2. INTRODUCTION
• Schistosomiasis is a metazoan parasitic
infection of man and other vertebrates,
caused by blood flukes of the genus
Schistosoma. The parasite is transmitted
through specific aquatic or amphibious
snail intermediate hosts in various
freshwater habitats. In terms of
socioeconomic and public health
importance it is regarded by the World
Health Organization as only second to
malaria among the parasitic diseases
affecting man in tropical and some sub-
tropical countries in the world.
3. • It is estimated that about 200 million peopl
e, mostly from rural and agricultural areas
are infected, and that 500 to 600 million pe
ople are at risk of contracting infection (W
HO 1985). The World Health Organization
further regards it as one of the main occup
ational risks encountered in rural areas of
developing countries, and that it is second
to none in terms of prevalence among the
water-borne diseases.
4. DISTRIBUTION
There are three major species of Schistosoma affecting
man in about 76 endemic countries in the world;
Schistosoma mansoni (Sambon, 1907) occurring in Africa,
the Arabian Peninsula, (e.g. Lebanon, Oman, Saudi Arabia,
and Yemen) the West Indies (e.g. Puerto Rico and Saint
Lucia) and South America (e.g. Brazil, Venezuela, Surinam
etc.);
Schistosoma haematobium (Bilharz, 1852) occurring in
Africa and the Middle East (e.g Iran, Iraq, Lebanon, Oman,
Saudi Arabia, Syria, and Yemen);
And Schistosoma japonicum (Katsurada, 1904) occurring
only in the Far East in countries such as the Philippines,
Indonesia, Thailand and China.
Other important but sparsely occurring types of
schistosomiasis affecting man are Schistosoma
intercalatum (Fisher, 1934) in parts of Central Africa, e.g.
Cameroon, Gabon, Zaire and Equatorial Guinea; and
Schistosoma mekongi in Southern Laos and Kampuchea
5.
6. MORPHOLOGY OF SCHISTOSOMA WORMS
The Schistosoma worms are dioecious, i.e. are
separate male and female worms, unlike all other
human flukes which are hermaphroditic. The male
is boat shaped with a groove (gynaecophoric
canal) at the middle in which the female is
attached. It is shorter and thicker than the female
(6-20mm), and the cuticle of the male has
tuberculations for all species except S. japonicum,
whose cuticle is smooth. There are two suckers,
the oral and ventral suckers. The genital pore is
situated just behind the ventral sucker. Males
usually have 4-9 testes depending on the species.
The number of testes is a useful factor in
separating species.
7. The female worms are thread like (long and
slender), measuring about 7-28 mm. They
appear dark due to the inclusion of changed
blood pigment in the gut. The ovary may be
situated in the anterior half, middle or the
posterior half of the body, depending on the
species. The number of eggs in the uterus
differs with the species: Schistosoma
mansoni usually has 1-2 eggs, Schistosoma
haematobium 10-50, Schistosoma
japonicum 50-200.
9. LIFE CYCLE OF SCHISTOSOMA
WORMS
• The schistosome worms, which are digenetic
trematodes, inhabit the blood vascular system, mainly
the mesenteric veins for S. mansoni and S. japonicum
and the vesical plexus veins for S. haematobium.
They lay from approximately 250 to more than 3000
eggs per day, depending on the species, and are said
to live for up to 20-30 years (Mansoni-Bar, 1987).
However, the mean life span is 3.3 years (Mansoni-
Bar, 1987).
• Eggs voided in either urine for S. haematobium or
stools (S. mansoni and S. japonicum) must get into
fresh water with the relevant intermediate hosts.
10.
11. Egg, miracidium, primary sporocyst, secondary
sporocyst, cercaria and the adult worm.
When viable eggs which are passed out in
human faeces for S. mansoni and S. japonicum
and in urine for S. haematobium get into
freshwater they hatch into free-swimming
miracidia.
The actively swimming miracidia must find and
penetrate a suitable freshwater snail intermediate
host. The miracidia are very host-specific in
regard to the snail species or strains in which
they will develop, though they can penetrate any
snail or other aquatic organisms in which they
may not, however, develop.
