[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 3 The Amebas
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CASE STUDY 3-1
UNDER THE MICROSCOPE—cont’d
Questions for Consideration
1. Which ameba is the most likely cause of Scooter’s infection?
(Objective 3-10B)
2. Name associated diseases linked with the responsible
parasite. (Objective 3-10C)
3. Construct the life cycle of the responsible parasite
emphasizing the epidemiology, route of transmission,
and infective and diagnostic stages. (Objective 3-10D)
4. What are the treatment options and how can this
parasitic infection be prevented or controlled? (Objective
3-10E)
5. What is the specimen of choice for the recovery of this
parasite? (Objective 3-10F)
6. Interpret the laboratory data given and propose specific
follow-up tests that could aid in the identification of the
infection. (Objective 3-10H)
FOCUSING IN
Protozoa are unicellular organisms and the
lowest form of animal life. In the subkingdom
Protozoa, there are three phyla of medical
interest in humans. The phylum Sarcomastigophora,
subphylum Sarcodina, includes the
pathogenic and nonpathogenic amebas. This
chapter describes the morphologic features, laboratory
diagnosis, life cycle, epidemiology and
clinical symptoms, treatment, and prevention
and control of nine ameba species, each of
which is known to infect humans.
MORPHOLOGY AND LIFE
CYCLE NOTES
The most important feature that separates
amebas from the other groups of unicellular Protozoa
is the means by which they move. Amebas
are equipped with the ability to extend their
cytoplasm in the form of pseudopods (often
referred to as false feet), which allows them move
within their environment. With one exception,
there are two morphologic forms in the amebic
life cycle—trophozoites, the form that feeds,
multiplies, and possesses pseudopods, and cysts,
the nonfeeding stage characterized by a thick
protective cell wall designed to protect the parasite
from the harsh outside environment when
deemed necessary. It is important to note here
that the nuclear characteristics of trophozoites
are basically identical to those of their corresponding
cysts.
Trophozoites are characteristically delicate
and fragile and, because of their ability to produce
and use pseudopods, motile. The life cycles of
all the intestinal amebas are similar. The most
common means whereby amebas are transferred
to humans is through ingestion of the infective
cyst in contaminated food or water. In most
cases, trophozoites are easily destroyed by the
gastric juices of the stomach. Trophozoites are
also susceptible to the environment outside the
host. Therefore, trophozoites are not usually
transmitted to humans. Excystation, the morphologic
conversion from the cyst form into the
trophozoite form, occurs in the ileocecal area of
the intestine. Replication only occurs in the trophozoite
stage; it is accomplished by multiplication
of the nucleus via asexual binary fission.
The conversion of trophozoites to cysts, a
process known as encystation, occurs in the
intestine when the environment becomes unacceptable
for continued trophozoite multiplication.
A number of conditions individually or in
combination may trigger encystation, including
ameba overpopulation, pH change, food supply
(too much or too little), and available oxygen
(too much or too little). Contrary to the trophozoites,
cysts are equipped with a protective cell
wall. The cell wall allows cysts to enter the
outside environment with the passage of feces
and remain viable for long periods of time. The
ingestion of the infective cysts completes the
typical intestinal amebic life cycle.
Because the basic life cycle is the same for each
of the intestinal amebas, a section dedicated to