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Entamoeba histolytica: Life Cycle, Diseases and laboratory diagnosis

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). 

Quadrinucleate cyst of Entamoeba histolytica

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

Life Cycle of Entamoeba histolytica
(Source: CDC)

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;

Flask Shaped Ulcer (Histopathology, UFPA, Araujo R.)

Diseases

  1. Non-invasive infection: In many cases, the trophozoites remain confined to the intestinal lumen of individuals who are thus asymptomatic carriers and cysts passers.
  2. Intestinal disease: In some patients the trophozoites invade the intestinal mucosa,
  3. Extra-intestinal disease : through the bloodstream, trophozoites invade extraintestinal sites such as the liver, brain, and lungs, with resultant pathologic manifestations.

Amoebic liver abscess

Amoebic liver abscess

Clinical Findings

Laboratory diagnosis

E.histolytica with ingested RBCs

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

Laboratory diagnosis methods: 

A. Microscopy: 

E. histolytica can be distinguished from other amoebas by two major criteria

  1. 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
  2. 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.