from 1977 to 1996. Analysis of the data showed that: a) the proportion of municipalities with prevalence above 25% was significantly higher in the coastal-forest zone than in the zone of transitional vegetation Agreste in the four evaluations made in the endemic area; b) the prevalence of infection decreased in both zones even when the interval between campaigns were more than five years. The last survey (1996) indicated a predominance of municipalities with prevalences below 25%. However, the majority of these municipalities had localities with prevalence above 50%. A proposal is presented for the identification of the problematic localities, where complementary measures to chemotherapy, such as systematic snail control, improved sanitation, health education and community mobilization, are still necessary.

/pdf/avaliacao-das-acoes-de-controle-da-esquistossomose-157ikg0jad.pdf

Avaliação das ações de controle da esquistossomose implementadas entre 1977 e 1996 na área endêmica de Pernambuco, Brasil

A long-term controlled field experiment is described from northeastern Brazil, in which the molluscicide Bayluscide was used as the sole means of control against Schistosoma mansoni infection.Bayluscide was effective in the reduction of all of the parameters used for evaluation of the disease: incidence, prevalence, intensity of the infection (egg counts) in man, snail population (dynamics and natural infection rate) and the infectivity of natural water bodies (as measured through the exposure of sentinel mice). A less pronounced reduction of the same parameters was seen in the untreated area.Comments are made on the use of molluscicides and on the present situation in schistosomiasis control in the social and economic context of northeastern Braeil.

D. P. P. Costa

Papers

A long-term schistosomiasis control project with molluscicide in a rural area of Brazil.