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Schistosomiasis
or Bilharziasis
Introduction
Schistosomiasis
or Bilharziasis, widespread disease caused by the infestation of the
human body by flukes commonly called blood flukes, of the genus Schistosoma.
In many tropical and subtropical countries these flukes cause serious diseases;
they rarely produce disease in temperate countries, even though they are
widespread. It is estimated that 150 to 200 million people throughout the world
are afflicted with diseases caused by blood flukes. Blood flukes spend most of
their life cycle in two hosts; the adult stage is spent in a mammal, usually
humans, and the immature stages are spent in certain snails. Eggs discharged
from the host hatch into larval forms in fresh water; from the water, the
larvae, miracidia, invade the snail that acts as an intermediate host.
The larval form of the parasite undergoes partial maturation in the snail, then
escapes back into the water, as mature larvae called cercariae. At this
stage they penetrate the skin of the host from the water and then migrate
through the blood vessels to specific capillaries as maturation completes. There
they remain and lay eggs.
Flukes of the genus Schistosoma parasitize two hosts.
The young hatch from their eggs in rivers and lakes and enter a specific kind
of aquatic snail, where they develop into tadpolelike larva called cercariae.
When the cercariae leave the snail, they burrow through
 the skin of a human
host swimming or wading in infested water. Adult flukes mature in the host’s
bloodstream and settle in the veins of the gut. Their eggs, deposited in the
lining of the human intestine and bladder, pass back into water via the
excretory system, and the cycle begins again. More than 200 million people
worldwide suffer from schistosomiasis (bilharzia), a disease characterized by
the abscesses and bleeding caused by the flukes’ infestation.
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