Molecular insights into seasonal trematode infections in Bithynia Snails: host lineages, parasite diversity, and Opisthorchis viverrini susceptibility in southern Lao PDR

Background Opisthorchiasis, caused by Opisthorchis viverrini , is a major public health concern in Southeast Asia. Despite control programs, O. viverrini infection persists and contributes to severe liver diseases, including cholangiocarcinoma. This study aimed to assess seasonal variation in trematode prevalence and diversity, evaluate the susceptibility of Bithynia siamensis sensu lato lineages II and III to O. viverrini infection, and examine the phylogenetic and haplotype network of identified trematode and their snail hosts in Champasak Province, southern Lao PDR. Methods Snail samples were collected quarterly in 2024 (February, May, August, and November) from Khong and Mounlapamok Districts using handpicking and scooping. Trematode infections were detected by the crushing method, identified morphologically, and confirmed by molecular analysis. DNA barcoding of nuclear and mitochondrial genes was used to verify trematode species and snail lineages. Results Of 1,764 Bithynia snails examined, 169 (9.58%) were infected. Five cercarial types were identified: amphistome (3.40%), xiphidiocercariae (2.78%), monostome (2.61%), mixed monostome and amphistome (0.34%), cystophorous (0.28%), and O. viverrini (0.17%). Infection rates of O. viverrini did not differ between lineages II and III, but other trematodes were significantly more frequent in lineage III (76.67%). Conclusions Trematode infection rates and species diversity in B. s. goniomphalos show marked seasonal variation in Champasak Province, southern Lao PDR. These findings highlight the complexity of host–parasite interactions and the role of environmental factors shaping transmission, providing insights for targeted prevention and control.