Entamoeba histolytica INTESTINAL PROTOZOA
Entamoeba coli unicellular eukaryotic organisms
• Giardia lamblia
• Dientamoeba fragilis
• Chilomastix mesnili
• Trichomonas hominis
• Trichomonas vaginalis
•(other body sites)
• Enteromonas hominis
• Retortamonas intestinalis
• Cryptosporidium hominis
• Cryptosporidium parvum
• Blastocystis hominis
• Balantidium coli
Fecal-Oral Transmission Factors
• poor personal hygiene
• food handlers Control/Prevention
• institutions • improve personal hygiene
• children in day care centers • especially institutions
• developing countries • treat asymptomatic carriers
• highly endemic • eg, family members
• poor sanitation • health education
• travelers diarrhea • hand-washing
• water-borne epidemics • sanitation
• food handling
• zoonosis • protect water supply
• Entamoeba = no
• treat water if questionable
• Cryptosporidium = yes
• Giardia = controversial
• not chlorine
Amoebiasis is caused by Entamoeba histolytica .
The organism formerly known as E. histolytica is now
known to consist of two distinct species :E. histolytica ,
which is pathogenic, and E. dispar ,which is non-
Cysts of the two species are identical, but can be
distinguished by molecular techniques after culture of
the trophozoite .E. histolytica can be distinguished
from all amoebae except E. dispar ,and from other
intestinal protozoa, by microscopic appearance.
Amoebiasis occurs world-wide, although much higher
incidence rates are found in the tropics and
The organism exists both as a motile
trophozoite and as a cyst that can survive
outside the body.
Cysts are transmitted by ingestion of
contaminated food or water, or spread
directly by person-to-person contact.
Trophozoites emerge from the cysts in the
small intestine and then pass on to the
colon, where they multiply .
Many individuals can carry the pathogen
without obvious evidence of clinical disease
(asymptomatic cyst passers).
This is may be due in some cases to the
misidentification of non-pathogenic E.
dispar as E. histolytica, and it is not clear
how often true E. histolytica infection is
In affected people E. histolytica
trophozoites invade the colonic epithelium,
probably with the aid of their own
cytotoxins and proteolytic enzymes.
The parasites continue to multiply
and finally frank ulceration of the
If penetration continues, trophozoites
may enter the portal vein, via which
they reach the liver and cause
This invasive form of the disease is
serious and may even be fatal .
• ulcers with raised borders
• little inflammation between lesions
The incubation period of intestinal amoebiasis is
highly variable and may be as short as a few days or
as long as several months.
The usual course is chronic, with mild intermittent
diarrhoea and abdominal discomfort. This may
progress to bloody diarrhoea with mucus, and is
sometimes accompanied by systemic symptoms such
as headache, nausea and anorexia.
Less commonly, infection may present as acute
amoebic dysentery, resembling bacillary dysentery or
acute ulcerative colitis.
Complications are unusual, but include toxic
dilatation of the colon, chronic infection with
stricture formation, severe haemorrhage,
amoeboma, and amoebic liver abscess.
Amoebomas, which develop most commonly in the
caecum or rectosigmoid region, are sometimes
mistaken for carcinoma. They may bleed, cause
obstruction or intussuscept.
Amoebic liver abscesses often develop in the absence
of a recent episode of colitis. Tender hepatomegaly, a
high swinging fever and profound malaise are
characteristic, although early in the course of the
disease both symptoms and signs may be minimal.
Microscopic examination of fresh stool or colonic
exudate obtained at sigmoidoscopy is the simplest
way of diagnosing colonic amoebic infection.
To confirm the diagnosis motile trophozoites
containing red blood cells must be identified: the
presence of amoebic cysts alone does not imply
disease. Sigmoidoscopy and barium enema
examination may show colonic ulceration but are
The amoebic fluorescent antibody test is positive in at
least 90% of patients with liver abscess and in 60-
70% with active colitis. Seropositivity is low in
asymptomatic cyst passers.
Metronidazole 800 mg three times daily for
5 days is given in amoebic colitis; a lower
dose (400 mg three times daily for 5 days)
is usually adequate in liver abscess.
Tinidazole is also effective: dehydroemetine
and chloroquine are alternative drugs, but
are rarely used. After treatment of the
invasive disease, the bowel should be
cleared of parasites with a luminal
amoebicide such as diloxanide furoate.
Amoebiasis is difficult to eradicate
because of the substantial human
reservoir of infection. The only
progress will be through improved
standards of hygiene and better
access to clean water.
Cysts are destroyed by boiling, but
chlorine and iodine sterilizing tablets
are not always effective.
Dr. Ayham Abulaila