Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba.
Predominantly infecting humans and other primates, E. histolytica is estimated to infect about
50 million people worldwide.
Previously, it was thought that 10% of the world population was infected, but these figures
predate the recognition that at least 90% of these infections were due to a second species,
E. dispar.
Mammals such as dogs and cats can become infected transiently, but are not thought to contribute
significantly to transmission.
Transmission The active stage exists only in the host and
in fresh loose feces; cysts survive outside the host in water, in soils, and on foods,
especially under moist conditions on the latter.
The cysts are readily killed by heat and by freezing temperatures, and survive for only
a few months outside of the host.
When cysts are swallowed they cause infections by excysting in the digestive tract.
The pathogenic nature of E. histolytica was first reported by Lösch in 1875, but it was
not given its Latin name until Fritz Schaudinn described it in 1903.
E. histolytica, as its name suggests, is pathogenic; infection can be asymptomatic or can lead
to amoebic dysentery or amoebic liver abscess.
Symptoms can include fulminating dysentery, bloody diarrhea, weight loss, fatigue, abdominal
pain, and amoeboma.
The amoeba can actually 'bore' into the intestinal wall, causing lesions and intestinal symptoms,
and it may reach the blood stream.
From there, it can reach different vital organs of the human body, usually the liver, but
sometimes the lungs, brain, spleen, etc.
A common outcome of this invasion of tissues is a liver abscess, which can be fatal if
untreated.
Ingested red blood cells are sometimes seen in the amoeba cell cytoplasm.
Genome The E. histolytica genome was sequenced, assembled,
and automatically annotated in 2005.
The genome was reassembled and reannotated in 2010.
The 20 million basepair genome assembly contains 8,160 predicted genes; known and novel transposable
elements have been mapped and characterized, functional assignments have been revised and
updated, and additional information has been incorporated, including metabolic pathways,
Gene Ontology assignments, curation of transporters, and generation of gene families.
The major group of transposable elements in E. histolytica are non-LTR retrotransposons.
These have been divided in three families called EhLINEs and EhSINEs.
EhLINE1 encode an endonuclease protein, which have similarity with bacterial Restriction
Endonuclease.
This similarity with bacterial protein indicates that Transposable Elements have been acquired
from prokaryotes by horizontal gene transfer in this protozoan parasite.
Pathogen interaction E. histolytica may modulate the virulence
of certain human viruses and is itself a host for its own viruses.
For example, AIDS accentuates the damage and pathogenicity of E. histolytica.
On the other hand, cells infected with HIV are often consumed by E. histolytica.
Infective HIV remains viable within the amoeba, although fortunately there has been no proof
of human reinfection from amoeba carrying this virus.
A burst of research on viruses of E. histolytica stems from a series of papers published by
Diamond et al. from 1972 to 1979.
In 1972, they hypothesized two separate polyhedral and filamentous viral strains within E. histolytica
that caused cell lysis.
Perhaps the most novel observation was that two kinds of viral strains existed, and that
within one type of amoeba the polyhedral strain had no detrimental effect but led to cell
lysis in another.
Although Mattern et al. attempted to explore the possibility that these protozoal viruses
could function like bacteriophages, they found no significant changes in Entamoeba histolytica
virulence when infected by viruses.
However, no newer published research has been conducted on this species since.
Extra intestinal ameobiasis Sometimes,the trophozoites may rupture the
wall of capillaries, enter the blood stream and primarily reach the liver where they may
cause abscesses.
From there, they may go to lungs, heart, brain, kidney, gonads, etc., and cause abscesses
in those parts leading to severe pathological conditions.
Diagnosis It can be diagnosed by stool samples, but
it is important to note that certain other species are impossible to distinguish by microscopy
alone.
Trophozoites may be seen in a fresh fecal smear and cysts in an ordinary stool sample.
ELISA or RIA can also be used.
Treatment There are many kinds of effective drugs.
This is just a short overview of a few of the different methods of treatments.
Intestinal Infection: Usually nitroimidazole derivatives are used because they are highly
effective against the trophozoite form of the amoeba.
Since they have little effect on amoeba cysts, usually this treatment is followed by an agent
that acts on the organism in the lumen.
Liver abscess: In addition to targeting organisms in solid tissue, primarily with drugs like
metronidazole and chloroquine, treatment of liver abscess must include agents that act
in the lumen of the intestine to avoid re-invasion.
Surgical drainage is usually not necessary except when rupture is imminent.
Asymptomatic Patients: For asymptomatic patients, non endemic areas should be treated by paromomycin,
and other treatments include diloxanide furoate and iodoquinol.
There have been problems with the use of iodoquinol and iodochlorhydroxyquin, so their use is
not recommended.
Diloxanide furoate can also be used by mildly symptomatic persons who are just passing cysts.
precyistic stage:- it is the non-feeding and non-pathogenic stage of entameoba histolytica
that is found in the lumen of the large intestine.
it is a small,spherical or oval, non-motile form.the cytoplasm of the pre-cyistic stage
stores glycogen,granules and chromatoid bars) which act as resrve food meterial
Additional images
See also List of parasites
References
External links Entamoeba histolytica image library
Entamoeba histolytica - Centers for Disease Control and Prevention
CDC DPDx Parasitology Diagnostic Web Site LSHTM Entamoeba Homepage
Entamoeba Genome Resource - AmoebaDB [1]
Entamoeba histolytica article from the US Food and Drug Administration's Bad Bug Book
[2]