Schistosoma haematobium detection in snails by DraI PCR and Sh110/Sm-Sl PCR: further evidence of the interruption of schistosomiasis transmission in Morocco

Abstract

BackgroundThis is the first study in Morocco to estimate snail infection rates at the last historic transmission sites of schistosomiasis, known to be free from new infection among humans since 2004. Screening of large numbers of snails for infection is one way to confirm that Schistosoma haematobium transmission has stopped and does not resurge.MethodsA total of 2703 Bulinus truncatus snails were collected from 24 snail habitats in five provinces of Morocco: Errachidia, El Kelaa des Sraghna, Tata, Beni Mellal, and Chtouka Ait Baha. All visible snails were collected with a scoop net or by hand. We used waders and gloves as simple precautions. Snails were morphologically identified according to Moroccan Health Ministry guide of schistosomiasis (1982).All snails were analyzed in pools by molecular tool, using primers from the newly identified repeated DNA sequence, termed DraI, in the S. haematobium group. To distinguish S. bovis and S. haematobium, the snails were analyzed by Sh110/Sm-Sl PCR that was specific of S. haematobium.ResultsThe results showed that snails from Errachidia, Chtouka Ait Baha, sector of Agoujgal in Tata and sector of Mbarkiya in El kelaa des Sraghna were negative for DraI PCR; but, snails from remaining snail habitats of El Kelaa des Sraghna, Tata and Beni Mellal were positive. This led to suggest the presence of circulating schistosome species (S. haematobium, S. bovis or others) within these positive snail habitats. Subsequently, confirmation with S. haematobium species specific molecular assay, Sh110/Sm-Sl PCR, showed that none of the collected snails were infected by S. haematobium in all historic endemic areas.ConclusionThe absence of S. haematobium infection in snails supports the argument of S. haematobium transmission interruption in Morocco.

Advances in the Diagnosis of Human Schistosomiasis

“…Moreover, determining the prevalence of schistosome species within snail hosts, including prepatent infections, is especially important for identifying specific transmission sites with seasonal and spatial patterns (310,311). These findings are important for detecting risk areas to guide surveillance and the use of interventions, as well as to determine the efficacy of ongoing control strategies (312). Hence, this approach is extremely helpful in monitoring control and elimination efforts.…”

“…It has been shown that PCR-based assays can detect prepatent schistosome infections and infection with a single miracidium (316)(317)(318). Amplification of different species-specific schistosome gene segments in snails, including the 18S rDNA gene of S. mansoni and the DraI 121-bp repeat sequence of S. haematobium, has been tested successfully and applied in community studies on a large scale (310,312,319,320). Modified PCR methods, including multiplex and nested PCRs, have also been used successfully for the identification of infected snails (314)(315)(316)(320)(321)(322).…”

Towards interruption of schistosomiasis transmission in sub-Saharan Africa: developing an appropriate environmental surveillance framework to guide and to support ‘end game’ interventions

“…More broadly, the application of DNA-based assays in medical malacology [72] also coincided with the rise in DNA diagnostic assays [19], such as real-time PCR approaches with TaqMan® probes, for medical diagnostics [73, 74]. It is precisely that these medical diagnostic assays have become accepted, standardised and largely routine, that they can be used to spur on interests in environmental surveillance of schistosomes by inspection of water filtrates, environmental samples as well as in field-caught or sentinel snails [65, 66], see Table 1.…”

Selecting accurate post-elimination monitoring tools to prevent reemergence of urogenital schistosomiasis in Morocco: a pilot study

“…In 2009 validation of interruption of transmission was commenced with the initiation of a national serological survey (using enzyme-linked immunoelectrotransfer blot, EITB) screening for human antibodies against S. haematobium in children, followed by a national molecular malacology survey analyzing the prevalence of infected snails (the intermediate host). The results confirmed interruption of transmission and indicated progress towards elimination as it showed that none of children or the collected snails was infected by S. haematobium [5, 6]. However, given that the exact parasite life spans and the distribution of the post-treatment antibody responses across the whole population are not fully understood [1, 7], prevention of reemerging required a vigilant survey strategy.…”

“…Interruption of transmission was claimed in 2004 and since then no new active autochthonous schistosomiasis cases have been reported. In 2015, serological and molecular-malacology survey showed that none of the children and the collected snails were infected by S. haematobium, however about 30% of snails were infected by S. bovis [6]. …”