Entamoeba histolytica is an ameba parasite that lives in the large intestine of humans. It feeds on intestinal cells and causes dysentery and colonic ulcers. The cyst form is resistant and spreads through contaminated food or water. It excysts in the intestine and the trophozoite form causes tissue damage and disease symptoms. Infection is diagnosed by examining stool samples microscopically. It is treated with metronidazole but prevention relies on improved sanitation and hygiene.

1. Entamoeba histolytica
General description:
Entamoeba histolytica is an ameba that feeds on cells in the human colon. It is the cause of
amebic dysentery (bloody diarrhea) as well as colonic ulcerations. The infection is also
referred to as amebiasis. If the organisms spread throughout the body via the bloodstream they
may cause abscesses in the liver or, less frequently, other organs.
Habitat:
Entamoeba histolytica is a parasite and lives in the mucous and sub-mucous layers of the large
intestine of man. It may occur in the liver and lungs. Rarely it invades brain, spleen etc.
Noor Zada, M.Sc Zoology, KUST, Kohat

2. The parasite gets its nutrition from the red blood cells and
tissue cells of their host. They produce a toxic substance
which dissolve the mucous and sub-mucous layers of the large
intestine. From the resultant absess it derives its nutrition.
This is evident from the fact that the stool of persons with
amoebic dysentery contain pure blood and mucus.
Nutrition required by Entamoeba histolytica:
Noor Zada, M.Sc Zoology, KUST, Kohat

3. Morphology:
The organism has two forms. The cyst is round and 10-20 micrometers in diameter,
and contains four nuclei when mature. It is resistant to desiccation and stomach
acid, and can survive long enough in the environment to be spread to other humans.
When the cyst reaches the large intestine, it excysts and the four nuclei present in
the cyst become four separate amebae, each of which undergoes binary fission
immediately; thus the ingestion of a single cyst leads to 8 trophozoites. The
trophozoite, 10-60 micrometers in diameter, is the active form of the organism and
it is in this form that the damage is done to the body.
Noor Zada, M.Sc Zoology, KUST, Kohat

4. Life-cycle of Entamoeba histolytica:
Entamoeba histolytica multiplies by binary fission in the trophic stage. They have the
capacity to encyst. Prior to encystment the parasite rounds up and eliminates food
vacuoles. A cyst wall develops and the nucleus divides first into 2 and then into 4.
The quadrinucleate cyst is the infective stage. It comes out with the feces of the host.
Through contaminated food or drink the infective cysts pass into the lower portion of
the small intestine of the new host.
There the cyst wall dissolves by the action of the intestinal enzymes and a 4 nucleate
amoeba emerges. Division of both the cytoplasm and the nuclei of the amoeba results
8 small amoebulae. They become active and transform into trophic form.
Noor Zada, M.Sc Zoology, KUST, Kohat

5. Noor Zada, M.Sc Zoology, KUST, Kohat

6. Noor Zada, M.Sc Zoology, KUST, Kohat

7. Transmission:
E. Histolytica is spread by the fecal-oral route. This is achieved through food or
water contaminated with cysts, oral-anal sexual contact, or occasionally directly in
childcare centers or institutions for the developmentally challenged. The disease is
found far more frequently in people from developing countries or travelers to such
areas than in developed countries.
Noor Zada, M.Sc Zoology, KUST, Kohat

8. Virulence:
Damage is caused by the lysis of epithelial cells, due in part to the insertion of pore-
forming proteins into the membrane of the cell. Neutrophils and non-activated
macrophages may also be killed and ingested by the organism, limiting the ability
of the immune system to deal with the disease. E. Histolytica is also capable of
phagocytosing red blood cells. Symptoms of infection vary widely, from an
asymptomatic carrier state, to mild discomfort and stools containing some blood or
mucous, to full-blown dysentery with bloody and mucoid stools.
Noor Zada, M.Sc Zoology, KUST, Kohat

9. Diagnosis:
E. Histolytica is diagnosed by the examination of slides prepared from fecal matter. It can be difficult
to diagnose E. Histolytica for several reasons. It is morphologically indistinguishable from E. Dispar, a
nonpathogenic species. There are also difficulties involved in spotting E. Histolytica in slides prepared
from stool specimens. The CDC morbidity and mortality weekly report from march of 1985 discusses
a "pseudo-outbreak" that occurred in california in 1983. 38 patients over the course of 3 months were
diagnosed with E. Histolytica, in comparison to a previous frequency of about 1 case per month. No
connection between the patients was found, nor were the patients part of the high-risk population of
travelers or homosexual males. Instead it was found that 36 of these cases were actually misdiagnosed.
Two of the patient samples were positive for protozoa other than E. Histolytica, and 34 of these stool
samples contained polymorphonuclear neutrophils and/or macrophages, which can be visually
confused with E. Histolytica cysts.
Noor Zada, M.Sc Zoology, KUST, Kohat

10. Control and treatment:
E. Histolytica is anaerobic and is sensitive to metronidazole when in the trophozoite
form. Metronidazole is not effective against the cyst form of the organism, and
therefore is followed up with iodoquinol or paromomycin to target the cysts.
Dehydroemetine, a treatment that requires hospitalization due to the need for close
supervision, and diloxanide furoate, which is used in conjunction with other
treatments in systematic cases, are only available through the center for disease
control and prevention. If liver infection occurs cholorquine may be used, in the
event that metronidazole is ineffective.
Noor Zada, M.Sc Zoology, KUST, Kohat

11. Entamoeba histolytica is a typical example of diseases that impact poor populations
in developing countries. This makes it far more difficult to fund research and
development for new treatments or vaccines, in spite of the fact that research has
shown some possibilities for vaccines. The presence of iga antibodies against E.
Histolytica indicates that a vaccine that brings about a mucosal immune response
could be effective. However, the lack of projected profit limits the interest of
pharmaceutical and biotechnology companies. Sanitation and hygiene are effective
controls but often cannot be applied in many poor nations. Until a vaccine is created
and distributed, entamoeba histolytica will remain an important disease in mortality
rates, especially among children in developing countries.
Noor Zada, M.Sc Zoology, KUST, Kohat