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Balantidium coli (also referred to as Balantioides coli) is a large ciliated intestinal protozoan found worldwide, but particularly in the tropics. Pigs are the main reservoir. The infection is rare in humans, and results from ingestion of cysts passed in stools of humans or swine. In the new host, the cyst wall dissolves and the trophozoite may invade intestinal mucosa, causing abscesses and ulcerations. Many infections are asymptomatic. Chronic infections may cause recurrent diarrhea or dysentery. Deaths have occurred from intestinal perforation and hemorrhage.

Dientamoeba fragilis is a protozoan that commonly causes infection without symptoms, so its role in human disease remains somewhat controversial. Symptoms attributed to the pathogen include diarrhea, abdominal pain, and anorexia. Other flagellates may infect humans, are generally considered nonpathogens, but may be markers for fecal-oral contamination.

Diagnosis of balantidiasis is established by finding trophozoites or cysts in stool, or in ulcer scrapings of the large bowel. D fragilis infection can be hard to diagnose because it only exists in the trophozoite form; the sensitivity of evaluation of three stools is 70–90%. The treatment of choice for balantidiasis is tetracycline, 500 mg orally four times daily for 10 days. The alternative is iodoquinol (diiodohydroxyquin), 650 mg orally three times daily for 21 days, and metronidazole, paromomycin, and nitazoxanide have also been used successfully. D fragilis infections have been successfully treated with metronidazole, iodoquinol, paromomycin, and tetracycline, but failures have been reported recently with metronidazole; all the regimens are the same as for balantidiasis.

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Ponce-Gordo  F  et al. Balantioides coli. Res Vet Sci. 2021;135:424.
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Stark  D  et al. Dientamoeba fragilis, the neglected trichomonad of the human bowel. Clin Microbiol Rev. 2016;29:553.
[PubMed: 27170141]  

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