Further morphological features, molecular characterization, and Wolbachia endosymbionts of Dirofilaria ursi Yamaguti, 1941 (Spirurida: Onchocercidae) from the Japanese black bear Ursus thibetanus japonicus Schlegel (Carnivora: Ursidae).

Dirofilaria ursi Yamaguti, 1941 (Spirurida: Onchocercidae) was originally described from the Japanese black bear Ursus thibetanus japonicus and later reported from the American black bear U. americanus in North America. Both D. ursi and D. ursi-like species have been implicated in human zoonosis in Japan and North America. This study aimed to revise the morphological characteristics of D. ursi in Japan and determine its phylogeny using multi-locus sequence analyses. We investigated the genotypes of Wolbachia endosymbionts harbored by Japanese D. ursi and compared them with those of American D. ursi to assess genetic variation. A pre-esophageal cuticular ring was observed in both juvenile and adult D. ursi specimens and compared with a similar structure previously reported in D. immitis to evaluate its morphological relationship within the genus. Molecular analyses placed D. ursi within the monophyletic clade ONC3, consisting of the genera Dirofilaria and Onchocerca in the subfamily Onchocercinae; Dirofilaria species formed a monophyletic clade in ONC3. Within the species cluster of the genus Dirofilaria, species of the subgenera Dirofilaria (cardiovascular species) and Nochtiella (species inhabiting subcutaneous and connective tissues) did not form separate monophyletic groups. Japanese D. ursi harbored Wolbachia endosymbionts of the supergroup C, and the genotypes exhibited a high sequence similarity to those identified in American D. ursi. Consequently, we consider that Japanese and American D. ursi are closely related at the molecular level, despite differences in host species and geographic distribution.