Abstract
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 Tarebiagranifera 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 (Loxogenoidesbicolor, Loxogenesliberum and Acanthatriumhistaense), (ii) armatae xiphidiocercariae cercariae (Maritreminoidescaridinae and M.obstipus); (iii) parapleurophocercous cercariae (Haplorchispumilio, H.taichui and Stictodoratridactyla); (iv) pleurophocercous cercariae (Centrocestusformosanus); (v) megarulous cercariae (Philophthalmusgralli); (vi) furcocercous cercariae (Cardicolaalseae, Alariamustelae and Transversotremalaruei); 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): Loxogenoidesbicolor, Loxogenesliberum, Maritreminoidesobstipus, Haplorchistaichui, Stictodoratridactyla, Centrocestusformosanus, Philophthalmusgralli, 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.
Highlights
Trematodes are endoparasitic platyhelminths that infect fishes, birds and other wildlife worldwide and mammals as well as humans
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
As outlined in the Introduction, this represents a serious threat to public health. Thiarid snails such as Melanoides tuberculata, T. granifera, Mieniplotia scabra and Sermyla riqueti have been reported as the intermediate hosts of a wide array of diverse trematodes, such as Haplorchis pumilio, H. taichui, Loxogenoides bicolor, Centrocestus formosanus, Acanthatrium hitaense, Haematoloechus similes, Cloacitrema philippinum, Transversotrema laruei, Stictodora tridactyla, Apatemon gracilis, Mesostephanus appendicalatus, Cardicola alseae and Alaria mustelae (Dechruksa et al 2007, Ukong et al 2007, Krailas et al 2011, 2014)
Summary
Trematodes (or flatworms) are endoparasitic platyhelminths that infect fishes, birds and other wildlife worldwide and mammals as well as humans. As liver flukes and intestinal flukes human infecting parasites are highly prevalent in Southeast Asian countries (Wongratanacheewin et al 2001, Chai et al 2005, 2013, Krailas et al 2014). Infections caused by these flukes have a major public health impact and are of economic importance in veterinary medicine. The prevalence of trematodes in Thailand remains a continuous problem (Krailas et al 2014)
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