Opisthorchis viverrini: an underestimated parasite in world health

Abstract

Infection with Opisthorchis viverrini and its associated cholangiocarcinoma (CCA) is an underestimated problem in the Mekong region of Southeast Asia, despite the widespread use of praziquantel and health education measures for parasite control. Although data from Cambodia, Laos and Vietnam are rare, data from Thailand often show wide-ranging variability in epidemiological parameters, including human morbidity and the prevalence and incidence of CCA. The recent discovery of high levels of population genetic variability in O. viverrini in different wetlands in Thailand and Laos, which indicates the presence of sibling species, suggests that we have underestimated the complexity of this epidemiological situation. Future research should determine the relationship between the genetic variability of O. viverrini and patterns of opisthorchiasis-related disease.

Figure 1

The life cycle of O. viverrini. Humans become infected by ingesting metacercariae in uncooked fish. The ingested metacercariae excyst in the duodenum and enter the bile duct, where they develop into sexually mature adult worms. Eggs are produced and discharged with bile fluid into the intestine and faeces. When eggs reach a body of freshwater and are ingested by an appropriate snail, miracidium hatch and develop into sporocysts and rediae. Different species of Bithynia snails serve as first intermediate hosts. The rediae gave rise to cercariae and, when exposed to appropriate cyprinid species of fish (the second intermediate hosts), the free-swimming cercariae penetrate into the tissues or skin of freshwater fish and become fully infective metacercariae, which completes the life cycle. Humans act as definite hosts, along with fish-eating carnivores (e.g. cats and dogs), which act as reservoir hosts.

Figure 2

A summary of the systematics of the O. viverrini species complex. Depicted are the numbers of genetic groups (in yellow boxes) detected within each of the currently detected sibling species; the magnitude of fixed genetic differences between genetic groups; the levels of fixed genetic differences that define each species, and correlations of species and genetic groups with distinct wetlands after the application of the molecular genetic technique of allozyme electrophoresis. Fixed genetic differences occur when samples being compared do not have any alleles in common at the enzyme locus being examined. In the case of O. viverrini, 33 independent enzyme loci were examined [36]. The geographical distribution of O. viverrini includes Thailand, Laos, Cambodia, Vietnam and, possibly, Myanmar (no data are currently available).