Chilomastix mesnili

Non-pathogenic, cosmopolitan parasite. The vegetative form usually colonizes the large intestine (especially the caecum) but can also live in the small intestine. It multiplies by longitudinal binary fission.


Living trophozoites have a stiff, jerky rotary movement. At room temperature the rotation slows until immobility and the parasite takes on a rounded shape. As it slows down, a spiral groove that crosses the center of the body can be seen.

Size: 12-20 μm long, 5-6 μm wide.

Morphology: pear-shaped (piriform), with only one nucleus in the anterior, wider, rounded end that is not visible in fresh specimens. In stained smears (trichrome, hematoxylin) the nucleus displays a small karyosome located centrally or against the nuclear membrane; peripheral chromatin is granular and can be evenly or irregularly distributed over the nuclear membrane. A deep groove is located at the anterior end, near the nucleus: this is the cytostome (the trophozoite mouth), which may extend as far as half way across the trophozoite body and is bordered by curved, shepherds-crook fibrils. The cytostome can often be seen in unstained smears. Many food vacuoles are located within the cytoplasm. In stained preparations, the trophozoite may appear rounded when viewed transversally and, hence, can be confused with an amoebic cyst.

Flagella: four. Three arise from a group of blepharoplasts located in front of the nucleus and are anteriorly directed. A fourth, short and thick flagellum is located inside the cytostome.


This is the infective stage.

Size: 6-10 μm (usual range, 7-9 μm).

Morphology: usually lemon-shaped, with an anterior knob, but often rounded if viewed transversally. Cysts from fresh samples are colorless, refractile, with a thick, clear cell wall; sometimes a nucleus can be seen whose diameter can be as large as half of the entire cyst. In formalin-fixed, stained wet mounts (and especially in permanent stained smears), the cytostome can be seen, displaying a typical “open safety pin” (shepherd’s crook) feature.