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Liver flukes: Clonorchiasis and opisthorchiasis

INTRODUCTION

The three major liver trematodes or flukes that infect humans are Clonorchis sinensis, Opisthorchis species, and Fasciola hepatica. Clonorchiasis, opisthorchiasis, and the North American liver fluke, Metorchis conjunctus, will be reviewed here. Fascioliasis is discussed separately. (See "Liver flukes: Fascioliasis".)

CLONORCHIS SINENSIS AND OPISTHORCHIS SPECIES

C. sinensis and Opisthorchis species are parasite flatworms. They are leaf-shaped and hermaphroditic, ranging in size from a few millimeters to several centimeters. The adult flukes allow differentiation of species, although the eggs of these parasites are all yellowish-brown in color, oval in shape, and measure approximately 20 to 30 µm by 15 µm, making species identification difficult. The eggs also resemble those of other intestinal flukes.

The adult flukes undergo asexual reproduction in snails, which are the intermediate host, and undergo sexual reproduction in definitive mammalian hosts. Humans are accidental hosts. Different genera of snails and different species of fish serve as the first and second intermediate hosts respectively for the different flukes. The geographical ranges and habitats of these fish and snail species contribute to the geographic distributions of human infection by each of these flukes. Distribution is also determined by local eating habits.

Epidemiology — C. sinensis, also known as the Chinese liver fluke, is endemic in the Far East, particularly in China, Japan, Taiwan, Vietnam and Korea [1,2]. It is a parasite of fish-eating mammals, with dogs and cats the most common reservoirs. The adult worms are flat and elongated, measuring 10 to 25 mm long and 3 to 5 mm wide. It is estimated that more than 7 million people are infected worldwide. Prevalence rates in endemic areas vary widely. As an example, prevalence rates in different provinces of China range from <1 to 57 percent [3]. One study in Korea showed a prevalence of 16 percent [4].

The two species that most frequently cause human opisthorchiasis are O. felineus and O. viverrini. They are common liver flukes of cats, dogs and other fish-eating mammals, which serve as reservoir hosts. O. felineus occurs in Southeast Asia and in Central and Eastern Europe, particularly in Siberia and other parts of the former Soviet Union. Prevalence rates of 40 to 95 percent in some areas, and infection of over 16 million people is estimated [5]. O. viverrini is endemic in Thailand, Kampuchea and Laos. Prevalence rates of over 24 to 90 percent in villages in Thailand and 40 to 80 percent in Laos have been reported, and it is estimated that over 10 million people are infected [5-7]. Both O. felineus and O. viverrini are usually 8 to 12 mm long, but O. felineus tends to be wider (2 to 3 mm compared to 1 to 2 mm).
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