Bulinus tropicus (Krauss, 1848)

Hammoud, Cyril, Kayenbergh, Annelies, Tumusiime, Julius, Verschuren, Dirk, Albrecht, Christian, Huyse, Tine & Bocxlaer, Bert Van, 2022, Trematode infection affects shell shape and size in Bulinus tropicus, International Journal for Parasitology: Parasites and Wildlife 18, pp. 300-311 : 308

4.1. Trematode infections in Bulinus tropicus from Lake Kasenda

The overall trematode infection prevalence (38.5%) and trematode species richness (11 taxa) observed in B. tropicus from Lake Kasenda were both high. However, care should be taken when comparing our results with values from other studies because various methods were used to diagnose and identify infections, which have different levels of sensitivity. More specifically, cercarial shedding may underestimate trematode prevalence because it does not detect immature infections and it is less sensitive than RD-PCR ( Born-Torrijos et al., 2014; Carolus et al., 2019) and high-throughput amplicon sequencing (HTAS, Hammoud et al., 2022). Furthermore, studies that rely on morphological identification of cercariae may underestimate species richness since diagnostic morphological characters are scarce in larval trematodes (Nolan and Cribb, 2005). Using cercarial shedding and morphological identification, Loker et al. (1981) reported infections by 2–7 parasite morphotypes with a prevalence ranging from 2.5 to 9.7% for several species of Bulinus from Tanzanian streams (sample size ranging from 186 to 1503 snails). Chingwena et al. (2002) analysed 4080 B. tropicus collected from streams and dams in Zimbabwe, 13.1% of which were infected by combinations of 5 morphotypes of cercariae, and Mohammed et al. (2016) reported that 46.2% of 1403 B. truncatus collected form irrigation canals in Sudan were infected by an assemblage of 8 morphotypes of cercariae. Beyond sampling size, various factors likely drive variation in observed trematode species richness among snail populations, including differences in susceptibility to infection among snail species and final host species richness ( Hechinger and Lafferty, 2005). Discussing these extensively is beyond our scope, but the infection dynamics reported here combined with the properties of the study system mentioned above render Bulinus tropicus from Lake Kasenda promising to study trematode-intermediate host dynamics in detail. Although our molecular approach towards linking parasites and their intermediate hosts has high sensitivity and allows accurate characterization of trematode taxa, we were still limited in producing specific trematode identifications, because trematodes are underrepresented in public DNA databases (see also Schols et al., 2020). We are, however, optimistic that integrative molecular and morphological characterizations of trematodes (e.g., Laidemitt et al., 2019) will improve future species-level identification, and trematode representation in existing databases. Despite this limitation, our study indicates that relying on multiple molecular markers increased our chances to find close matches in DNA databases and it allowed us to link sequences of different markers belonging to the same trematode species, which can subsequently help attributing GenBank data of unverified/uncertain origin to trematode genera or even species a posteriori.