Trematodes, or flatworms, are a group of morphologically and biologically heterogeneous organisms that belong to the phylum Platyhelminthes. Human infection with trematodes occurs in many geographic areas and can cause considerable morbidity and mortality. The dependence on one drug—praziquantel—for treatment of most infections caused by helminths, including trematodes, raises the specter of developing resistance in these worms; several instances of reduced drug efficacy have already been reported.
For clinical purposes, significant trematode infections of humans may be divided according to tissues invaded by adult flukes: blood, biliary tree, intestines, and lungs (Table 219-1). Trematodes share some common morphologic features, including macroscopic size (from one to several centimeters); dorsoventral, flattened, bilaterally symmetric bodies (adult worms); and the prominence of two suckers. Except for schistosomes, all human parasitic trematodes are hermaphroditic. Their life cycles involve a definitive host (mammalian/human), in which adult worms initiate sexual reproduction, and an intermediate host (snails), in which asexual multiplication of larvae occurs. More than one intermediate host may be necessary for some species of trematodes. Human infection is initiated either by direct penetration of intact skin or by ingestion. Upon maturation within humans, adult flukes initiate sexual reproduction and egg production. Helminth ova leave the definitive host in excreta or sputum and, upon reaching suitable environmental conditions, they hatch, releasing free-living miracidia that seek specific snail intermediate hosts. After asexual reproduction, cercariae are released from infected snails. In certain species, these organisms infect humans; in others, they find a second intermediate host to allow encystment into metacercariae—the infective stage.
Table 219-1 Major Human Trematode Infections |Favorite Table|Download (.pdf)
Table 219-1 Major Human Trematode Infections
|Schistosoma mansoni||Skin penetration by cercariae released from snails||Africa, South America, Middle East|
|S. japonicum||Skin penetration by cercariae released from snails||China, Philippines, Indonesia|
|S. intercalatum||Skin penetration by cercariae released from snails||West Africa|
|S. mekongi||Skin penetration by cercariae released from snails||Southeast Asia|
|S. haematobium||Skin penetration by cercariae released from snails||Africa, Middle East|
|Biliary (Hepatic) Flukes|
|Clonorchis sinensis||Ingestion of metacercariae in freshwater fish||Far East|
|Opisthorchis viverrini||Ingestion of metacercariae in freshwater fish||Far East, Thailand|
|O. felineus||Ingestion of metacercariae in freshwater fish||Far East, Europe|
|Fasciola hepatica||Ingestion of metacercariae on aquatic plants or in water||Worldwide|
|F. gigantica||Ingestion of metacercariae on aquatic plants or in water||Sporadic, Africa|
|Fasciolopsis buski||Ingestion of metacercariae on aquatic plants||Southeast Asia|
|Heterophyes heterophyes||Ingestion of metacercariae in freshwater or brackish-water fish||Far East, North Africa|
|Paragonimus westermani||Ingestion of metacercariae in crayfish or crabs||Global except North America and Europe|
The host-parasite relationship in trematode infections is a product of certain biologic features of these organisms: they are multicellular, undergo several developmental changes within the host, and usually result in chronic infections. In general, the distribution of worm infections in ...