Microscopy

Cysts
Mature Entamoeba histolytica/Entamoeba dispar cysts have 4 nuclei that characteristically have centrally located karyosomes and fine, uniformly distributed peripheral chromatin.  Cysts usually measure 12 to 15 µm.

A	B

A: Line drawing of an E. histolytica/E. dispar cyst.
B: E. histolytica/E. dispar in a concentrated wet mount stained with iodine.  The cysts are usually spherical and often have a halo.  The cyst in A appears uninucleate.

C	D

C, D: E. histolytica/E. dispar cysts stained with trichrome.  Two to three nuclei are visible in the focal plane (black arrows), and the cysts contain chromatoid bodies with typically blunted ends (red arrows).  The chromatoid body in C is particularly well demonstrated.

Trophozoites
Entamoeba histolytica/Entamoeba dispar trophozoites have a single nucleus, which have a centrally placed karyosome and uniformly distributed peripheral chromatin.  This typical appearance of the nucleus is not always observed as some trophozoites can have nuclei with an eccentric karyosome and unevenly distributed peripheral chromatin.  The cytoplasm has a granular or "ground-glass" appearance.  E. histolytica/E. dispar trophozoites usually measure 15 to 20 µm (range 10 to 60 µm), tending to be more elongated in diarrheal stool.

Erythrophagocytosis (ingestion of red blood cells by the parasite) is the only morphologic characteristic that can be used to differentiate E. histolytica from the nonpathogenic E. dispar.  However, erythrophagocytosis is not typically observed on stained smears of E. histolytica.

E	F

E: Line drawing of an E. histolytica/E. dispar trophozoite.
F: E. histolytica/E. dispar trophozoite in a direct wet mount stained with iodine.

G	H

G: E. histolytica/E. dispar trophozoite, measuring approximately 16.7 µm, stained with trichrome.  The image was taken at 1000× magnification and contributed by the Kansas Department of Health and Environment.
H: E. histolytica trophozoite.  The specimen was preserved in poly-vinyl alcohol (PVA) and stained in trichrome.  PCR was performed on this specimen to differentiate between E. histolytica and E. dispar.

I

I: Trophozoite of E. histolytica with ingested erythrocytes stained with trichrome.  The ingested erythrocyte appears as a dark inclusion.  Erythrophagocytosis is the only characteristic that can be used to differentiate morphologically E. histolytica from the nonpathogenic E. dispar.

For images of other intestinal amebae that may be mistaken for E. histolytica/E.dispar, please click here.