Detecting and identifying Schistosoma infections in snails and aquatic habitats: A systematic review

Abstract

Background

We were tasked by the World Health Organization (WHO) to address the following question: What techniques should be used to diagnose Schistosoma infections in snails and in the water in potential transmission sites? Our goal was to review and evaluate the available literature and provide recommendations and insights for the development of WHO’s Guidelines Development Group for schistosomiasis control and elimination.

Methodology

We searched several databases using strings of search terms, searched bibliographies of pertinent papers, and contacted investigators who have made contributions to this field. Our search covered from 1970 to Sept 2020. All papers were considered in a PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) framework, and retained papers were grouped by technique and subjected to our GRADE (Grading of Recommendations, Assessment, Development and Evaluations) evidence assessment profile determined in consultation with WHO. We also considered issues of sensitivity, specificity, coverage, cost, robustness, support needs, schistosome species discrimination, and relevant detection limits.

Principal findings

Our PRISMA process began with the perusal of 949 articles, of which 158 were retained for data extraction and evaluation. We identified 25 different techniques and for each applied a GRADE assessment considering limitations, inconsistency, imprecision, indirectness, and publication bias. We also provide advantages and disadvantages for each category of techniques.

Conclusions

Our GRADE analysis returned an assessment of moderate quality of evidence for environmental DNA (eDNA), qPCR and LAMP (Loop-mediated isothermal amplification). No single ideal diagnostic approach has yet been developed, but considerable recent progress has been made. We note a growing trend to use eDNA techniques to permit more efficient and replicable sampling. qPCR-based protocols for follow-up detection offer a versatile, mature, sensitive, and specific platform for diagnosis though centralized facilities will be required to favor standardization. Droplet digital PCR (ddPCR) can play a complementary role if inhibitors are a concern, or more sensitivity or quantification is needed. Snail collection, followed by shedding, is encouraged to provide specimens for sequence verifications of snails or schistosomes. LAMP or other isothermal detection techniques offer the prospect of less expensive and more distributed network of analysis but may face standardization and verification challenges related to actual sequences amplified.

Ability to detect schistosome infections in snails or in the water is needed if control and elimination programs hope to succeed. Any diagnostic techniques used need to be regularly verified by the acquisition of DNA sequences to confirm that the detected targets are of the expected species. Further improvements may be necessary to identify the ideal schistosome or snail sequences to target for amplification. More field testing and standardization will be essential for long-term success.

Author summary

Global efforts are underway to reach the goal of elimination of schistosomiasis as a public health problem by 2030. A crucial step in elimination programs is the verification of elimination, including surveillance of former transmission foci. This systematic review assessed and evaluated a wide range of diagnostic tools for detection of Schistosoma parasites in snails and water. Our analysis revealed that along with standard snail shedding methods, molecular methods such as PCR, qPCR and LAMP are becoming the widely adopted standard approaches to detect schistosomes in snails. Recent developments in eDNA methods are further enabling novel surveillance capabilities for snails and schistosomes in water and are likely to become more widely adopted. While there is currently a plethora of techniques to choose from, there is a clear need for further field testing and development of standardized protocol for the most promising among them, including eDNA, ddPCR, qPCR and LAMP methods. Future studies focused on field-worthy detection approaches and their efficacy and sensitivity in the field will be a corner stone in development of control and elimination programs.