A vascular filarial nematode in sika deer (Cervus nippon): Morphological and molecular characterization of Elaeophora (Nematoda: Onchocercidae) in Japan

<i>Elaeophora</i> (Nematoda: Onchocercidae), a filarial nematode infecting the blood vessels of ruminants and horses, is transmitted by tabanid flies. <i>Elaeophora elaphi</i> was previously detected in wild sika deer in Wakayama Prefecture, Japan in 2009; however, detailed information on this species is scarce. In 2023, 26 <i>Elaeophora</i> worms were collected from the hepatic vasculature of eight deer in Nara, Mie, Kyoto and Gifu Prefectures of Japan and analyzed. Species identification was performed by morphological and genetic analyses. Additionally, multi-gene analysis of seven genes was performed to determine their taxonomic position within the family Onchocercidae. The specimens were identified as <i>E. elaphi</i> based on their morphological characteristics. Analyses of 18S rRNA and cytochrome <i>c</i> oxidase subunit 1 genes revealed no variations, indicating that species belonged to the same lineage. Multi-gene analysis revealed that the species belonged to the subfamily Onchocercinae, showing a close relationship with the tick-borne filarial nematodes of the genera <i>Monanema</i>, <i>Acanthocheilonema</i>, <i>Litomosoides</i>, <i>Cruorifilaria</i>, <i>Yatesia</i>, and <i>Cercopithifilaria</i>. This study demonstrated the widespread distribution of <i>E. elaphi</i> in Japan and provided insights into its genetic relationship with other onchocercid species. Further research is necessary to determine the ecological and epidemiological implications of this parasite.