Disease. amebiasis, amebic dysentery, amebic hepatitis

Geographic distribution. Worldwide in tropical and temperate zone

Infection rate. The actual incidence of amebiasis in the worldwide remains unknown. Survey indicate that the infection rates vary from 0.2 to 50%, being directly correlated with sanitary conditions.

Life cycle. Human is the principal host and the source of infection. The infective cysts pass out through the feces, and are immediately infective. Cyst-contaminated food or water cause infections. Excystment takes place in the lower part of the small intestine. The immature amebas move downward to the large intestine, where they establish a site of infection. Reproduction occurs by binary fission.

Morphology. The majority of trophozoites measure from 15 to 30 ㎛. The hyaline ectoplasm, sharply separated from the endoplasm, constitutes about one-third of the entire trophozoite. The thin, fingerlike ectoplasmic pseudopodia are extended rapidly. Hematoxylin staining reveals a nuclear membrane, the inner surface of which is lined with uniform chromatin. The small, deeply staining karyosome is centrally located. The round or oval cysts measure 10 to 20 ㎛ in diameter, and contain one to four nuclei and sausage-shaped chromatoid bodies.

Pathology and clinical symptoms. The lesions produced by E. histolytica are primarily intestinal, and secondarily extraintestinal. The intestinal lesions are confined to the large intestine, frequently cecal and sigmoidorectal regions. The typical flask-like primary ulcers to large necrotic areas are produced. In acute amebiasis, there is severe dysentery with numerous small stools containing blood, mucus and necrotic mucosa accompanied by acute abdominal pain, tenderness and fever. Chronic amebiasis is characterized by recurrent attacks of dysentery with gastrointestinal disturbance. In extraintestinal amebiasis, the liver is invaded chiefly, resulting amebic hepatitis or liver abscess. It is characterized by an enlarged, tender liver, with pain in the upper right hypochondrium. Less frequently, lung abscess, splenic abscess, brain abscess or cutaneous amebic lesions are seen.

Diagnosis. Diagnosis may be made by the microscopic identification of cysts in solid feces or trophozoites in diarrheic stool. The parasite may be detected in aspirated material of liver abscess. Serologic tests, such as IFA, ELISA, for specific antibodies to E. histolytica are very helpful in diagnosis of invasive amebiasis.

Prevention. Environmental sanitation is necessary to prevent water and food contamination. Boiling or filtration of drinking water is a safe and effective way of prevention, and avoidance of consuming contaminated food is essential.

Comments. There is a non-pathogenic E. dispar, having a same morphology with E. histolytica. It can be distinguished by molecular biological techniques such as PCR.

Cyst of Entamoeba histolytica, 5-20 ㎛ in size. Chromatoid bodies are often present with thick rodlike masses. The number of nuclei is 1-4.

Cyst of Entamoeba histolytica showing a large nucleus with centrally located small karyosome (I-H stain, 1000 x).

Trophozoite stage from culture of Entamoeba histolytica. The living trophozoites vary in size from about 10 to 60 um in diameter, depending on their degree of activity and various other conditions. The nucleus is spherical, with a diameter about one-fifth or one-sixth
that of the entire ameba. It contains a small, distinct central karyosome surrounded by an unstained "halo". Iron-Hematoxylin stain x400

Live trophozoites of Entamoeba histolytica. Note the extending pseudopodia from the ectoplasm.

Cultured Entamoeba histolytica trophozoite.

Trophozoite of Entamoeba histolytica. The nucleus is characterized by the presence of a small, compact, centrally located karyosome (I-H stain, 1000 x). B. Cultured trophozoite shows pseudopodia.

Amebic appendicitis in human. H&E, X400.

Amebic liver abscess in human. H&E, X100.

Amebic liver abscess in human. Trophozoites of Entamoeba histolytica are well observed. H&E, X400.

Amebic colon abscess in human. H&E, X40.

Amebic colon abscess in human. H&E, X100.

Amebic colon abscess in human. Trophozoites of Entamoeba histolytica can be distinguished from necrotic tissues. H&E, X400.

Amebic lung abscess in human. H&E, X100.

Amebic lung abscess in human. Trophozoites of Entamoeba histolytica are well observed in necrotic tissues. H&E, X400.

Experimental liver abscess formation following intraperitoneal injection of Korean strain (YS-27) of Entamoeba histolytica in a Mongolian gerbil.
(A) A liver abscess showing an area of necrosis,
(B) An magnified view of necrotic region. Note the presence of Entamoeba histolytica trophozoites.

Entamoeba histolytica in intestine, Trichrome stain(A).

Trophozoite of Entamoeba histolytica in intestine. Nuclei with typical central karyosome are visible (H&E, 1000x).