Retortamonas intestinalis

Cosmopolitan parasite for which a possible pathogenic role is still under investigation. The vegetative form colonizes the colon, and multiplies by longitudinal binary fission.


The active trophozoite moves jerkily both clockwise and anticlockwise. In watery stools this motility can last for a long time; as time passes, the trophozoite immobilizes and takes on a rounded shape.

Size: 5-12 μm long, 3-5 μm wide.

Morphology: when viewed laterally, it has a bird-shaped profile, but the usually observed forms are pear-shaped (piriform) or oval. In fixed specimens, it generally has a round shape. The nucleus is located at the anterior end but is often not visible; it has a prominent karyosome and a nuclear membrane coated with a thin layer of peripheral chromatin. The cytostome, located near the nucleus, is small, not very deep and barely visible.

Flagella: two, one being shorter and thicker; they are located at the anterior end, anteriorly directed; they arise from two blepharoplasts. They can be seen in stained wet mounts or better in smears stained with Giemsa, May-Grünwald-Giemsa or Field stains; with trichrome or hematoxylin, they rarely stain and, even then, only faintly.


This is the infective stage.

Size: 4-7 μm long, 3-4 μm wide. These are the smallest cysts of flagellate protozoan found in man.

Morphology: oval or pear-shaped (piriform), similar to cysts of Chilomastix mesnili. From fresh samples, the cysts are transparent, refractile, with a relatively thick cell wall. In stained wet mounts or permanent stained smears, it is possible to see an elongated nucleus wrapped in a U-shaped flagellum.