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Entamoeba polecki (Nonpathogen)

Organism:
This organism belongs to the amebae, is a nonpathogen, and causes no disease.  Both the trophozoite (usual size, 10-12 µm) and cyst forms (usual size, 5-11 µm) can be found in clinical specimens.

Few cases are reported, possibly in part because the organism resembles E. histolytica/E. dispar, E. moshkovskii and E. coli. The trophozoites resemble E. coli in that the cytoplasm is granular, containing ingested bacteria, and the motility tends to be sluggish. The nuclear morphology is almost a composite of those of E. histolytica/E. dispar, E. moshkovskii, and E. coli. Without some of the cyst stages for comparison, it would be very difficult to identify this organism to the species level on the basis of the trophozoite alone.

       
 
E. polecki cysts; note single nucleus and inclusion body

Life Cycle:
Large bowel, organisms passed in feces; common in pigs and monkeys, humans (New Guinea very common)

Acquired:
Fecal-oral transmission via cyst form; contaminated food and water

Epidemiology:
Worldwide, human-to-human transmission or animal to human transmission

Clinical Features:
None

Clinical Specimen:
Intestinal:  Stool

Laboratory Diagnosis:
Intestinal:  Ova and Parasite examination (concentration, permanent stained smear)

Organism Description:
Trophozoite:  Evenly arranged nuclear chromatin, central karyosome, fine cytoplasm (will not contain ingested RBCs); looks like all other Entamoeba species
Cyst:  May contain irregular shaped chromatoidal bars, a single nucleus with an inclusion body.  This mass tends to be round or oval and is not sharply defined on the edges. The material, which is not glycogen, remains on the permanent stained smear and stains less intensely than nuclear material or chromatoidal bars.  Identification to the species level is not possible unless there are cysts present containing the typical inclusion body; chromatoidal bars may or may not be present.

Laboratory Report:
Without confirmation using the specific immunoassay to detect the true pathogen, E. histolytica, the report must indicate:  Entamoeba histolytica/E. dispar group (indicate cysts and/or trophozoites); make sure your clients know what this report means in terms of pathogens/nonpathogen (based on trophozoite morphology).  If cysts are seen containing a single nucleus and the typical inclusion body, then the organism could be identified to the correct species:  Entamoeba polecki

Report Comment:  Submit a fresh stool if you want confirmation of the true pathogen (Entamoeba histolytica).  The laboratory will then test the fresh stool (fresh, frozen, some acceptable in Cary-Blair) for the presence of the true pathogen, Entamoeba histolytica, antigen.  If confirmation of E. histolytica is not performed, then the physician will usually treat if the patient is symptomatic.

Treatment:
None, if it has determined via specific testing that the organism is one of the nonpathogenic Entamoeba spp.
Control:
Improved hygiene, adequate disposal of fecal waste, adequate washing of contaminated fruits and vegetables

Comments:
E. histolytica (true pathogen, cause of amebiasis) cannot be differentiated from the nonpathogen, E. dispar and many of the other Entamoeba spp. (although when trophozoites are found to contain ingested RBCs, it is more likely to be E. histolytica and will be reported as such).  E. dispar, E. moshkovskii, and E. polecki do not normally ingest RBCs.
Entamoeba polecki is found worldwide, and in certain areas of the world, such as Papua, New Guinea, it is the most common intestinal ameba of humans. Few cases are reported, possibly in part because the organism resembles E. histolytica/E. dispar, E. moshkovskii and E. coli.