Molecular and morphological identification of helminthes found in Japanese smelt, Hypomesus transpacificus nipponensis, with notes on new host records of Eustrongylides ignotus and Raphidascaris gigi

As an adult, Eustrongylides ignotus, recognized as a zoonotic parasite, is a parasitic nematode found in the gastrointestinal tract of fish-eating birds. As larvae, they are found in the connective tissue or body cavities of freshwater fish. In Japan, E. ignotus and E. tubifex have been found in three avian species, but their distribution in intermediate and paratenic hosts remains unclear. For this study, 16 commercial raw Japanese smelts, Hypomesus transpacificus nipponensis, were used for parasitological examination. A pinkish red worm collected from the abdominal wall of one smelt (6.3%) was identified as E. ignotus based on the high sequence similarity (99.9%) of the partial 18S ribosomal RNA gene and the morphology of its cephalic and caudal ends. Furthermore, a Raphidascaris-like nematode was found in the intestines of five smelts examined (31.3%) and was identified as R. gigi in view of the absence of lateral cuticular alae. Eustrongylides ignotus and R. gigi have been reported in various freshwater fish, but not in smelts. Therefore, this finding of E. ignotus and R. gigi in H. transpacificus nipponensis represents a new host record.