Entamoeba histolytica is an enteric protozoan parasite with worldwide distribution. It is responsible for amoebic dysentery (bloody diarrhea) and invasive extraintestinal amebiasis (such as liver abscess, peritonitis, pleuropulmonary abscess). Other species of Entamoeba-E.hartmanii, E.coli and E.dispar does not cause diseases but their trophozoite is difficult to distinguish from those of E.histolytica by light microscopy.
Mode of transmission: Feco-oral route, via the ingestion of contaminated food or water containing mature quadrinucleate cyst of Entamoeba histolytica. Trophozoites if ingested would not survive exposure to the gastric environment.
Mneomonic: EntAmoebaHistoLytica i.e. (Remember:
- Ent: Enterocytes (a cell of the intestinal lining);
- Amoeba: Protozoa;
- Histo: Tissue;
- Lytica: lysis.)
As this protozoan parasite lyse the cells of intestinal tract; there will be bleeding; so the stool contains blood and mucus (Amoebic dysentry).
Infective form: Mature quadrinucleate cyst; it is spherical in shape with refractile wall (Note: Giardia lamblia cyst also has four nuclei, but the cyst is oval in shape).
Geographical distribution: Worldwide, more common in the tropics and sub tropics, especially in areas with poor sanitation (developing and under-developed countries).
Habitat: Trophozoites of E. histolytica live in the mucosal and submucosal layers of the large intestine of man. Life cycle of Entamoeba histolytica has two stage: motile trophozoite and non-motile cyst. Trophozoites are found in intestinal lesions, extra-intestinal lesions and diarrheal stools where as cyst predominate in non-diarrheal stools.
Life cycle of Entamoeba histolytica
Infection by Entamoeba histolytica occurs by ingestion of mature quadrinucleate cysts in fecally contaminated food, water, or hands.The quadrinucleate cyst is resistant to the gastric environment and passes unaltered through the stomach
- When they cyst of E.histolytica reaches caecum or lower part of ileum excystation occurs and an amoeba with four nuclei emerges and that divides by binary fission to form eight trophozoites.
- Trophozoites migrate to the large intestine and lodge in to the submucosal tissue.
- Trophozoites grow and multiply by binary fission in large intestine (Trophozoite phase of life cycle is responsible for producing characteristics lesion of amoebiasis).
- Certain number of trophozoites are discharged in to the lumen of the bowel and are transformed into cystic forms.
- The cysts thus formed are unable to develop in the same host and therefore necessitate a transference to another susceptible host.The cysts are passed in the feces.
Note: Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment. Cysts are not highly resistant and are readily killed by boiling. But they are resistant to chlorination or can be removed by filtration. Trophozoites can also be passed in diarrheal stools, but are rapidly destroyed once outside the body.
Trophozoite is responsible for disease conditions;
- The trophozoites invade the colonic epithelium and secrete enzymes that cause localized necrosis. Little inflammation occurs at the site.
- As the lesion reaches the muscularis layer, a typical “flask shaped” ulcer forms, that can undermine and destroy large areas of intestinal epithelium.
- Progression into submucosa leads to invasion of the portal circulation by the trophozoites.
- Non-invasive infection: In many cases, the trophozoites remain confined to the intestinal lumen of individuals who are thus asymptomatic carriers and cysts passers.
- Intestinal disease: In some patients the trophozoites invade the intestinal mucosa,
- Extra-intestinal disease : through the bloodstream, trophozoites invade extraintestinal sites such as the liver, brain, and lungs, with resultant pathologic manifestations.
Amoebic liver abscess
- About 2-10% of individuals infected with E. histolytica suffers from hepatic complications.
- In 50% of cases history of ameobic dysentery many not seen.
- The trophozoites of E.histolytica are carried as emboli by the radicles of the portal vein from the base of amoebic ulcer in the large intestine.
- Capillary system of the liver acts as efficient filter and holds the
- Trophozoites multiply inside liver cells and carry on cytolytic actions.
- This leads to obstruction to the circulation and produce thrombosis of the portal venules (sinusoids) resulting in anaemic necrosis of the liver cells
- Progressive destruction of concentric layers of liver cells occurs.
- Large sized abscess is formed by coalescence of miliary abscess.
Amoebic liver abscess
- Amebic abscess of the liver is characterized by right upper-quadrant pain, weight loss, fever, and a tender enlarged liver.
- Right-lobe abscesses can penetrate the diaphragm and cause lung disease (pulmonary amoebiasis)
- Other metastatic lesions:
- Cerebral amoebiasis
- Amoebic pericarditis
- Cutaneous amoebiasis
- Splenic Abscess etc.
- Most cases of amebic liver abscess occur in patients who have not had overt intestinal amebiasis.
- Aspiration of the liver abscess yields brownish-yellow pus with consistency of anchovy-paste.
- Acute intestinal amebiasis
- dysentery (i.e. bloody, mucus containing diarrhea)
- lower abdominal discomfort,
- flatulence and
- Chronic amebiasis : low grade symptoms such as occasional diarrhea, weight loss and fatigue also occurs.
- Roughly 90% of infected individuals have asymptomatic infection but they may be carriers,
- Ameboma, a granulomatous lesion may form in the cecal or rectosigmoid areas of the colon in some patients. These lesions resemble an adenocarcinoma of the colon and must be distinguished from them.
Diagnosis of intestinal amebiasis rests on finding either trophozoites in diarrheal stools or cysts in formed stools. Diarrheal stools should be examined within one hour of collection to see the ameboid motility of the trophozoite. The trophozoite characteristically contain ingested red blood cells.
Characteristics of Stool
- Macroscopic appearance of stool: Offensive dark brown semisolid stool, acid in reaction, admixed with blood, mucus and much fecal matter.
General microscopic examination:
- Presence of charcot-Leyden crystals. Scanty cellular exudates, and consists of only the nuclear masses (“pyknotic bodies).
- E. histolytica infection is distinguished from bacillary dysentery by the lack of high fever and absence PMN leukocytosis
Laboratory diagnosis methods:
E. histolytica can be distinguished from other amoebas by two major criteria
Nature of the nucleus of the trophozoite.
The E. histolytica nucleus has a small central nucleolus and fine chromatin granules along the border of the nuclear membrane. The nuclei of other amebas are quite different. Note: The trophozoites of Entamobea dispar, a nonpathogenic species of Entamoeba, are morphologically indistinguishable from those of E. histolytica
Cyst size and number of its nuclei.
Mature cysts of E. histolytica are smaller than those of Entamoeba coli and contain four nuclei, where as E. coli cysts have eight nuclei.
B. Antigen detection: detection of E.histolytica antigen in the stool
C. Serologic testing is useful for the diagnosis of invasive amebiasis.
C. Detection of nucleic acid of this protozoan parasite by PCR based assay.