Trematodes, or flatworms, are a group of helminths that belong to the phylum Platyhelminthes. The adult flatworms share some common characteristics, such as macroscopic size (from one to several centimeters); dorsoventrally flattened, bilaterally symmetric bodies; and two suckers—oral and ventral. Except for schistosomes, which have separate sexes, all human parasitic trematodes are hermaphroditic. Their life cycles involve a mammalian/human definitive host, in which sexual reproduction by adult worms takes place, and an intermediate host (snails), in which asexual multiplication occurs. Some species of trematodes have more than one intermediate host.
Humans are infected either by direct penetration of intact skin (schistosomiasis) or by ingestion of raw freshwater fish, crustaceans, or aquatic plants with metacercariae—the infective larval stage.
Significant trematode infections of humans may be divided according to the location of the adult worms: blood, liver (biliary tree), intestines, or lungs (Table 229-1). Adult worms do not multiply within the mammalian host but can live for up to 30 years. Infections are often chronic.
TABLE 229–1Major Human Trematode Infections ||Download (.pdf) TABLE 229–1 Major Human Trematode Infections
|Trematode ||Transmission Route ||Geographic Distribution |
|Blood Flukes |
|Intestinal schistosomiasis |
|Schistosoma mansoni ||Skin penetration by cercariae released from snails (Biomphalaria spp.) ||Africa, Brazil, Venezuela, Surinam, the Caribbean (low risk) |
|Shistosoma japonicum ||Skin penetration by cercariae released from snails (Oncomelania spp.) ||China, Indonesia, Philippines |
|Schistosoma guineensis and Schistosoma intercalatum ||Skin penetration by cercariae released from snails (Bulinus spp.) ||Rain forest areas of Central Africa |
|Schistosoma mekongi ||Skin penetration by cercariae released from snails (Neotricula aperta) ||Several districts of Cambodia and Lao People’s Democratic Republic (PDR) |
|Urogenital schistosomiasis |
|Schistosoma haematobium ||Skin penetration by cercariae released from snails (Bulinus spp.) ||Africa, Middle East, Corsica (France) |
|Liver Flukes |
|Clonorchis sinensis ||Ingestion of metacercariae in freshwater fish ||Asia, including Republic of Korea, China, Taiwan, Vietnam |
|Opisthorchis viverrini ||Ingestion of metacercariae in freshwater fish ||Northeast Thailand, Lao PDR, Cambodia, Vietnam |
|Opisthorchis felineus ||Ingestion of metacercariae in freshwater fish ||Former Soviet Union, Kazakhstan, Ukraine, Turkey |
|Fasciola hepatica ||Ingestion of metacercariae on aquatic plants or in water ||Worldwide |
|Fasciola gigantica ||Ingestion of metacercariae on aquatic plants or in water ||Africa, Asia |
|Intestinal Flukes |
|Fasciolopsis buski ||Ingestion of metacercariae on aquatic plants ||Bangladesh, China, India, Indonesia, Lao PDR, Malaysia, Taiwan, Thailand, Vietnam |
|Echinostoma spp. ||Ingestion of freshwater fish, frogs, mussels, snails ||China, India, Indonesia, Japan, Malaysia, Russia, Republic of Korea, Philippines, Thailand |
|Heterophyes heterophyes, several other species ||Ingestion of metacercariae in freshwater or brackish-water fish ||Egypt, Greece, Islamic Republic of Iran, Italy, Japan, Republic of Korea, Sudan, Tunisia, Turkey |
|Lung Flukes |
|Paragonimus westermani ||Ingestion of metacercariae in crayfish or crabs ||Tropical and subtropical areas of eastern and southern Asia and sub-Saharan Africa |
|Paragonimus kellicotti ||Ingestion of metacercariae in crayfish or crabs ||North America |
Although it is relatively rare to encounter patients with trematode infections in the United ...