Monophyly of Opisthorchis viverrini populations in the lower Mekong Basin, using mitochondrial DNA nad1 gene as the marker.

The liver fluke, Opisthorchis viverrini, causes serious public-health problems in the Lower Mekong Basin. This study aimed to clarify whether O. viverrini populations may be genetically divided into sub-specific taxa. We collected 6 populations of O. viverrini from different places in Cambodia, Lao PDR, and Thailand, along both sides of the Mekong River, and analyzed the population structure of these using the mitochondrial nad1 gene as a marker. The results of the DNA polymorphism measurements, by theta-w (thetaw) and -pi (thetapi) values, neutrality tests, and mismatch distribution, suggested that the population of O. viverrini has expanded under the influence of purifying selection and selective sweep. The analysis of molecular variance (AMOVA) test revealed no significant genetic differences among the O. viverrini populations on opposite sides of the Mekong River. O. viverrini haplotypes occurred in multiple populations, and no distinct geographical clade. The star-like haplotype network confirmed a demographic expansion of the O. viverrini population. Overall, the genetic data from these populations suggested that the postulated existence of an O. viverrini species complex should be rejected. The bio-geographical diversity of O. viverrini populations should be explored further, using other appropriate markers and a wider range of samples from geographically different areas.

Discovery of Opisthorchis viverrini metacercariae in freshwater fish in southern Cambodia.

Small liver flukes, Clonorchis sinensis and Opisthorchis viverini, are fish-borne trematodes (FBTs) causing significant public health problems in Asia. While C. sinensis is distributing mainly in far east Asia, O. viverini is distributing in Indochina peninsula. Recently, however, the geographical distributions of those small liver flukes were proven to be far wider than expected. Nevertheless, little is known about the epidemiology of small liver flukes in Cambodia. The present study is, therefore, aimed at clarifying the status of small liver fluke infections in various species of freshwater fish in southern Cambodia. A total of 1479 freshwater fish, 1316 (89%) comprised of 20 different species of the cyprinoid family and 163 (11%) belonging to 8 families of non-cyprinoids, were collected during May 2007 and February 2008, and the presence of small liver fluke metacercariae was examined by the compression method. Small liver fluke metacercariae were found in 10 species of cyprinoids with the infection rate ranging 2.1-66.7% and the mean intensity of infection of 1.0-15.0 (range: 1-65). For the speciation, adult worms were obtained by experimental infection in hamsters. The small liver fluke found in this study were identified as Opisthorchis viverrini by the morphological features of adult worms, and this identification was confirmed by partial COI sequencing of the metacercariae.