Trematoda (flukes) and Cestoda (tapeworms) are the two large classes of parasites in the phylum Platyhelminthes. The most important trematodes are Schistosoma species (blood flukes), Clonorchis sinensis (liver fluke), and Paragonimus westermani (lung fluke). Schistosomes have by far the greatest impact in terms of the number of people infected, morbidity, and mortality. Features of the medically important trematodes are summarized in Table 55–1, and the medically important stages in the life cycle of these organisms are described in Table 55–2. Three trematodes of lesser importance, Fasciola hepatica, Fasciolopsis buski, and Heterophyes heterophyes, are described at the end of this chapter.
TABLE 55–1Features of Medically Important Trematodes (Flukes) |Favorite Table|Download (.pdf) TABLE 55–1 Features of Medically Important Trematodes (Flukes)
|Trematode ||Mode of Transmission ||Main Sites Affected ||Intermediate Host(s) ||Diagnostic Features of Eggs ||Endemic Area(s) ||Treatment |
|Schistosoma mansoni ||Penetrate skin ||Veins of colon ||Snail ||Large lateral spine ||Africa, Latin America (Caribbean) ||Praziquantel |
|Schistosoma japonicum ||Penetrate skin ||Veins of small intestine, liver ||Snail ||Small lateral spine ||Asia ||Praziquantel |
|Schistosoma haematobium ||Penetrate skin ||Veins of urinary bladder ||Snail ||Large terminal spine ||Africa, Middle East ||Praziquantel |
|Clonorchis sinensis ||Ingested with raw fish ||Liver ||Snail and fish ||Operculated ||Asia ||Praziquantel |
|Paragonimus westermani ||Ingested with raw crab ||Lung ||Snail and crab ||Operculated ||Asia, India ||Praziquantel |
TABLE 55–2Medically Important Stages in Life Cycle of Trematodes (Flukes) |Favorite Table|Download (.pdf) TABLE 55–2 Medically Important Stages in Life Cycle of Trematodes (Flukes)
|Organism ||Insect Vector ||Stage That Infects Humans ||Stage(s) in Humans Most Associated with Disease ||Important Stage(s) Outside of Humans |
|Schistosoma mansoni, Schistosoma haematobium, Schistosoma japonicum ||None ||Cercariae penetrate skin ||Adult flukes living in mesenteric or bladder veins lay eggs that cause granulomas ||Miracidium (ciliated larvae) infect snails → cercariae infect humans |
|Clonorchis ||None ||Larvae in undercooked fish ||Adult flukes live in biliary ducts ||Eggs ingested by snails → cercariae infect fish |
|Paragonimus ||None ||Larvae in undercooked crab ||Adult flukes live in lung ||Eggs ingested by snails → cercariae infect crab |
The life cycle of the medically important trematodes involves a sexual cycle in humans (definitive host) and asexual reproduction in freshwater snails (intermediate hosts) (Figure 55–1). Transmission to humans takes place either via penetration of the skin by the free-swimming cercariae of the schistosomes (Figures 55–2D and 55–3) or via ingestion of cysts in undercooked (raw) fish or crabs in Clonorchis and Paragonimus infection, respectively.
Schistosoma species. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are infected at step 2 when free-swimming cercariae penetrate human skin. Cercariae differentiate into adult worms (two sexes) that migrate to the mesenteric veins (Schistosoma mansoni and Schistosoma japonicum) or the venous plexus of the urinary bladder (Schistosoma haematobium). The adult worms lay eggs, which ...