Back to Home Page of CD3WD Project or Back to list of CD3WD Publications

Home - English - French - German - Italian - Portuguese - Spanish
Bilharziasis (also called schistosomiasis) is one of the most widespread human
diseases caused by parasites. This entry explains in general terms what is
necessary for personal protection from bilharzia and for ridding an area of the
disease. Further information from the references given is needed. Cooperation
with government or other programs is essential.
An estimated 150 to 250 million people suffer from the disease. It is found in
much of Africa, the Tigris and Euphrates valleys, parts of Israel, northern Syria,
Arabia, Iran, Iraq, parts of Puerto Rico, Venezuela, Dutch Guiana, Brazil, Lesser
Antilles, Dominica, Taiwan and parts of China, the Philippines, Japan, and a few
villages in southern Thailand.
A basic understanding of the life cycle of the parasites, called schistosomes, and
the characteristics of each phase is the first step in preventing the disease (see
Figure 1).

fg1x186.gif (600x600)

The disease has been found, besides in humans, in baboons, monkeys, rodents,
water buffalo, horses, cattle, pigs, cats, and dogs. When water is contaminated by
urine or feces from a victim of the disease, the eggs contained in these hatch out
larvae that penetrate certain types of fresh-water snails. In the snail host, the
larvae develop into cercariae, which work their way out of the snail and become
free-swimming; this is the form that infects people. It can survive in water for a
few days under favorable conditions.
The disease is contracted by contact with water containing cercariae. Typical
ways are bathing, drinking, washing teeth, washing pots and clothes, walking
through water, irrigating, and cultivating crops. Once the parasite has contacted a
host, five minutes may be enough for it to penetrate the skin.
It is important to note that bilharziasis cannot be passed from human to human;
it depends on the snail intermediary. A victim must live in or have visited an
area where the parasite is found.
At the spot where the parasite penetrates the host, a red itching eruption lasting
several days usually develops. After the host is infected, symptoms relate particularly
to the large bowel, the lower urinary tract, liver, spleen, lungs, and the
central nervous system. The most characteristic symptoms are bladder and colon
irritation, ulceration, and bleeding. Three to 12 weeks after infection, a victim
will likely develop fever, malaise, abdominal pain, cough, itchy skin, sweating,
chills, nausea, vomiting, and sometimes mental and neurological symptoms. Later
developments may include frequent painful urination with blood in the urine,
dysentery with blood and pus in the stool, loss of weight, anemia, and enlargement
of the liver and spleen. Numerous complications are possible.
Typically the acute phase subsides and host and parasite live together over a
period of years, sometimes as long as 30, with the host suffering a variety of
symptoms of intermittent and variable types. Bladder and bowel troubles are the
most characteristic symptoms in this period.
The variety of vague and general symptoms is considerable and may not be very
specific. Examination of urine and/or feces is very important; special concentration
techniques may be necessary to reveal the eggs. Tissue tests and skin tests
can be used by medically-trained personnel to identify the disease.
The disease can be treated with drugs, but only well-trained persons should
undertake to treat a victim. Supportive treatment, which includes good diet,
nursing care, rest, and treatment of other ailments and infections, is important.
The disease can be prevented by:
o   Using uncontaminated water-a properly built sealed well or an improved
    sealed spring is safe. (See section on "Water Resources.")
o   However, it is important to remember that all water used must be safe.
    Never bathe in or touch water you wouldn't drink. Avoid suspected water. If
    it is necessary to use questionable water, boil it, or treat it with iodine or
    chlorine. If you must enter suspected waters, wear rubber gloves and wading
    boots, and put repellent on your skin; insect repellent (either diethyl
    toluamide or dimethyl phthalate), benzyl benzoate, cedar wood oil, or
    tetmosol give effective protection for about eight hours if applied to the
    skin before contact with the water. In case of accidental contact, rub your
    skin immediately with a dry cloth. Once cercariae have penetrated the skin,
    no preventive measures are possible.
o   Chlorination-Chlorine kills cercariae slowly, but properly chlorinated water
    systems are almost always free of the larvae. Use 2 halazone tablets in a
    liter (quart) of clear water; 4 tablets if the water is cloudy. In a water
    system, use 1 part per million chlorine. Iodine is even more lethal to
    cercariae. See section on "Chlorination of Polluted Water."
o   Filtering-Cercariae are just big enough to be seen with the unaided eye, and
    can be filtered from the water. However, dependence on filtration is
    questionable, since improperly made or operated filters will not only allow
    cercariae to pass, but may even provide a place for the host snail to live. In
    short, filtering is a poor technique.
o   Storage-Storing water at temperatures over 21C (70F) completely isolated
    from snail hosts for four days will allow the cercariae to die; at cooler
    temperatures they may live as long as six days. This is seldom a practical
