Parasitology Training Manual

Introduction to the Entamoeba species


The following amoeba will be studied here:

Entamoeba histolytica/dispar
Entamoeba coli
Entamoeba hartmanni
Entamoeba polecki

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Overview

Differentiation between the Entamoeba species is based on a number of factors and characteristics. Size, number of nuclei and the general morphology of the nuclei i.e. peripheral chromatin and density of the karyosome are the most important factors. Other characteristics such as motility are useful but impractical as most stool specimens are received fixed. An identification should not be based on only one characteristic, but on that of "the whole picture".

At the present time there is only one of the Entamoeba species that is considered pathogenic. Entamoeba histolytica, is the causative agent of amebic dysentery and amebic liver abscess. Entamoeba dispar morphologically similar to E. histolytica is considered non-pathogenic at this time. Serology or molecular biology is required to differentiate between the two species. 16

You may view a chart that outlines the differences between the Entamoeba species.

Note:

Finding of any other Entamoeba species organisms should be considered an indicator of fecal contamination.

Some literature recommends a series of 3 stools in SAF be examined to diagnose amebiasis.(10)(1)(11)(13)(18) Recently literature is recommending only 1 specimen be examined initially. Depending on the result of the testing and the condition of the patient(symptomology) examination of more specimens may be suggested.(12)(14)(17)

Entamoeba species Life Cycle

Entamoeba species exist in only two forms. The non-infective trophozoite stage and the ineffective cyst stage. Ineffective cysts are ingested by man. Some time after passing through the stomach and small bowel they excyst to form motile trophozoites. It is these trophozoites of E. histolytica that can penetrate, invade and colonize intestinal mucosa. As the trophozoite travels down the large bowel at some point when conditions are right(not certain what those conditions are at present) it encysts into the ineffective cyst stage and is excreted with the feces.

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