Parachordodes or Gordius worms have parasitic larval stages, and final hosts include the mantis and some carnivorous or omnivorous species of Orthoptera (Acrididae: grasshoppers, Gryllidae: crickets, etc.), and Coleoptera (Carabidae: ground beetles). The caudal end of the male is either bifurcated or has a dorsoventral groove in Gordiidae and Parachordodinae, but not in Chordodinae. When intermediate hosts are eaten by the final hosts, larvae develop to juveniles and grow to adult size. Adults of Parachordodes (Chordodidae: Parachordodinae), Gordius (Gordiidae), or the genera of the subfamily Chordodinae (Chordodidae), also commonly known as hair worms, are free-living freshwater inhabitants, and their larvae are first free-living and then infect intermediate hosts. Parachordodes or Gordius worms belong to the phylum Nematomorpha (Gordiida, Gordioidea). Despite their worldwide distribution, detailed life cycles of Nematomorpha have not been completely documented. Nematomorpha is a phylum of animals that has been poorly studied. DNA sequencing of the Parachordodes worm does not appear on the database therefore, more information on the gene sequences of the genus Parachordodes from humans, animals, or intermediates is required. which has already been reported in humans in Japan. DNA analysis on 18S rRNA partial sequence arrangements was also carried out and both worms were assumed to be close to the genus Paragordionus based on tree analysis, and far from Gordius sp. (Nematomorpha: Chordodidae) based on the characteristic morphologies of cross sections and areoles in the cuticle. They were identified as Parachordodes sp.

Both worms were males having bifurcated posterior ends and male gonads in cross sectional specimens. Two gordiid worms were collected in the vomit and excreta of an 80-year-old woman in November 2009 in Kyoto city, and in the mouth of 1-year-old boy in December 2009 in Nara city, Japan, respectively. The present study was performed to describe 2 human cases infected by the horsehair worm, Parachordodes sp., in Japan.

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