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: Cyst of E. histolytica/E. dispar in an unstained concentrated wet mount of stool.  Notice the chromatoid body with blunt, rounded ends (arrow).
B: Cyst of E. histolytica/E. dispar in a concentrated wet mount stained with iodine.

C	D

C: Cyst of E. histolytica/E. dispar stained with trichrome.  Three nuclei are visible in the focal plane (black arrows), and the cyst contains a chromatoid body with typically blunted ends (red arrow).
D: Cyst of E. histolytica/E. dispar stained with trichrome.  Notice the chromatoid body with blunt, rounded ends (arrow).

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, F: Trophozoites of E. histolytica/E. dispar in a direct wet mount stained with iodine.

G	H

G: Trophozoite of E. histolytica/E. dispar, 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: Trophozoite of E. histolytica.  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.

J	K

J: Trophozoites of E. histolytica in a colon biopsy specimen, stained with hematoxylin and eosin (H&E).
K: Trophozoites of E. histolytica in a perianal biopsy specimens, stained with H&E.

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