.

Geographic distribution. Worldwide.

Infection rate. About 6% of the world population

Life cycle. This parasite lives in the cecum and colon of humans, chimpanzees, orangutans, monkeys, and pigs. The trophozoites mutiply by binary fission. Transmission occurs by ingestion of cysts. Fecal contamination of drinking water is major route of transmission.

Morphology. The pear-shaped trophozoites measure 6-24 ㎛ in length by 3-10 ㎛ in width. Three flagella are extended from the anterior end. A spiral groove crosses over the middle half of the body. A slit-like cytosome, enclosing a fourth flagellum, is located in the anterior portion of the body. A nucleus with central karyosome is located near the front end. The lemon-shaped cysts measure 6-10 ㎛ in diameter. The cyst wall is relatively thick. It has hyaline protrusion. The single nucleus and the cytostome remain in the cysts.

Pathology and clinical symptoms. Usually considered to be nonpathogenic but may cause intestinal disorders like diarrhea in heavy infected cases.

Diagnosis. Detection of cysts or trophozoites in the stool.

Prevention. Chlorization and filtration of drinking water.

Comments. The structures described in the morphology section are not easily recognizable. Both optimally stained smear and experienced technician are needed for the accurate diagnosis.

Cyst of Chilomastix mesnili showing lemon shape with a single nucleus and curved cytostomal fibril (I-H stain, 1000x).