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CLONORCHIASIS AND OPISTHORCHIASIS

The trematodes Clonorchis sinensis, Opisthorchis viverrini, and O. felineus are members of the family Opisthorchidae and are important causes of liver disease in people within endemic areas. Notably, and somewhat uniquely among parasitic species, C. sinensis and O. viverrini are classified as group 1 carcinogens due to their association with cholangiocarcinoma and other neoplasms of the biliary system.1 Infections with these “fishborne liver flukes” are widely distributed in countries in the Far East and Southeast Asia, and countries of the former Soviet Union. They present serious public health problems in certain localized areas of China, Korea, Thailand, Laos, Cambodia, Vietnam, and several countries of the former Soviet Union and increasingly into Europe. The most recent estimate of global burden indicates 45 million infections in the endemic Asian and European range; this includes 35 million C. sinensis cases, 10 million O. viverrini cases, and 1.2 million O. felineus cases.2 Up to 680 million people may be at risk for infection.2 Globally, clonorchiasis and opisthorciasis are associated with an estimated 522,863 and 188,346 disability-adjusted life years, which is relatively high among parasitic helminths.3 With increased travel and migration of populations at risk and importation of indigenous uncooked foods contaminated with these parasites, infections have also occurred in nonendemic areas.

Life Cycle

The flukes C. sinensis, O. viverrini, and O. felineus follow a typical multihost trematode life cycle, requiring two intermediate hosts and a definitive host to complete their life cycle. The host range for these species is summarized in Table 129-1. Infected definitive hosts pass embryonated eggs, containing miracidia, into the environment. These eggs are ingested by snails, the first intermediate hosts, and the miracidia are released. Within the snail tissues, the miracidia develop into rediae and then cercariae—long-tailed, motile forms which are infective to the second intermediate host. Cercariae emerge from the snail into water, and upon contact with a suitable intermediate host (nearly always fish of the family Cyprinidae, such as carp, true minnows, and barbs) they penetrate the skin of the fish, shed their tails, and encyst in the skeletal muscle and viscera as metacercariae. When infected raw freshwater fish containing the encysted stages of the parasite are eaten, the larvae are set free in the duodenum of the definitive host. The larvae enter the bile ducts within a few hours after being ingested.2 In about 3–4 weeks, the hermaphroditic adult flukes (Fig. 129-1) reach maturity and begin shedding eggs into the bile ducts. The estimated daily egg output of these flukes varies substantially; the typical egg output for an individual C. sinensis in a human host is estimated about 4000 eggs/day, but outputs of > 400,000 eggs/day in very high burden infections have been noted.4 Limited studies on O. viverrini suggest a typical egg output of anywhere between 3000 and 36,000 eggs/day.5 The complete life cycle, ...

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