Cryptic Biomphalaria spp. diversity, spatial population structure and contrasting schistosome infection patterns in the Lake Albert region.

Biomphalaria snails are intermediate hosts of Schistosoma mansoni, and their distribution predicts where intestinal schistosomiasis may occur. Here, we investigated the genetic diversity and distribution patterns of Biomphalaria spp. in the Lake Albert region and assessed their role in the transmission of schistosomes. Snails (n = 6869) were screened for schistosome infections using cercariae shedding and molecular techniques. In total, 29,670 Biomphalaria snails were collected, 90% of which were in the upland sites. For a subset of snails (n = 184) we sequenced a partial cytochrome c oxidase subunit one (COX1) fragment for phylogeographic analyses. Biomphalaria sudanica, Biomphalaria pfeifferi and Biomphalaria stanleyi were identified, with the former two species having haplotypes shared at the lake and upland sites. We also report the presence of a cryptic lineage, tentatively named B. cf. pfeifferi, awaiting further description. Molecular assays revealed higher overall prevalence of S. mansoni (1.9%) and S. rodhaini (1.12%) among Biomphalaria spp. (n = 1619) than inferred from cercarial shedding (0.66%). However, S. mansoni occurred exclusively at lake and nearby stream sites and was hosted predominantly by B. sudanica (24.7%, 22/89) and B. pfeifferi (5.4%; 5/147), while S. rodhaini, hosted mainly by B. cf. pfeifferi, was widely distribution in upland sites allopatric to S. mansoni. The wet season (October and November) had the highest S. mansoni prevalence. We argue that the lack of S. mansoni in upland sites reflects a more nuanced situation than simply the absence of suitable host snails, and suggest taxonomic revision of the B. pfeifferi complex.