Current assessment of the systematics and population genetics of Opisthorchis viverrini sensu lato (Trematoda: Opisthorchiidae) and its first intermediate host Bithynia siamensis sensu lato (Gastropoda: Bithyniidae) in Thailand and Southeast Asia

The group 1 carcinogen, the liver fluke Opisthorchis viverrini is the causative agent of opisthorchiasis and subsequent bile duct cancer (cholangiocarcinoma; CCA), which is an important public health problem in Southeast Asia. Bithynia snails are known to be the sole intermediate host of O. viverrini, and distributed throughout endemic areas of opisthorchiasis. Since 2001, the genetic variation investigation of O. viverrini has progressively been investigated. Comprehensive genetic variation studies of O. viverrini and Bithynia snails were undertaken and consecutively published in 2007 by Saijuntha and colleagues. These studies provided genetic evidence that O. viverrini and Bithynia snails are both species complex with evidence of co-evolution. Later, several studies have provided data in support of this finding, and have continuously to date reinforced that both O. viverrini and Bithynia are species complexes. Moreover, studies have shown that genetic variation of O. viverrini is related to geographical, temporal, fish host species including geographical genetic variation of its snail host, Bithynia siamensis sensu lato. This is significant and important in our understanding of the evolution and phylogenetic relationships between species within the O. viverrini and Bithynia species complexes. A comprehensive knowledge of the systematics and population genetics of O. viverrini and Bithynia snails provides a sound basis to instigate and develop effective prevention and control programs targeting opisthorchiasis and CCA in the endemic areas of Southeast Asia. Thus, this review examines the historical series of investigations of the systematics and population genetics of O. viverrini including Bithynia spp. in Southeast Asia since molecular genetic investigations commenced some 20 years ago. •Systematic and population genetic investigations of Opisthorchis viverrini and its Bithynia snail host are reviewed.•Studies of the parasite and snail host by time periods since the past two decades are updated.•In both O. viverrini and Bithynia snails, many cryptic species have been discovered.•The further requirement for comprehensive molecular genetic analyses of O. viverrini and Bithynia snail complexes across their ranges is discussed.