Taxonomy and Morphology
Trematodes, members of Phylum Platyhelminthes, also called flukes, cause a variety of clinical infections in humans worldwide. The parasites are named trematodes because of their conspicuous suckers, which are the organs of attachment (trematode means "pierced with holes"). All of the flukes that cause infections in humans are contained in the group called "digenetic trematodes."
Depending on their habitat in the infected host (generally a vertebrate), flukes can be classified as blood flukes, liver flukes, lung flukes, and intestinal flukes. Flukes causing most human infections are Schistosoma species (blood fluke), Paragonimus westermani (lung fluke), and Clonorchis sinensis (liver fluke). Some less clinically important flukes are Fasciola hepatica and Opisthorchis viverrini, which are liver flukes, and Fasciolopsis buski, Heterophyes heterophyes, and Metagonimus yokogawai, which are all intestinal flukes.
Features of Digenic Trematodes
• The sexes of the parasites are not separate (monecious). In other words,they are mostly hermaphroditic with the male and female reproductive organs existing complete in each fluke. One exception is the schistosomes, which are diecious.
• The flukes are oviparous and lay diagnostically operculated eggs. Once again, an exception is schistosome eggs, which are not operculated.
• They are unsegmented, dorsoventrally flattened, and leaf-shaped.
• The alimentary canal is incomplete, with the anus being absent.
• The excretory system is bilaterally symmetric.
• They bear two suckers, one on the ventral surface of the body (ventral sucker) and one around the mouth (oral sucker). These serve as organs of attachment for the fluke.
• Class Trematoda
o Subclass Digenea (the digenetic trematodes)
There are three main species of Oriental liver fluke that affect humans:
• Clonorchis sinensis is common in China, Taiwan, Korea, Japan and Vietnam, and is often carried by domestic dogs and cats. The reservoir for this disease is various freshwater snails, while the vector is usually uncooked freshwater fish. C. sinensis is the most common "Oriental liver fluke" infection.
• Opisthorchis felineus is widespread in Eastern Europe, particularly in Poland, and is carried by a variety of wild and domestic animals.
• Opisthorchis viverrini is exxtremely common in Thailand, especially the north, and again is mostly carried by domestic dogs and cats.
C. sinensis is oblong, flat, transparent, and relatively small (10-25 mm in length, 3-5 mm in width). It has a pointed anterior and rounded posterior end.