Eliminating the snail intermediate host is at present the most effective single
method of controlling bilharziasis. The following methods are recommended:
o   Use a sealed, covered well or properly developed spring for a water supply.
    Make sure it is covered; this prevents access of organic matter that snails
    eat, cuts out light that would allow plants to grow for snail food, and
    prevents infected people from bathing in or contaminating the water.
o   If surface water must be used, put long-lasting (copper) screens on the
    intake; draw lake water far from vegetated shorelines, and preferably 2.4m
    (8') deep; take stream water from a fast moving spot.
o   Be sure filters and reservoir tanks are kept covered and dark and keep them
o   Since snails prefer the stagnant water of canals, irrigation ditches, and
    dams, control has been possible where the water level in ditches has been
    varied, where it has been turned off completely for periods, and where
    canals have been lined with cement or pipes have been used. Although the
    latter is initially expensive, it pays dividends not only in better health, but
    also in less water evaporation.
o   Poison the snails with copper sulfate, copper chromate, or other copper
    salts. Use a dose of 15-30 parts per million by weight of copper and try to
    hold the copper-treated water over the snails for 24 hours. All or most of
    the aquatic vegetation should be stripped from the stream bed or pool before
    treatment. Results for other than small controlled pools have been poor.
    Before attempting to treat streams, lakes, or other natural waters, study the
    reference material and seek experienced help.
Education is a major step in a continuing campaign against bilharziasis. Basic
steps involved in improving your local waters so they will not spread the disease
are as follows:
o   Inform yourself. Study this article, locate reference material cited below,
    consult any available health officials.
o   Learn to identify dangerous snails; for Africa, Professor Mozley's book is
    very helpful. To find the percentage of snails harboring schistosomes, collect
    a large sample of suspects (use rubber gloves, repellant, and snail scoop),
    put individually in test tubes or glass jars of water. Those shedding cercariae
    are readily detected, as the cercariae (0.5mm long and easily visible to
    the naked eye) are released in clouds. This test reveals only the snails
    harboring mature cercariae. Observe precautions at all times when collecting
    and handling snails!
o   Find dangerous snails locally, collect (again using rubber gloves, repellent,
    and snail scoop) and kill them. Mail empty shells to an expert to confirm
    your identification. Visit the expert if possible. Find out about government
    or other programs and participate in these.
o   Make a personal survey on foot (wearing boots) of local waters, using maps
    and keeping exact records to locate all dangerous snails. Local people can
    often help here. Aerial photographs are also helpful.
o   Survey types and intensity of bilharzia present in populace. Differences may
    help localize infection points. Keep special records for three- to six-year-olds,
    who are the most recently infected; these records will show most
    accurately the incidence of new infections.
o   Educate the public as much as possible, and get them to participate in the
    program. Better sanitation facilities, medical care, and improved nutrition are
    critical, but improved sanitary facilities are worthless if nobody uses them.
    Encourage people to live in villages away from infected waters, and to
    construct culverts or bridges at places where paths cross streams. The
    number of such crossings should be reduced. Any improvement should cater
    to local customs or offer an attractive alternative.
o   Personally supervise, participate in, and measure the effectiveness of
    poisoning the snails.
o   Take continuing steps to destroy the natural breeding places of snails,
    particularly at sites where humans and snails congregate. For example, the
    place where a stream crosses a road is a focal point: people stop to drink
    and bathe; they cook and wash out pots, providing food for snails. The
    culvert and embankments slow and impound the water, making ideal breeding
    conditions. Finally, a favorite sheltered place to defecate is under a bridge.
    Filling in places where water stands, changing drainage patterns, and
    eliminating snail food sources are possible techniques.
o   Maintain a continuing surveillance of focal spots and repeat poisoning
    periodically when necessary.
Mozley, Alan. The Snail Hosts of Bilharzia in Africa: Their Occurrence and
Destruction. London: H. K. Lewis & Co. Ltd.
Schistosomiasis, Bulletin No. 6. London: The Ross Institute, The London School of
Hygiene and Tropical Medicine.
Mason V. Hargett, M.D., Hamilton, Montana
Dr. Guy Esposito
Dr. Thomas W. M. Cameron, Montreal, Canada
Other References:
Craig, C. F. and Faust. Clinical Parasitology. Philadelphia: Lea and Fibeger, 1964.
Hinman, E.H. World Eradication of Infectious Diseases. Springfield Illinois:
Charles C. Thomas, 1966.
Markell, Edward K. and M. Voge. Medical Parasitology. Philadelphia: W.B. Saunders
Co., 1965.
The Merck Manual of Diagnosis & Therapy. Rahway, New Jersey: Merck.
Manson, Patrick. Tropical Diseases. Baltimore: William & Wilkins Co., 1966.
In addition, up-to-date information can be obtained from the World Health
Organization, Geneva, Switzerland.