Entamoeba polecki

Cosmopolitan amoeba, commensal in pigs and monkeys. Humans are probably infected by accidental ingestion of cysts in the stool of these animals   In fact, infection with Entamoeba polecki is more common in breeders and butchers than in the general population.  In countries with a high prevalence of infection and where man lives in close contact with pigs (for example, Papua New Guinea), interhuman transmission is possible.

Trophozoite

Size: 10 to 25μm; usual range, 12-18μm.

Motility: similar to that of Entamoeba coli.

Nucleus: rarely visible in fresh specimens. In fixed, stained specimens, the peripheral chromatin is often thin and evenly distributed, but it may also appear in the form of more or less regular granules. The karyosome, in central or eccentric position, is of variable size. The general appearance of the nucleus is similar to that of Entamoeba histolytica and hence the identification of Entamoeba polecki is possible only when typical cysts are seen (see below).

Cytoplasm: foamy, due to numerous food vacuoles (even of large dimensions) containing coarse residues, like those of Entamoeba coli.

In feces, trophozoites are less frequently seen than cysts.

Cyst

Size: 10 to 24 μm; usual range, 10-14 μm.

Shape: round or oval.

Nucleus: one or, very rarely, two. The diameter of the nucleus ranges from 25% to 30% of that of the cyst.  This characteristic facilitates the distinction of Entamoeba polecki cysts from mononucleate cysts of Entamoeba histolytica, whose nuclear diameter is 40%-50% of that of the entire cyst.

-     Karyosome and peripheral chromatin: similar to those of the trophozoite nucleus.

Some little vacuoles that are only poorly stained by iodine solution can be seen in the cyst.

Chromatoid bodies: present in most cysts and in great number (up 30 or more). Variable in shape and size: round, oval, elongated.  Can be clearly seen with Sargeaunt's stain, Bailenger's stain or permanent stains.

Inclusion mass: this is an important diagnostic element, albeit of as yet unknown nature. It can be round or of irregular shape, and has a variable size.  It does not contain glycogen and therefore cannot be stained with Lugol's iodine solution or MIF.  With trichrome staining, it takes on a variable color (from grayish purple to green), while with hematoxylin staining it becomes grayish.

Images