12. The miracidia die within 10 to 30 hours if they fail to
penetrate, as their energy gets exhausted. The free
swimming smiracidia and cercariae do not feed.
After penetrating a suitable snail host the miracidia
go through the asexual primary and secondary
sporocyst stages with subsequent production of
cercariae.
S. mansoni miracidia develop in snails of
Biomphalaria species, S. haematobium in species of
Bulinus and S. japonicum in species of Oncomelania.
The cercariae emerge between 4 to 12 weeks
(depending on temperature) after the penetration of
the miracidia.
15. Man becomes infected when cercariae shed by the
infected snails penetrate the skin. During
penetration the cercariae lose their tails and
transform into schistosomula just under the
surface of the skin of their new host where they
remain for two to three days. The schistosomula
subsequently enter the lymphatic and blood
vessels from where they are carried in the blood
stream to the heart and lungs, and finally to the
liver where they develop into young adults. Here
the worms pair up.
Schistosomes feed on blood particles through
anaerobic glycolysis
16. The long slender females get enclosed within the
gynaecophoric canal of the males, a state in which they live
during their lifetime, and the worm pairs migrate to their
areas of predilection, which is the mesenteric veins for S.
mansoni, S. japonicum and the veins of the vesical plexus
for S. haematobium, where the egg laying commences.
Some of the eggs work their way through the wall of the
vessels where they are laid, using mechanical and
enzymatic means, to the sites of their excretion (either
intestines for S. mansoni and S. japonicum or the urinary
bladder for S. haematobium). They have to reach fresh
water in order to continue the life cycle.
17. PATHOLOGY
Eggs trapped in the host tissues are the main
causative agent of disease, mainly due to some
immunological changes in the host induced by the
egg antigens. These antigens stimulate the
formation of granulomas, causing changes in the
walls of the bladder and ureters in urinary
schistosomiasis (S. haematobium) ureteric
obstruction and occasionally cancer of the bladder;
while in the intestinal schistosomiasis (S. mansoni
and S. japonicum) the granulomas surrounding the
eggs in the large bowels cause colitis. Some of the
eggs, especially in S. mansoni, are carried back
through the blood stream and trapped in the liver.
Granuloma formation around these eggs follows,
causing a coarse periportal fibroses which may
result in the enlargement of the liver due to the
blockage to the hepatic portal flow, leading to
18. Hepatosplenomegaly in a 15 year old girl,showing
scarification. Nzoila School - Kambu
Hepatosplenomegaly in a 15 year old girl. Nzoila Pry. School, Kibwezi. 2002
ENLARGED LIVER
SCAR MARKS
ENLARGED SPLEEN
20. Other clinical manifestations
include
• 1) Invasive stage
• Cercarial dermatitis (Swimmer's itch). This
may appear 24 hours after first infection
but seldom lasts more than 48 hours. It
rarely affects people indigenous to an
endemic area.
21. • 2) Acute phase
• Allergic manifestations such as headache,
oedema, cough, dysenteric symptoms, pruritis
and urticaria occur 3-8 weeks after infection with
S. japonicum. This is known as the Katayama
syndrome and may be accompanied by mild
abdominal pain and eosinophilia. The condition
is possibly caused by an excess of antigen when
eggs are first produced.
22. • 3) Chronic phase
• As described in the first paragraph in
'Pathology'.
23. DIAGNOSES
• PARASITOLOGICAL. The detection of eggs in
faeces or urine is the simplest and most direct
method of diagnoses in heavy infections. In the
case of single sex infection the presence of
worms would not be detected by this method, in
which case serological tests can be carried out if
necessary.
• For intestinal schistosomiasis formal ether
concentration method can be used. For more
sensitivity or in control programmes Kato thick
smear method is preferable. In the urinary
schistosomiasis sedimentation and filtration
techniques are carried out. Direct centrifugation
of a sample of urine is commonly practiced in
most laboratories.
