Opisthorchis viverrini: the carcinogenic human liver fluke

Abstract

Opisthorchiasis caused by Opisthorchis viverrini remains a major public health problem in many parts of Southeast Asia, including Thailand, Lao PDR, Vietnam and Cambodia. The infection is associated with a number of hepatobiliary diseases, including cholangitis, obstructive jaundice, hepatomegaly, cholecystitis and cholelithiasis. Multi-factorial etiology of cholangiocarcinoma, mechanical damage, parasite secretions, and immunopathology may enhance cholangiocarcinogenesis. Moreover, both experimental and epidemiological evidences strongly implicate liver fluke infection as the major risk factor in cholangiocarcinoma, cancer of the bile ducts. The liver fluke infection is induced by eating raw or uncooked fish products that is the tradition and popular in the northeastern and northern region, particularly in rural areas, of Thailand. The health education programs to prevent and control opisthorchiasis are still required in the high-risk areas.

Figure 1

Life cycle of O viverrini. Embryonated eggs are discharged in the biliary ducts and in the stool (1). Eggs are ingested by a suitable snail intermediate host (2). Each egg releases a miracidia (2a), sporocysts (2b), rediae (2c), and cercariae (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, where it encysts as a metacercaria (3). Humans are infected through ingestion of undercooked or raw freshwater fishes (4). The metacercaria excysts in the duodenum (5) and ascends the biliary tract through the ampulla of Vater. Maturation to adulthood takes approximately one month (6). (Adapted from: http://www.dpd.cdc.gov/DPDx/HTML/Opisthorchiasis.htm).

Figure 2

Adult worm of human liver flukes: a: O viverrini; b: O felineus; c: C sinensis. The average size of O viverrini worm is 7.0 (5.4-10.2) x 1.5 (0.8-1.9) mm[8]. (Adapted from Ash, 1997).

Figure 3

Preparation of koi pla, pla som, pla ra and jaewbhong. koi pla is prepared with the fish-based meals with local herbs, spices, condiments, rice, and vegetables. Pla som is moderately fermented or stored for a few days to weeks.Pla ra is fermented fish sauce, popular in Northeastern Thai cuisine. It is made by combining fish, salt, and roasted rice, fermenting in a pot for one to three months. jaewbhong is made by combining pla ra and chili.

Figure 4

Prevalence of O viverrini infection in Thailand[15], OV: Percentage of O viverrini infection.

Figure 5

Proposed mechanisms of Opisthorchis-derived CCA initiation. (Adapted from Sripa et al[30]).