First confirmed human case of Diphyllobothrium stemmacephalum infection and molecular verification of the synonymy of Diphyllobothrium yonagoense with D. stemmacephalum (Cestoda: Diphyllobothriidea).

Several tapeworm species in the genus Diphyllobothrium Cobbold, 1858 have uncertain taxonomic positions, leading to taxonomic confusion as well as misdiagnosis of infections. Taxonomic revision based on DNA sequence analysis is considered necessary to resolve the taxonomy of several cases, including that between Diphyllobothrium stemmacephalum, the type species of the genus, and Diphyllobothrium yonagoense. Diphyllobothrium yonagoense was synonymized with D. stemmacephalum based on morphological observations by Andersen (1987), however no molecular studies have been undertaken to verify the validity of this synonymization. In the present study, the first human case confirmed molecularly as D. stemmacephalum infection is reported, and the validity of the synonymization of D. yonagoense with D. stemmacephalum was assessed based on molecular phylogenetics. Diphyllobothrium stemmacephalum and D. yonagoense grouped into the same clades with high bootstrap confidence values for both cox1 and nad3. Genetic distances between the two taxa were very small (0.000-0.012 and 0.000-0.017 for cox1 and nad3, respectively) and were considered to fall within the range of intraspecific variation. Using these molecular analyses, this study verified molecularly that D. yonagoense is a junior synonym of D. stemmacephalum. Further, the closer phylogenetic relationship between D. stemmacephalum and Diplogonoporus species rather than other diphyllobothriids, including Diphyllobothrium nihonkaiense and Diphyllobothrium latum, was corroborated. The genus name for D. nihonkaiense and D. latum is also discussed.