Enteromonas hominis

Non-pathogenic, cosmopolitan parasite.

Trophozoite

The active vegetative form moves rapidly and jerkily.

Size: 3-6 μm. The smallest intestinal flagellate.

Morphology: pear-shaped (piriform) or oval when motile but rounded in fixed specimens. The nucleus is small (~1 μm), anteriorly located, with a central karyosome and a thin nuclear membrane. The cytostome is absent.

Flagella: the flagella originate from a group of blepharoplasts located in front of the nucleus. According to O. da Fonseca’s original description (1915), two flagella are anteriorly located: one is linear, while the other is doubled for two-thirds of its length. However, this doubled flagellum is rarely visible and, generally (according to Wenyon and O'Connor, 1917), three free flagella are seen anteriorly. A fourth flagellum is adjacent to the body of the trophozoite and extends beyond the posterior part, taking on the appearance of a free flagellum. In wet preparations of fresh stool, flagella are difficult to see, while they stain well with Giemsa’s, May-Grünwald-Giemsa’s or Field’s stains.

Cyst

This represents the infective stage.

Size: 6-8 μm long, 3-4 μm wide.

Morphology: oval or elliptical; it can be mistaken for Endolimax nana because it displays similar features and size. Due to its thin, barely refractile cyst wall can be difficult to see the parasite in direct mount examination. The cyst has 1-4 nuclei that can be seen in both permanent stained smears (e.g. treated with trichrome or hematoxylin) and in wet mounts stained with Lugol's iodine solution, MIF solution, or Bailenger's stain). In the most common binucleate cysts, the nuclei are located at opposite ends; in the less frequent tetranucleate cysts, a pair of nuclei is located at each end.

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