An age old preference for eating uncooked fish dishes like “koi-pla” puts people in SE Asia at risk of ingesting trematodes that can cause a type of liver cancer called cholangiocarcinoma (cancer of the bile ducts), say researchers.
Banchob Sripa (Khon Kaen University) and colleagues discuss the mechanisms by which the food-borne trematode Opisthorchis viverrini (the SE Asian liver fluke) causes cholangiocarcinoma. The fluke is endemic to Thailand, Lao People’s Democratic Republic, Vietnam, and Cambodia. In Thailand alone, 6 million people are thought to be infected with the fluke.
Embryonated eggs are discharged in the biliary ducts and in the stool (1). Eggs are ingested by a suitable snail intermediate host (2); there are more than 100 species of snails that can serve as intermediate hosts. Each egg releases a miracidium (2a), which goes through several developmental stages (sporocyst [2b], redia [2c], and cercaria [2d]. The cercaria is released from the snail and after a short period of free-swimming time in water, it penetrates the flesh of a freshwater fish, such as Cyclocheilichthys armatus or Puntius leiacanthus, where it encysts as a metacercaria (3). Humans are infected through ingestion of undercooked, salted, pickled, or smoked freshwater fishes (4). After ingestion, the metacercaria excysts in the duodenum (5) and ascends the biliary tract through the ampulla of Vater. Maturation to adulthood (right, O. viverrini; left, C. sinensis) takes approximately one month (6). The adult flukes (measuring 10–25 mm by 3–5 mm) reside in the small and medium-sized biliary ducts. In addition to humans, carnivorous animals can serve as reservoir hosts. (Adapted from http://www.dpd.cdc.gov/DPDx/HTML/Opisthorchiasis.htm.)
Currently, more than 600 million people are at risk of infection with these trematodes [1]. O. viverrini is endemic in Southeast Asian countries, including Thailand, Lao People's Democratic Republic, Vietnam, and Cambodia [2], and C. sinensis infection is common in rural areas of Korea and China. Opisthorchiasis has been extensively studied in Thailand, where an estimated 6 million people are infected with the liver fluke (calculated from overall 9.4% prevalence within the population in 2001) [3].
Infection with these food-borne parasites is prevalent in areas where uncooked cyprinoid fish are a staple of the diet. Due to poor sanitation practices and inadequate sewerage infrastructure, people infected with O. viverrini and C. sinensis pass parasite eggs in their faeces into natural water reservoirs, where the parasite eggs are eaten by intermediate host snails, for example, aquatic snails of the genus Bithynia, the first intermediate host of O. viverrini. After hatching, free swimming parasites, called cercariae, are released from the infected snails. Cercariae then locate their next intermediate host, cyprinoid fishes, encyst in the fins, skin, and muscles of the fish, and become metacercariae. The metacercariae are infective to humans and other fish-eating mammals upon ingestion of raw or undercooked fish in dishes such as koi-pla, and in turn the parasite's life cycle is completed.
Liver fluke induces cholangiocarcinomaSripa B, Kaewkes S, Sithithaworn P, Mairiang E, Laha T, et al. (2007) . PLoS Med 4(7): e201.