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.

Simultaneous genotyping of snails and infecting trematode parasites using high‐throughput amplicon sequencing

“…We statistically tested for differences in read support for each category of ASVs (primary, secondary and contamination) per marker in infected vs. uninfected snails using pairwise Wilcoxon tests with Bonferroni correction for multiple comparisons in R. In addition, we evaluated the reliability of the diagnostics inferred with the HTAS workflow by comparing it with the results of an infection rapid‐diagnostic PCR (infection RD‐PCR, Schols et al, 2019). We opted for this method as it allows us to simultaneously detect the presence of any trematode within the snail and verify whether some of these trematodes belong to the genus Schistosoma , which is not possible with other existing qPCR or ddPCR assays (Kamel et al, 2021). The method consists of multiplexing three primer pairs in a PCR and evaluating the infection diagnostic based on amplicon size discrimination using gel electrophoresis.…”

“…The fragmentary nature of current knowledge on parasite communities within lymnaeid or planorbid snail hosts (but see Soldánová et al, 2012 or Laidemitt et al, 2019) can to a large extent be ascribed to technical research limitations. First, the traditional parasite shedding method to detect infections is highly time‐consuming (Born‐Torrijos et al, 2014; Kamel et al, 2021). Because typically only a limited fraction of the snails in a population are infected (overdispersion, Curtis et al, 2002), large numbers of snail specimens must be screened to adequately characterize the trematode community (Eppert et al, 2002; Loker et al, 1981), limiting the scalability of these methods.…”

“…Cercarial genotyping resolves this issue but is labour‐intensive as multiple cercariae need to be sequenced from each snail to detect all co‐infecting strains and species (Bakuza et al, 2017; Eppert et al, 2002; Thiele & Minchella, 2013; Van den Broeck et al, 2011), which limits scalability in co‐infection studies. Recent methodological advances include the development of molecular xenomonitoring tools such as PCR assays that identify parasite infections in snails to family, genus or species level (see Kamel et al, 2021 for a review). Compared to shedding, these tend to be more sensitive for detecting infections (an additional 10%–60%; Born‐Torrijos et al, 2014) and they allow cost‐efficient screening of large snail populations when combined with a pooling design (Carolus et al, 2019).…”

Scratching the Itch: Updated Perspectives on the Schistosomes Responsible for Swimmer’s Itch around the World

“…Sequences for commonly used reference barcode genes, such as cox1 or ITS2, are obtained and compared to available databases such as GenBank. The shedding method is used the world over, whether to survey for human or non-human schistosomes [ 135 ].…”

“…Pros and cons of these and other methods have been discussed [ 135 , 137 ]. For the purposes of documenting biodiversity, having the actual sample of the infected gastropod and the schistosome cercariae it is shedding provide enduring samples for natural history collections.…”