The cross sectional studies at the epidemiological area in Binh Dinh province was conducted to define the prevalence and risk factors of the small liver tape-worm disease caused by O. viverrini species in both last hosts, such as human and duck, and the intermediate hosts, such as snail and fish. In addition, the gene sequence in mitochondrial of the identified small liver tape-worms was decoded in order to define the genetic position of this small liver tape-worm species in the phylogenetic tree of the Opisthorchiidae family in relationship with the Opisthorchis species in this family that was identified prior. The studied result showed that the infection rate of human with O. viverrini in the surveyed communities in Binh Dinh province was 11.4% and the infection rate of duck with Opisthorchis sp BD 2013 was 34.3%. The two snail species, such as Bithynia siamensis goniomphalos and B. funiculata were determined to be the first intermediate hosts and 10 fresh water fish species, belonging to Cyprinidae family were determined to be the second intermediate hosts of the small liver tape-worm (O. viverrini) in Viet Nam. Of which, the infection rate of Carassius auratus species with (the raw meat of this fish species was often eaten by the communities in the surveyed region) O. viverrini was highest. Also, the studied result indicated that O. viverrini and Opisthorchis sp BD2013 were identified in the same 1st intermediate host (snail species Bithynia funiculata) and 2nd and 3rd intermediate hosts (fish species Esomus metallicus, Puntius brevis and Rasbora aurotaenia). The result of analysing genes (nad1, cob and cox1) in mitochondrial and phylogenetic tree indicated that Opisthorchis sp BD2013 was an independent small tape-worm species, it related closely to O. viverrini. Although, the Opisthorchis sp BD2013 species was not found in human, but the sympatric distribution and sharing the same intermediate hosts in the same epidemiological area of these small liver tape-worm species indicated that the risk of creating a complicated epidemiology of the small liver tape-worm infection due to crossing and gene transfer between two closely relation small liver
tape-worm species may occur in the same last host.