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Background  Members of the genus  Paragonimus  require at least three hosts in their life-cycles. The obligatory first intermediate hosts are freshwater snails. In Vietnam, although seven  Paragonimus  species have been recorded, the natural first intermediate hosts of almost all species have not been confirmed. The aim of this study was, therefore, to investigate snail hosts of  Paragonimus  species in Vietnam, and to identify  Paragonimus  species at intramolluscan stages.    Methods  Freshwater snails were collected from streams in Yen Bai and Quang Tri Provinces, where high prevalences of  Paragonimus  metacercariae in crab hosts have been reported. Snails were morphologically identified and then examined individually for  Paragonimus  cercariae using shedding and crushing methods. Chaetomicrocercous cercariae, the morphological class to which  Paragonimus  cercariae belong, were collected for morphological description and molecular species identification by analyses of  ITS 2 sequences. The infected snail species were identified based on analyses of nucleotide sequences of the  cox 1 gene.    Results  Three snail species were found to be infected with  Paragonimus  cercariae at low infection rates, ranging between 0.07–1.0%. The molecular analyses identified them as  Sulcospira quangtriensis  and 2 species of subfamily Triculinae. In a phylogenetic tree, these two triculine snails were related to the genera  Gammatricula  and  Tricula  with low posterior probabilities. Thus we named them as Triculinae sp. 1 and Triculinae sp. 2. Cercariae from the three snail species,  Sulcospira quangtriensis , Triculinae sp. 1 and Triculinae sp. 2, were molecularly identified as  Paragonimus westermani ,  P. heterotremus  and  P. proliferus , respectively. The cercariae of the three species are morphologically similar to each other, but their daughter rediae can be distinguished by the length of the intestine and the number of cercariae per redia. The rediae of  P. westermani  have a long intestine and each contain 6–8 cercariae. In contrast, those of  P. heterotremus  and  P. proliferus  have a short intestine and each redia contain 10–12 and 5–6 cercariae, respectively.    Conclusions  Three snail species,  Sulcospira quangtriensis , Triculinae sp. 1 and Triculinae sp. 2, serve as the first intermediate hosts of  P. westermani ,  P. heterotremus  and  P. proliferus , respectively, in Vietnam. The length of the intestine of rediae and the number of cercariae per redia are valuable characteristics for distinguishing between larvae of these  Paragonimus  species.

First intermediate hosts of Paragonimus spp. in Vietnam and identification of intramolluscan stages of different Paragonimus species