Balantidiasis — DTM&H MCQ

Correct answer: B — Tetracycline

The correct answer is B, Tetracycline. This vignette describes balantidiasis caused by Balantidium coli, the largest protozoan parasite of humans, which is zoonotically acquired from pigs and produces a trophozoite with a characteristic kidney-shaped macronucleus on stool microscopy. Chronic or intermittent bloody diarrhoea in a pig farmer with this microscopy finding is the classic discriminator pointing to Balantidium coli rather than to Entamoeba histolytica or helminth infection. Tetracycline is the first-line agent because it reliably clears the ciliate from the colonic mucosa and is used in the standard treatment regimens for this organism. Metronidazole is a recognised alternative but is second-line, reinforcing that tetracycline is the preferred single best answer here. Why the other options are wrong: E. Chloroquine: this is an antimalarial with no activity against ciliate protozoa, and there is no malaria diagnosis in this stem. C. Albendazole: this is a benzimidazole anthelmintic effective against nematodes and some cestodes, not against ciliated protozoa such as Balantidium coli. A. Praziquantel: this treats trematode and cestode infections (e.g. schistosomiasis, tapeworms) by disrupting their tegument, a mechanism irrelevant to a protozoan ciliate. D. Metronidazole gel: topical metronidazole gel is used for conditions like bacterial vaginosis or rosacea; it has no role in treating a systemic bowel protozoal infection, and even systemic metronidazole is only second-line here. Key point: pig exposure plus a large ciliated protozoan with a kidney-shaped macronucleus on stool microscopy signals Balantidium coli, for which tetracycline is the first-line treatment.

Reference: CDC, Clinical Treatment of Balantidium coli infection (parasite guidance aligned with UK DTMH/Manson's Tropical Diseases teaching), https://www.cdc.gov/balantidium/hcp/clinical-care/index.html