C H A P T E R 1 2
Antiprotozoal agents
171
The disease is transmitted while the protozoan is
in the cystic stage in faecal matter, from which it can
enter water and the ground. It can be passed to other
humans who drink this water or eat food that has been
grown in this ground. The cysts are swallowed and pass,
unaffected by gastric acid, into the intestine. Some of
these cysts are passed in faecal matter, and some of them
become trophozoites that grow and reproduce. The tro-
phozoites migrate into the mucosa of the colon, where
they penetrate into the intestinal wall, forming erosions.
These forms of
Entamoeba
release a chemical that dis-
solves mucosal cells, and eventually they eat away tissue
until they reach the vascular system, which carries them
throughout the body. The trophozoites lodge in the liver,
lungs, heart, brain, and so on.
Early signs of amoebiasis include mild to fulmi-
nate diarrhoea. In the worst cases, if the protozoan is
able to invade extraintestinal tissue, it can dissolve the
tissue and eventually cause the death of the host. Some
individuals can become carriers of the disease without
having any overt signs or symptoms. These people seem
to be resistant to the intestinal invasion but pass the
cysts on in the stool.
Leishmaniasis
Leishmaniasis
is a disease caused by a protozoan that is
passed from sand flies to humans. The sand fly injects an
asexual form of this flagellated protozoan, called a pro-
mastigote, into the body of a human, where it is rapidly
attacked and digested by human macrophages. Inside
the macrophages, the promastigote divides, develop
ing many new forms called amastigotes, which keep
dividing and eventually kill the macrophage, releas-
ing the amastigotes into the system to be devoured by
more macrophages. Thus, a cyclic pattern of infection is
established. These amastigotes can cause serious lesions
in the skin, the viscera or the mucous membranes of the
host.
Environment
(contaminated food,
water, hands)
Ingested by host
(humans, primates,
other mammals)
Enters intestinal tract
(produced daughter cells—
trophozoites—and cysts)
Bloodstream
and tissue
Liver, lungs, heart,
brain and spleen
Fatal to host
Fatal to
trophozoites
Faeces of host
Cysts
Metronidazole
works here
Metronidazole
works here
Metronidazole,
chloroquine
works here
FIGURE 12.3
Life cycle of
Entamoeba histolytica
and the
sites of action of metronidazole
and chloroquine, which are
used to treat amoebiasis. Cysts
ingested by the host enter the
intestinal tract and produce
trophozoites. Trophozoites enter
the bloodstream to reach tissue.
Trophozoites enter the liver,
lungs, heart, brain and spleen,
which can be fatal to the host.
Trophozoites are excreted in the
stool and die. Cysts excreted in
the stool contaminate water and
can be ingested by the host.
