From Allozymes to Genomics: Reframing the Systematics and Population Structure of Opisthorchis viverrini and Its...

The carcinogenic liver fluke Opisthorchis viverrini underlies one of the world's heaviest burdens of bile duct cancer, yet for decades it was treated as a single, genetically uniform parasite whose transmission was shaped mainly by environment and human behavior. However, advances in molecular biology have fundamentally reshaped this conceptual model. Evidence accumulated over the past three decades demonstrates that O. viverrini comprises geographically structured populations linked to hydrological connectivity, host distribution, and long-term evolutionary processes across interconnected river systems of mainland Southeast Asia, particularly within the Lower Mekong Basin. This review synthesizes research on the systematics and population structure of O. viverrini and its Bithynia snail hosts, tracing the transition from early allozyme studies to contemporary DNA-based and genomic approaches. Collectively, mitochondrial, nuclear, microsatellite, and intron markers reveal strong spatial structuring among parasite populations, while genetic patterns observed in snail hosts show partial geographic concordance with parasite population structure, suggesting that both may be influenced by shared hydrological organization, ecological isolation, and host connectivity across endemic aquatic systems. Population structure is strongly scale-dependent, with local panmixia often occurring within connected aquatic systems but pronounced differentiation emerging across broader geographic regions. Together, these findings indicate that transmission dynamics are shaped not only by environmental and behavioral factors, but also by evolutionary and landscape-level processes influencing host and parasite connectivity. Finally, we emphasize the increasing significance of population genomics and landscape genetics in understanding how transmission systems persist, disperse, reconnect, and respond to environmental change across endemic landscapes.