C H A P T E R 1 3
Anthelmintic agents
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Whipworm infections
Whipworms
are transmitted when eggs found in the
soil are ingested. Whipworms attach to the wall of
the colon. In large numbers, they cause colic and bloody
diarrhoea. In severe cases, whipworm infestation may
result in prolapse of the intestinal wall and anaemia
related to blood loss.
Threadworm infestation
Threadworms
can cause more damage to humans than
most of the other helminths. Threadworms are trans-
mitted as larvae found in the soil and inadvertently
ingested. The larvae mature into worms, and, after
burrowing into the wall of the small intestine, female
worms lay eggs. These eggs hatch into larvae that invade
many body tissues, including the lungs, liver and heart.
In very severe cases, death may occur from pneumonia
or from lung or liver abscesses that result from larval
invasion.
Ascaris
Worldwide,
Ascaris
infection is the most prevalent hel-
minthic infection. It may occur wherever sanitation is
poor. Eggs in the soil are ingested with vegetables or
other improperly washed foods. Many individuals are
unaware that they have this infestation unless they see
a worm in their stool. However, others become quite ill.
Initially, the individual ingests fertilised roundworm
eggs, which hatch in the small intestine and then make
their way to the lungs, where they may cause cough,
fever and other signs of a pulmonary infiltrate. The
larvae then migrate back to the intestine, where they
grow to adult size (i.e. about as long and as big around
as an earthworm), causing abdominal distension and
pain. In the most severe cases, intestinal obstruction by
masses of worms can occur.
Hookworm infections
Hookworm
eggs are found in the soil, where they
hatch into a larva that moults and becomes infective to
humans. The larvae penetrate the skin and then enter
the blood and within about a week reach the intestine.
Hookworms attach to the small intestine of infected
individuals. The worms suck blood from the walls of
the intestine, damaging the intestinal wall and leading
to severe anaemia with lethargy, weakness and fatigue.
Malabsorption problems may occur as the small intesti-
nal mucosa is altered. Treatment for anaemia and fluid
and electrolyte disturbances is an important part of the
therapy for this infection.
Infections caused by platyhelminths
The
platyhelminths
(flatworms) include the cestodes
(tapeworms) that live in the human intestine and the
flukes (schistosomes) that live in the intestine and that
also invade other tissues as part of their life cycle.
Because schistosomes invade tissues, they are dis-
cussed in the following section on tissue-invading worm
infections.
Cestodes
Cestodes
are segmented flatworms with a head, or
scolex, and a variable number of segments that grow
from the head. Cestodes enter the body as larvae that
are found in undercooked meat or fish; they sometimes
form worms that are several metres long. Persons with a
tapeworm may experience some abdominal discomfort
and distension, as well as weight loss because the worm
eats ingested nutrients. Many infected people require a
great deal of psychological support when they excrete
parts of the tapeworm or when the worm occasionally
exits through the mouth or nose.
TISSUE-INVADING WORM INFECTIONS
Some of the worms that invade the body exist outside of
the intestinal tract and can seriously damage the tissues
they invade. Because of their location within healthy
tissue, they can also be more difficult to treat.
Trichinosis
Trichinosis
is the disease caused by ingestion of the
encysted larvae of the roundworm,
Trichinella spiralis
,
in undercooked pork. Once ingested, the larvae are
deposited in the intestinal mucosa, pass into the blood-
stream and are carried throughout the body. They can
penetrate skeletal muscle and can cause an inflamma-
tory reaction in cardiac muscle and in the brain. Fatal
pneumonia, heart failure and encephalitis may occur.
The best treatment for trichinosis is prevention.
Because the larvae are ingested by humans in under-
cooked pork, freezing pork meat, monitoring the food
eaten by pigs and instructing individuals about properly
cooking pork can be most beneficial.
Filariasis
Filariasis
refers to infection of the blood and tissues
of healthy individuals by worm embryos, which enter
the body via insect bites. These thread-like embryos,
or filariae, can overwhelm the lymphatic system and
cause massive inflammatory reactions. This may lead to
severe swelling of the hands, feet, legs, arms, scrotum or
breast—a condition called elephantiasis.
Schistosomiasis
Schistosomiasis
(see Figure 13.1) is a platyhelmin-
thic infection by a fluke that is carried by a snail. This
disease is a common problem in parts of Africa, Asia
