Evidence of human-associated genetic similarity and a cryptic lineage in wild boar-derived Ascaris from Ishikawa Prefecture in Japan.

Background: Two species of Ascaris nematodes infect humans: Ascaris lumbricoides, a human parasite, and Ascaris suum, which primarily infects pigs. Since these species are morphologically similar, molecular techniques are necessary for species identification in human Ascaris cases. A common method involves analyzing nucleotide sequences in the internal transcribed spacer 1 (ITS-1) region, particularly at positions 133 and 246. Although Ascaris nematodes have also been detected in wild boars, they are often classified as A. suum without molecular analysis due to their genetic similarity to pigs. In this study, we conducted molecular identification of Ascaris specimens collected from wild boars in Ishikawa Prefecture to examine their potential origin.

Methods: Six Ascaris specimens from wild boars in Ishikawa Prefecture were analyzed by PCR and sequencing of the ITS-1 and COX1 regions. ITS-1 sequences were aligned to reference data, and phylogenetic analysis was performed using COX1 sequences.

Results: Alignment analysis of the ITS-1 region revealed a nucleotide deletion at position 129, with guanine (G) at position 133 and thymine (T) at position 246. This sequence was 100% identical to the reference A. lumbricoides sequence derived from humans. However, phylogenetic analysis of the COX1 region revealed that these wild boar-derived genotypes belonged to a clade that has not been identified in human-derived A. lumbricoides.

Discussion: The ITS-1 sequences of Ascaris from wild boars were identical to those of "lumbricoides" genotypes, suggesting possible past transmission from humans. However, COX1-based phylogenetic analysis revealed a distinct clade, indicating a potentially novel lineage within wild boars. These findings highlight the limitations of relying solely on ITS-1 for determining host origin and suggest that wild boars may serve as reservoirs of zoonotic Ascaris in Japan.

Conclusions: This study highlights the presence of "lumbricoides" genotypes sequences in wild boars and their potential role in human ascariasis. Reevaluation of unexplained cases in relation to wild boar distribution is warranted, along with enhanced attention to zoonotic transmission risks.