[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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APPENDIX D Answers to Test Your Knowledge (Review Questions)
intestinal; Iodamoeba butschlii, intestinal;
Entamoeba gingivalis, atrial; Naegleria
fowleri, atrial; Acanthamoeba
spp., atrial.
3-5. The life cycles of all of the intestinal
amebas are similar, with one exception.
Amebic life cycles require two morphologic
forms, the trophozoite and the cyst.
Trophozoites are characteristically delicate,
fragile, and motile. 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 very susceptible to the environment
outside the host. Therefore, trophozoites
are not usually transmitted to humans.
Excystation occurs in the ileocecal area
of the intestine. Replication only occurs
in the trophozoite stage by multiplication
of the nucleus via asexual binary fission.
Encystation occurs in the intestine when
the environment becomes unacceptable
for continued trophozoite multiplication.
A number of conditions solely 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 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.
3-6. Entamoeba gingivalis typically lives
around the gum line of the teeth in the
tartar and gingival pockets of unhealthy
mouths. In addition, E. gingivalis trophozoites
have been known to inhabit
the tonsillar crypts and bronchial mucus.
There is no known cyst stage of E. gingivalis.
Infections of E. gingivalis are
contracted via mouth-to-mouth (kissing)
and droplet contamination, which may
be transmitted through contaminated
drinking utensils.
Naegleria fowleri is the only ameba
with three known morphologic forms—
ameboid trophozoite, flagellate trophozoite,
and cyst. The ameboid trophozoites
of N. fowleri are the only form known to
exist in humans. Replication of the
ameboid trophozoites occurs by simple
binary fission. The ameboid trophozoites
transform into flagellate trophozoites in
vitro after being transferred to water
from a tissue or culture. The flagellate
trophozoites do not divide but, instead,
lose their flagella and convert back into
the ameboid form, in which reproduction
resumes. The cyst form is known to exist
only in the external environment. It
appears that the entire life cycle of N.
fowleri occurs in the external environment.
Humans primarily contract this
ameba by swimming in contaminated
water. The ameboid trophozoites enter
the human body through the nasal mucosa
and often migrate to the brain, causing
rapid tissue destruction. Some infections
may be caused by inhaling dust infected
with N. fowleri cysts. The mechanics of
how, when, and where the transition of
the cysts to ameboid trophozoites occurs
after the cysts have been inhaled by a
human have not been described.
3-7. A
3-8. A. Entamoeba histolytica is the only
known pathogenic intestinal ameba.
Symptoms range from asymptomatic
carrier state to symptomatic intestinal
amebiasis (amebic colitis) to symptomatic
extraintestinal amebiasis.
B. Infections with E. coli are usually
asymptomatic.
C. Patients who contract N. fowleri
resulting in colonization of the nasal
passages are usually asymptomatic.
Primary amebic meningoencephalitis
(PAM) occurs when the ameboid
trophozoites of N. fowleri invade