Schistosomiasis is caused by digenetic blood trematodes. The three main
species infecting humans are Schistosoma haematobium, S. japonicum,
and S. mansoni. Two other species, more localized
geographically, are S. mekongi and S. intercalatum. In
addition, other species of schistosomes, which parasitize birds and
mammals, can cause cercarial dermatitis in humans.
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
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
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
Human contact with
water is thus necessary for infection by schistosomes. Various animals,
such as dogs, cats, rodents, pigs, horse and goats, serve as reservoirs
for S. japonicum, and dogs for S. mekongi.
is found in parts of South America and the Caribbean, Africa, and the
Middle East; S. haematobium in Africa and the Middle East; and
S. japonicum in the Far East. Schistosoma mekongi and S.
intercalatum are found focally in Southeast Asia and central West
infections are asymptomatic. Acute schistosomiasis (Katayama's fever)
may occur weeks after the initial infection, especially by S. mansoni
and S. japonicum. Manifestations include fever, cough, abdominal
pain, diarrhea, hepatospenomegaly, and eosinophilia. Occasionally
central nervous system lesions occur: cerebral granulomatous disease may
be caused by ectopic S. japonicum eggs in the brain, and
granulomatous lesions around ectopic eggs in the spinal cord from S.
mansoni and S. haematobium infections may result in a
transverse myelitis with flaccid paraplegia. Continuing infection may
cause granulomatous reactions and fibrosis in the affected organs, which
may result in manifestations that include: colonic polyposis with bloody
diarrhea (Schistosoma mansoni mostly); portal hypertension with
hematemesis and splenomegaly (S. mansoni, S. japonicum,
S. mansoni); cystitis and ureteritis (S. haematobium) with
hematuria, which can progress to bladder cancer; pulmonary hypertension
(S. mansoni, S. japonicum, more rarely S. haematobium);
glomerulonephritis; and central nervous system lesions.
Microscopic identification of eggs in stool or urine is the most
practical method for diagnosis. Stool examination should be performed
when infection with S. mansoni or S. japonicum is
suspected, and urine examination should be performed if S.
haematobium is suspected.
Eggs can be present in the stool in infections with all Schistosoma
species. The examination can be performed on a simple smear (1 to 2 mg
of fecal material). Since eggs may be passed intermittently or in small
amounts, their detection will be enhanced by repeated examinations
and/or concentration procedures (such as the formalin - ethyl acetate
technique). In addition, for field surveys and investigational
purposes, the egg output can be quantified by using the Kato-Katz
technique (20 to 50 mg of fecal material) or the Ritchie technique.
Eggs can be found in the urine in infections with S. haematobium
(recommended time for collection: between noon and 3 PM) and with S.
japonicum. Detection will be enhanced by centrifugation and
examination of the sediment. Quantification is possible by using
filtration through a Nucleopore®
membrane of a
standard volume of urine followed by egg counts on the membrane.
Tissue biopsy (rectal biopsy for all species and biopsy of the bladder
for S. haematobium) may demonstrate eggs when stool or urine
examinations are negative.
detection can be useful in both in clinical management (e.g., recent
infections) and for epidemiologic surveys.
comparison with other intestinal parasites
and effective drugs are available for the treatment of schistosomiasis.
The drug of choice is praziquantel for infections caused by all
Schistosoma species. Oxamniquine has been effective in treating
infections caused by S. mansoni in some areas in which
praziquantel is less effective.