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A comprehensive investigation, incorporating both morphological and molecular analyses, has unveiled the existence of a hitherto unknown nematode species, Paracapillaria (Ophidiocapillaria) siamensis sp. nov., residing in the intestine of the monocled cobra, Naja kaouthia , in the central region of Thailand. This study integrates morphological characteristics, morphometric examination, scanning electron microscopy and molecular phylogenetic analysis (COI , 18S rRNA and ITS1 genes). The findings place the newly described species within the subgenus Ophidiocapillaria , elucidating its distinctive characteristics, including a frame-like proximal spicule shape, approximate lengths of 19 000 and 22 500 μm with approximate widths of 90 and 130 μm for males and females, 39‒45 stichocytes, elevated lips without protrusion, a dorsal bacillary band stripe with an irregular pattern of bacillary cells and evidence of intestinal infection. These features serve to differentiate it from other species within the same subgenus, notably Paracapillaria (Ophidiocapillaria) najae De, 1998, a species coexisting P. siamensis sp. nov. in the monocled cobra from the same locality. This study addresses the co-infection of the novel species and P. najae within the same snake host, marking the second documented instance of a paracapillariid species in the monocled cobra within the family Elapidae. The genetic characterization supports the formal recognition of P. siamensis sp. nov. as a distinct species, thereby underscoring its taxonomic differentiation within the Capillariidae family. This research identifies and characterizes the new nematode species, contributing valuable insights into the taxonomy of this nematode. [ABSTRACT FROM AUTHOR]

The parasitic nematode Paracapillaria (Ophidiocapillaria) najae De, 1998, found in the Indian cobra Naja naja is redescribed and re-illustrated in the present study. The monocled cobra Naja kaouthia was discovered to be a new host for this parasite in central Thailand. A comprehensive description extending the morphological and molecular characteristics of the parasites is provided to aid species recognition in future studies. The morphometric characters of 41 parasites collected from 5 cobra specimens are compared with those described in the original studies. Phylogenetic analyses using mitochondrial cytochrome c oxidase subunit 1 and nuclear 18S ribosomal RNA genes were performed to provide novel information on the systematics of P. najae. Similar characteristics were observed in the examined nematode samples, despite being found in different hosts, confirming their identity as P. najae. The molecular genetic results support the species status of P. najae , indicating P. najae is well defined and separated from other related nematode species in the family Capillariidae. Morphological descriptions, genetic sequences, evolutionary relationships among capillariids and new host and distribution records of P. najae are discussed. Paracapillaria najae specimens found in the Thai cobra had some morphological variation, and sexual size dimorphism was also indicated. Paracapillaria najae was found to infect various cobra host species and appeared to be common throughout the Oriental regions, consistent with its hosts' distribution. [ABSTRACT FROM AUTHOR]