However, infection of American troops of Puerto Rican origin is quite
common, and eggs of S. mansonia are quite frequently found in routine stool
examination of such persons after entry into the military service.
In the majority of these cases, the condition is asymptomatic, but it is of
military significance in that these soldiers may require treatment and sometimes
hospitalization.
c. Life Cycle of Schistosomes. Schistosomes are modified forms of flukes.
The life cycles of the three species of Schistosoma are very similar to those of other
flukes.
(1) Adult female worms. The adult female worms:
Lodge in the veins of infected humans and domestic and wild
animals, including rodents and monkeys.
Lay eggs that ultimately find their way into feces or urine
If these eggs gain access to fresh water, the first intermediate host (a
particular species of snail, different for each worm) may become infected either by
eating the eggs or by being penetrated by the larva (miracidia) that emerge from the
eggs as free-swimming cercariae.
(2) In the snail. Within the snail:
Many cercariae develop, which then escape from the snail and go
back into the water.
As many as 100,000 cercariae (the form infective for man) may be
formed from a single miracidium.
(3) Adult flukes in humans. The free-swimming cercariae may then enter
man directly through the skin or mucous membranes (while the person is drinking,
working in, walking in, or swimming in contaminated water).
The adult flukes then develop within the human blood vessels.
Eggs attached to the vessel walls cause lesions and tissue necrosis.
The worms may lodge in the vessels of many of the major organs.
The worms are usually found in those serving the mesentery,
intestines, and pelvis, depending, to some extent, on the species of the schistosome
parasite.
MD0152
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