Studies on zoonotic onchocerciasis in Japan

Zoonotic onchocerciasis, an emerging human infection caused by animal parasitic Onchocerca species, has recently increased worldwide. We have investigated the transmission of Onchocerca japonica, a parasite of wild boar and a causative agent of zoonotic onchocerciasis in Japan. In this article, the search for the natural vectors (blood-sucking insects) of O. japonica is reviewed. Inoculation experiments demonstrated that six anthropophilic black fly species are suitable for the development of O. japonica microfilariae to the infective larvae. To determine the vector of O. japonica, natural infections of wild-caught black flies with filarial larvae were investigated. Morphometric and molecular identification of the infective larvae detected showed that anthropophilic Simulium bidentatum is a natural vector of O. japonica. In addition, metabolites of the four larval stages of O. japonica, i.e., microfilariae in naturally infected wild boar, and first-stage larvae, second-stage larvae, and third-stage larvae (infective larvae) in experimentally infected black flies were analyzed. Multivariate analysis of the data showed a clear distinction between the four larval stages and hydroxylamine was proposed as a potential biomarker candidate to detect infective larvae in black flies. Finally, a study on speciation of black flies is reviewed by focusing on the Simulium rufibasis subgroup in Asia.