Figure 2-4. Life cycle of a typical fluke.
Typically, the adult parasite, which resembles a flat, leaf-shaped worm,
lives in the body of the definitive host--man or animal.
Eggs, when passed in the feces of the host, require water to survive.
The eggs hatch to liberate free-swimming miracidia, which must
penetrate the body of an intermediate host (snail) or perish.
In some cases, the miracidium is eaten by the snail. Within the body of
the snail, miracidia develop, sometimes through several generations, into cercariae--
free-swimming larvae that leave the snail and seek either a second intermediate host
where they encyst as metacercariae, or as in the case of Schistosoma a definitive host
whose intact skin is penetrated by the cercariae.
The cercariae then migrate to their final body organs and develop into
The second intermediate host is a fish or crab. In one genus of flukes
(Fasciola), the cercariae encyst as metacercariae on vegetation.