Main snail species in Africa and their identification.

In Africa and adjacent regions, two snail genera, Biomphalaria and Bulinus, are responsible for transmission of Schistosoma mansoni and Schistosoma haematobium, respectively.¹⁴ Within each snail genus, different species have their own geographical distributions and compatibilities with different strains and species of Schistosoma. As a result, transmission of schistosomiasis varies both within and between endemic areas.¹⁵ The specificity of the schistosome–snail interaction means that in a particular endemic region, there are typically only one or two species of snails responsible for local transmission. Therefore, it is possible to define places where Schistosoma transmission takes place by mapping the occurrence of compatible snails at human–water contact sites (HWCS).

Numerous techniques are available for snail identification. Regional identification keys, such as those produced by the Danish Bilharzia Laboratory, predominantly use shell morphology to identify to family and genus level, although radulae and soft tissue morphology may be needed to delimit further.

Molecular identification methods such as cytochrome c oxidase 1 mitochondrial gene (Cox1) sequences are available for numerous freshwater snails (e.g., Refs. ^16–18), and reference data are available in online databases such as GenBank and barcode of life data system (BOLD).¹⁹ As the same Cox1 region has been sequenced within and between species, this is a good comparison gene for general species identification. However, correct primer selection is critical when dealing with different geographical isolates, in particular for the species of Bulinus.¹⁷ Other genes used for phylogenetic analyses and taxonomic purposes include the ribosomal internal transcribed spacer (ITS), 16S, and 18S.^18,20 As sequencing technologies progress, the precision of snail identifications can be expected to improve by inclusion of more marker genes, mitochondrial genomes,²¹ or entire genomes.²²