Exploring the evolutionary potential of parasites: Larval stages of pathogen digenic trematodes in their thiarid snail host Tarebia granifera in Thailand

Minute intestinal flukes from several distinct families of endoparasitic platyhelminths are a medically important group of foodborne trematodes prevalent throughout Southeast Asia and Australasia. Their lifecycle is complex, with freshwater snails as primary intermediate hosts, with infecting multiple species of arthropods and fish as second intermediate hosts, and with birds and mammals including humans as definitive hosts. In Southeast Asian countries, the diversity of snail species of the Thiaridae which are frequently parasitized by trematode species is extremely high. Here, the thiarid Tarebia granifera in Thailand was studied for variation of trematode infections, by collecting the snails every two months for one year from each locality during the years 2004–2009, and during 2014–2016 when snails from the same localities were collected and new localities found. From ninety locations a total of 15,076 T. granifera were collected and examined for trematode infections. With 1,577 infected snails the infection rate was found to be 10.46 %. The cercariae were categorized into fifteen species from eight morphologically distinguishable types representing several distinct families, viz. (i) virgulate xiphidiocercariae (Loxogenoides bicolor, Loxogenes liberum and Acanthatrium histaense), (ii) armatae xiphidiocercariae cercariae (Maritreminoides caridinae and M. obstipus); (iii) parapleurophocercous cercariae (Haplorchis pumilio, H. taichui and Stictodora tridactyla); (iv) pleurophocercous cercariae (Centrocestus formosanus); (v) megarulous cercariae (Philophthalmus gralli); (vi) furcocercous cercariae (Cardicola alseae, Alaria mustelae and Transversotrema laruei); as well as (vii) echinostome-type cercariae, and (viii) gymnocephalous-type cercariae. In addition, a phylogenetic marker (internal transcribed spacers 2, ITS2) was employed in generic and infrageneric level classifications of these trematodes, using sequences obtained from shed cercariae isolated from T. granifera specimens of the second study period collected in various regions in Thailand. We obtained ITS2 sequences of cercariae from nine species (of seven types): Loxogenoides bicolor, Loxogenes liberum, Maritreminoides obstipus, Haplorchis taichui, Stictodora tridactyla, Centrocestus formosanus, Philophthalmus gralli, as well as from one species each of echinostome cercariae and gymnocephalous cercariae. Thus, this analysis combines the parasites' data on morphology and geographical occurrence with molecular phylogeny, aiming to provide the groundwork for future studies looking into more details of the parasite-snail evolutionary relationships