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Chemotherapeutic agents
and certain South American and Caribbean countries
that have climates and snails conducive to the life cycle
of schistosomes.
Eggs that are excreted in the urine and faeces of
infected individuals hatch in fresh water into a form that
infects a certain snail. In the snail, larvae known as cer-
cariae develop. The snail sheds the cercariae back into the
freshwater pond or lake. People become infected when
they come in contact with the infested water. The larvae
attach to the skin and quickly burrow into the blood-
stream and lymphatics. Then they move into the lungs,
and later to the liver, where they mature into adult worms
that mate and migrate to the intestines and urinary
bladder. The female worms then lay large numbers of
eggs, which are expelled in the faeces and urine, and the
cycle begins again.
Signs and symptoms may include a pruritic rash,
often called swimmer’s itch, where the larva attaches to
the skin. About 1 or 2 months later, affected individuals
may experience several weeks of fever, chills, headache
and other symptoms. Chronic or severe infestation
may lead to abdominal pain and diarrhoea, as well as
blockage of blood flow to areas of the liver, lungs and
central nervous system (CNS). These blockages can lead
to liver and spleen enlargement, as well as signs of CNS
and cardiac ischaemia. (See Critical thinking scenario
for a case study of a person diagnosed with chronic
schistosomiasis.)
■■
Helminths are worms that cause disease by invading
the human body. Some helminths invade body tissues
and can seriously damage lymphatic tissue, lungs,
CNS, heart or liver.
■■
Pinworms are the most frequent cause of helminth
infection in Australia and New Zealand, and
roundworms called
Ascaris
are the most frequent
cause of helminth infections throughout the world.
■■
Education is important for decreasing the stress
and anxiety that may occur when individuals are
diagnosed with a worm infestation.
KEY POINTS
Eggs hatch in fresh-
water stream or lake
and infect snails.
Within the snails, larvae
(cercariae) develop.
Cercariae are
shed into water.
Cercarial larvae burrow
into the skin of humans
coming in contact
with the water.
Larvae enter the
bloodstream and go
to lungs and liver,
where they mature.
Adults move to intestines
and urinary bladder. Females
lay masses of eggs.
Eggs excreted
in human waste.
Larvae
Hair
follicle
FIGURE 13.1
Life cycle of schistosomes.
