protozoan species in the genus Entamoeba infect humans, but not
all of them are associated with disease. Entamoeba histolytica
is well recognized as a pathogenic ameba, associated with intestinal and
extraintestinal infections. The other species are important because
they may be confused with E. histolytica in diagnostic
Cysts are passed
Infection by Entamoeba histolytica occurs by ingestion of mature
fecally contaminated food, water, or hands. Excystation
in the small intestine and trophozoites
released, which migrate to the large intestine. The trophozoites
multiply by binary fission and produce cysts
which are passed in the feces
Because of the protection conferred by their walls, the cysts can
survive days to weeks in the external environment and are responsible
for transmission. (Trophozoites can also be passed in diarrheal stools,
but are rapidly destroyed once outside the body, and if ingested would
not survive exposure to the gastric environment.) In many cases, the
trophozoites remain confined to the intestinal lumen (
noninvasive infection) of individuals who are asymptomatic carriers,
passing cysts in their stool. In some patients the trophozoites invade
the intestinal mucosa (
intestinal disease), or, through the bloodstream, extraintestinal sites
such as the liver, brain, and lungs (
extraintestinal disease), with resultant pathologic manifestations. It
has been established that the invasive and noninvasive forms represent
two separate species, respectively E. histolytica and E.
dispar, however not all persons infected with E. histolytica
will have invasive disease. These two species are morphologically
indistinguishable. Transmission can also occur through fecal exposure
during sexual contact (in which case not only cysts, but also
trophozoites could prove infective).
higher incidence of amebiasis in developing countries. In
industrialized countries, risk groups include male homosexuals,
travelers and recent immigrants, and institutionalized populations.
A wide spectrum,
from asymptomatic infection ("luminal amebiasis"), to invasive
intestinal amebiasis (dysentery, colitis, appendicitis, toxic megacolon,
amebomas), to invasive extraintestinal amebiasis (liver abscess,
peritonitis, pleuropulmonary abscess, cutaneous and genital amebic
must be differentiated from other intestinal protozoa including: E.
coli, E. hartmanni, E. gingivalis, Endolimax nana,
and Iodamoeba buetschlii (the nonpathogenic amebas);
Dientamoeba fragilis (which is a flagellate not an ameba); and the
possibly pathogenic Entamoeba polecki. Differentiation is
possible, but not always easy, based on morphologic characteristics of
the cysts and trophozoites. The nonpathogenic Entamoeba dispar,
however, is morphologically identical to E. histolytica, and
differentiation must be based on isoenzymatic or immunologic analysis.
Molecular methods are also useful in distinguishing between E.
histolytica and E. dispar and can also be used to identify
E. polecki. Microscopic identification of cysts and trophozoites in
the stool is the common method for diagnosing E. histolytica.
This can be accomplished using:
wet mounts and permanently stained preparations (e.g., trichrome).
from fresh stool: wet mounts, with or without iodine stain, and
permanently stained preparations (e.g., trichrome). Concentration
procedures, however, are not useful for demonstrating trophozoites.
In addition, E.
histolytica trophozoites can also be identified in aspirates or
biopsy samples obtained during colonoscopy or surgery.
methods for discriminating between E. histolytica and E.
comparison with other intestinal parasites
Bench aid for
infections, iodoquinol, paromomycin, or diloxanide furoate (not
commercially available in the U.S.) are the drugs of choice. For
symptomatic intestinal disease, or extraintestinal, infections (e.g.,
hepatic abscess), the drugs of choice are metronidazole or tinidazole,
immediately followed by treatment with iodoquinol, paromomycin, or