Genetic diversity, population structure, and benzimidazole resistance profiling of Ascaris lumbricoides in northwestern Thailand.

Ascaris lumbricoides remains one of the most prevalent soil-transmitted helminths worldwide, with ongoing transmission in endemic regions despite long-standing control programs. This study investigated the genetic diversity, population structure, and benzimidazole resistance of A. lumbricoides across three endemic regions of Thailand using mitochondrial (cox1 and nad1) and nuclear (β-tubulin) markers. Haplotype network analysis identified 13 haplotypes within the Thailand dataset, including three newly identified haplotypes, indicating measurable genetic diversity at the local scale. When analyzed in a global context incorporating reference sequences, substantially higher haplotype diversity was observed, reflecting broad genetic diversity across geographic regions. Despite this diversity, no clear geographic structuring was detected among Thai populations, suggesting ongoing gene flow and population connectivity. Phylogenetic analyses further demonstrated that all isolates clustered within a single major lineage and shared haplotypes with Ascaris suum (infecting pig), supporting genetic overlap and potential zoonotic transmission. Analysis of the β-tubulin gene revealed exclusively wild-type alleles at codons 167, 198, and 200, with no evidence of mutations associated with benzimidazole resistance. Overall, this study demonstrates measurable genetic diversity in Thailand's A. lumbricoides populations, with no evidence of clear geographic structuring. These findings provide baseline molecular data to support future surveillance of transmission dynamics and anthelmintic resistance.