Schistosomiasis
is a disease caused by penetration of parasitic worms in the skin. The parasite
that penetrates the human skin is called cercaria which resides in a freshwater
snail. Freshwater include pond, rivers, streams, or lakes. The infection cannot
be acquired by contact with salt water of oceans and seas due to chlorination.
A person can get infected by walking, swimming, or having any other contact
with freshwater in disease endemic countries. Cercaria contaminates the water
by swimming toward the human host and infecting it. More than 200 million people are infected
worldwide. Schistosomiasis is second only to malaria as the parasitic disease
that cause most devastating. There are five types of Schistosomiasis
distributed wouldwide. Out of five species three main ones infecting humans are
S. haematobius, S. japonicum and S. mansoni which
are found mainly in tropical and subtropical areas. Ever since the increase in
tourism throughout Africa, sub-Saharan Africa, and South America there has been
an increase in the number of schistosomiasis cases.
Complex Life
cycle
1. Contact
with the parasite in a freshwater.
2. Parasite
enters the skin takes 3 days to reach blood capillaries.
3. Over the
next 7 days the parasite migrates to the lungs, heart then the liver.
4. In liver
they mature into adult worms and form male and female.
5. The
average life span of the worm is 5 years, but can live up to 20 years.
6. The eggs
release penetrates the bladder wall and goes in the urine.
7. Eggs are
eliminated in urine. In freshwater they can develop into mature blood flukes.
8. The eggs
hatch and release miracidia which search for a snail host.
9. Between
4-6 weeks the infected snail release 3000 cercariae per day.
10.
Cercariae penetrate human skin exposed in the water thus the life cycle begins
again.
Areas where
human schistosomiasis is found:
S. mansoni: Africa, South America and
Caribbeans. An estimated 85% of the world’s cases of schistosomiasis are in
Africa.
S. haematobium is distributed throughout
Africa and sub-Saharan Africa including the great lakes. And areas of the
Middle East
S. japonicum: Found in Indonesia and
parts of China and Southeast Asia
S. mekongi: Found in Cambodia and Laos
parts of Central and West Africa
Image
displays the distribution of the cases of Schistosomiasis due to Schistosoma
haematobium, Schistosoma japonicum, and Schistosoma
mekongi.
Image displays
the distribution of the cases of Schistosoma
mansoni and Schistosoma intercalatum.
Advice for
Travelers
Travelers
can avoid getting contact with the cercariae larvae by avoiding walking,
swimming, or having any other contact with freshwater in disease endemic
countries. Travelers should research on areas containing Schistosomasis
epidemic before visiting them. If contact is unavoidable a person must take
protective measures and undergo a checkup at a certified health center with
knowledge in tropical medicine for recommendations. Always drink safe water by
boiling water which can kill many bacteria and parasites. Take bath in hot
water. Another way of avoiding contact with cercariae larvae in freshwater is
by vigorously rubbing of the body with a towel. This brief method may or may
not prevent the parasite form penetrating the skin, but it is effective most of
the time. No vaccine is available. The drug of choice for all species of
schistosomiasis is praziquantel.
References
Corachan M.
Schistosomiasis and international travel. Clin Infect Dis. 2002 Aug
15;35(4):447–449.
Centers for
Disease Control and Prevention. National Center for Health Statistics. Health
Data Interactive. 9 July. 2012.
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