SN5 SHORT NOTE 5M ★ High-yield

Describe Balantidium coli and the non-pathogenic intestinal protozoa.

Microbiology · 2nd Professional — Para-Clinical

Balantidium Coli

The largest protozoan parasite of man and the only ciliate that causes human disease
Trophozoite — 50 to 200 micrometres, oval, covered in cilia giving rapid rotatory motility, with a cytostome (mouth), a large kidney-shaped macronucleus and a small micronucleus, and two contractile vacuoles
Cyst — the infective form, spherical, thick-walled
The pig is the natural reservoir, so the disease is a zoonosis and occurs in pig handlers, abattoir workers and communities living close to pigs — which includes parts of India and the north-east
Pathology — it invades the colonic mucosa with the aid of hyaluronidase, producing ulcers that closely resemble the flask-shaped ulcers of amoebiasis
Clinical features — most infections are asymptomatic; symptomatic disease gives dysentery with blood and mucus, indistinguishable clinically from amoebiasis. Perforation and, rarely, extraintestinal spread may occur
Diagnosis — the trophozoite is so large and so actively motile that it is readily seen in a wet mount of fresh stool; excretion is intermittent, so repeated specimens are needed
Treatment — tetracycline is the drug of choice, with metronidazole as an alternative

CLINICAL PEARL

Balantidiasis looks exactly like amoebiasis until you look down the microscope. The dysentery is similar and the ulcers are similar — but Balantidium is enormous, ciliated and unmistakable in a wet mount, and the treatment is tetracycline rather than metronidazole. The clue in the history is contact with pigs, which should prompt the request and the stool examination.

Non-pathogenic Intestinal Protozoa

Organism	Distinguishing features	Why it matters
Entamoeba dispar and E. Moshkovskii	Morphologically identical to E. Histolytica; distinguished only by antigen detection or PCR	The most important non-pathogen — E. Dispar is about 10 times commoner than E. Histolytica, so most "amoebic cysts" reported on microscopy represent harmless infection and do not warrant treatment
Entamoeba coli	Larger; a cyst with up to eight nuclei when mature; an eccentric karyosome and coarse irregular peripheral chromatin; chromatoid bars with splintered ends; sluggish non-directional motility and it does not ingest red cells	Commonly reported and regularly mistaken for E. Histolytica, leading to unnecessary treatment
Entamoeba hartmanni	Resembles E. Histolytica but is smaller (under 10 micrometres); careful measurement distinguishes it	Non-pathogenic
Endolimax nana, Iodamoeba butschlii	Small; Iodamoeba has a large iodine-staining glycogen vacuole	Non-pathogenic commensals
Chilomastix mesnili	A flagellate with a lemon-shaped cyst and a spiral groove	Non-pathogenic; confused with Giardia
Blastocystis hominis	Vacuolated forms of variable size	Pathogenicity remains disputed; commonly found in both symptomatic and asymptomatic people, and treatment is controversial
Dientamoeba fragilis	A flagellate without flagella, usually binucleate; NO cyst stage; may be transmitted within pinworm eggs	Increasingly accepted as a genuine cause of chronic diarrhoea and abdominal pain, and easily missed on routine microscopy

Applied Aspects

Do not treat "amoebic cysts" reported on stool microscopy without further evidence; most represent E. Dispar, and reflexive treatment of every positive report is a major source of unnecessary metronidazole use in India
Nuclear detail and chromatoid body shape are what distinguish the Entamoeba species, and a report should describe them rather than simply saying "cysts seen"
Erythrophagocytosis in a trophozoite is the one reliable morphological marker of invasive E. Histolytica, and should be looked for and reported specifically
Ask about pig contact in dysentery that does not respond to metronidazole, and request a wet mount looking for Balantidium
The finding of a non-pathogenic protozoan is still informative, because it proves faecal-oral exposure — and therefore that the patient may also have acquired something that matters
Correlate the laboratory finding with the clinical picture; a parasite in the stool of a patient whose symptoms do not fit is more likely a passenger than the cause, and continuing to search may be more useful than treating it

All questions in Parasitology I — Protozoa

Every Microbiology chapter →