Infections caused by the fish-borne zoonotic liver fluke, Opisthorchis viverrini, remain a crucial health issue in southeast Asian (SEA) countries such as Vietnam, Cambodia, Thailand, and Lao PDR (International Agency for Research on Cancer (IARC), 1994; Sripa, 2003). Approximately 8-10 million people, primarily in Lao PDR and northern and northeastern Thailand, have opisthorchiasis (Sripa et al., 2021). People are infected by eating undercooked freshwater fish harboring metacercariae of the fluke. Chronic infection with O. viverrini induces prolonged inflammation that can lead to the development of cholangiocarcinoma (CCA) (Sripa et al., 2018). Although the link between O. viverrini infection and CCA is well established, fewer than 1% of infected patients develop CCA (Mairiang et al., 2006), suggesting the possibility that there is a carcinogenic strain of O. viverrini circulating with non-carcinogenic ones (Saijuntha et al., 2007). Indeed, there have been many studies proving that there are genetically distinct populations of O. viverrini, endemic in Thailand and Lao PDR. The presence of at least two sibling species of fluke has been proposed by several authors (Saijuntha et al., 2007, Namsanor et al., 2020, Sota et al., 2022). Sota et al. (2022) recently demonstrated that one population of putative O. viverrini disseminates mainly in humans (“human-specific” population) and the other mainly in cats (“cat-specific” population). Although the terms “cat-specific” and “human-specific” are used for convenience in this paper, these two populations do not exhibit absolute specificity for the mammalian host. For the time being, we also refer to each population as representing O. viverrini. Little is known about the circulation of infective stages of these different populations in the environment.

Sequences of the mitochondrial cytochrome c oxidase subunit 1 (cox1) and NADH dehydrogenase subunit 1 (nad1) genes have been used to examine the genetic variation among O. viverrini populations (Ando et al., 2001, Saijuntha et al., 2008, Buathong et al., 2017, Pitaksakulrat et al., 2018, Namsanor et al., 2020, Sota et al., 2022). Recently, Namsanor et al. (2020) suggested that the metacercaria is the best stage of the life cycle for population genetic analysis. Thus, in this study, we will explore the second intermediate hosts of cat- and human-specific O. viverrini populations using the cox1 and nad1 genes as markers.

The findings of this investigation will add to our knowledge of how each population is disseminated and transmitted. We aimed to confirm the existence of the two populations of O. viverrini that are largely found in humans or in cats, as previously proposed. The results will promote more targeted and effective opisthorchiasis eradication programs. If intermediate hosts of distinct populations differ, then this will simplify field collection of each and facilitate future work to clarify their carcinogenic potential and other properties.