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Eggs are eliminated
with feces or urine
.
Under optimal conditions the eggs hatch and release miracidia
, which
swim and penetrate specific snail intermediate hosts
. The
stages in the snail include 2 generations of sporocysts
and the
production of cercariae
. Upon
release from the snail, the infective cercariae swim, penetrate the skin of
the human host
, and
shed their forked tail, becoming schistosomulae
. The
schistosomulae migrate through several tissues and stages to their residence
in the veins (,
). Adult
worms in humans reside in the mesenteric venules in various locations, which
at times seem to be specific for each species
. For
instance, S. japonicum is more frequently found in the superior
mesenteric veins draining the small intestine
,
and S. mansoni occurs more often in the superior mesenteric veins
draining the large intestine
.
However, both species can occupy either location, and they are capable of
moving between sites, so it is not possible to state unequivocally that one
species only occurs in one location. S. haematobium most often
occurs in the venous plexus of bladder
, but
it can also be found in the rectal venules. The females (size 7 to 20 mm;
males slightly smaller) deposit eggs in the small venules of the portal and
perivesical systems. The eggs are moved progressively toward the lumen of
the intestine (S. mansoni and S. japonicum) and of the bladder
and ureters (S. haematobium), and are eliminated with feces or urine,
respectively
. Pathology
of S. mansoni and S. japonicum schistosomiasis includes:
Katayama fever, hepatic perisinusoidal egg granulomas, Symmers’ pipe stem
periportal fibrosis, portal hypertension, and occasional embolic egg
granulomas in brain or spinal cord. Pathology of S. haematobium
schistosomiasis includes: hematuria, scarring, calcification, squamous cell
carcinoma, and occasional embolic egg granulomas in brain or spinal cord. Human contact with water is thus necessary for infection by schistosomes. Various animals, such as dogs, cats, rodents, pigs, hourse and goats, serve as reservoirs for S. japonicum, and dogs for S. mekongi.
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