Molecular identification of infective larvae of three species of Onchocerca found in wild-caught females of Simulium bidentatum in Japan.

Wild female black flies attracted to a man or an idling automobile were collected at Oita, Japan where five cases of zoonotic onchocerciasis had occurred. Among the five Simulium species captured, 2% of Simulium bidentatum, the predominant species, were infected with filarial larvae. There were at least two types of infective larvae, types A and B, based on morphometric observation. Moreover, molecular analysis of the mitochondrial cytochrome c oxidase subunit 1 (CO1) gene revealed that types A and B were represented by a single unknown species of Onchocerca and two species, i.e., Onchocerca dewittei japonica from wild boar, the causative agent of zoonotic onchocerciasis in Japan, and an undescribed Onchocerca sp. from wild boar, respectively. Phylogenetic analysis based on the sequences of the mitochondrial 12S ribosomal RNA (12S rRNA) gene also showed that type A is likely to be an unknown species of Onchocerca. Natural infection of black flies with infective larvae of O. dewittei japonica and O. sp. was demonstrated for the first time. The present study strongly suggests that S. bidentatum plays a role as a vector in the transmission of zoonotic onchocerciasis due to O. dewittei japonica in Japan.