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Epidemiology of Nematodes, Cestodes, and Trematodes 129

Source: Centers for Disease Control and Prevention (CDC)

FIGURE 5-10 Illustration shows the life cycle of O. felineus and O. viverrini. Not that a mollusk is involved as a host

Adult worms are 10 to 20 mm long; the male has an and portal routes of circulation. In about three weeks
unusual lamelliform (arranged in thin plates or scales) these forms mature and reach the mesenteric (S. japoni-
shape with marginal folds forming a canal in which the cum and S. mansoni) or the bladder (S. hematobium)
slender female worm resides. Unlike other trematodes, vessels where they live and ovulate for the duration of the
schistosomes may be divided by gender as they have host’s life. Eggs germinate as they pass through the ves-
separate sexes. sel wall into the intestine or bladder and are excreted in
feces (S. japonicum and S. mansoni) or urine (S. hema-
Life Cycle of Schistosomes tobium). When in fresh water the larval miracidia hatch
out of the egg and swim about until they find an appro-
The life cycle of the schistosomes require fresh water in priate snail. After two generations of multiplication in the
order to infect a host; interior lakes in the endemic areas snail, the fork-tailed cercariae emerge into the water and
of the world are often contaminated (Figure 5-11). Man infect another human.
is infected by cercaria in fresh water by skin penetration. Symptoms may initially be merely a nuisance and
After entering the body, the cercaria travel through the scarcely noticed. Penetration by the cercariae may cause
venous vessels of their human hosts to the heart, lungs, transient dermatitis (swimmer’s itch). The symptoms