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.