Phylum Ciliophora. Balantidium coli
Balantidium coli, the only ciliate that infects people and the largest protozoan parasite of humans, how its trophozoite and cyst are built, how it passes from pigs to people, and balantidiasis, from silent carriage to dysentery.
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- Checked 27 Oct 2026
Point at a name or a part to light it.
Balantidium coli is the only ciliate that infects humans, and the largest protozoan parasite of humans. The trophozoite, 40 to 200 µm long, is covered in cilia, with a mouth at its narrower front end, a large bean-shaped macronucleus and a small micronucleus. The cyst, 50 to 70 µm, is round in a double wall and is the infective form.
Simplified: not to scale: the trophozoite drawn large and the cyst smaller beside it; the cilia drawn only round the cell's edge, as if seen in section, and sparser than they are; the cilia's wave played far slower than life, and the cell drawn still rather than spinning; a few food vacuoles drawn; the many small vesicles, mucocysts and the rest of the cytoplasm left out; the cyst drawn with its parts clear to see; in a stool sample its insides are often hard to make out.
Contents4 sections
At a glance
- Balantidium coli is a ciliate, the only one known to infect people, and the largest protozoan parasite of humans. It lives in the large intestine.
- Its trophozoite is an oval cell 40 to 200 µm long, covered in cilia, with a mouth at its narrower front end, a large bean-shaped macronucleus and a small micronucleus. Its cyst, 50 to 70 µm, is round in a double wall.
- Pigs are the main reservoir. People swallow the cysts in water or food contaminated with faeces, most often a pig's.
- Most infections cause no symptoms. When the trophozoites invade the colon's wall they make ulcers, with diarrhoea or a dysentery like amoebic dysentery, which can perforate the colon.
- It is found in fresh stool, spinning on its cilia. Treatment is tetracycline, or metronidazole; prevention is clean water and keeping pig faeces out of it.
Balantidium, the ciliate of the gut
The Ciliophora are protozoa covered in cilia, short hairs that beat in waves and drive the cell along. Most live free in water; Balantidium coli is the only one known to infect people; some recent work renames it Balantioides or Neobalantidium coli, though the new names are not settled. It is also the largest protozoan parasite of humans, and like the amoebae it lives in the large intestine, as a trophozoite that feeds and multiplies and a cyst that passes the infection on.
Ciliates have two kinds of nucleus. The large macronucleus holds many copies of the genes and runs the cell's everyday life. The small micronucleus holds the copy that is passed on: in conjugation, two cells join and exchange material from it, mixing their genes without making new cells.
The trophozoite and the cyst
| Trophozoite | Cyst | |
|---|---|---|
| What it does | lives, feeds and multiplies in the large intestine; can invade the colon's wall | survives outside the body and infects |
| Shape and size | oval, narrower at the front, 40 to 200 µm long | round or slightly oval, 50 to 70 µm, in a double wall |
| Surface | covered in cilia | its wall; inside a young cyst the ciliate can sometimes be seen turning |
| Nuclei | a bean-shaped macronucleus and a small micronucleus | the same two |
The parts to know:
- Cilia: short hairs in rows over the whole surface. They beat in waves, and the cell swims fast, spiralling as it goes. That spinning swim is how it is recognised in a fresh stool.
- Peristome and cytostome: the mouth, at the narrower front end. A groove lined with dense cilia, the peristome, sweeps bacteria and particles down to the cytostome, the cell mouth, where they are taken in as food vacuoles.
- Macronucleus and micronucleus: the macronucleus is large and bean- or sausage-shaped, often visible even without a stain. The micronucleus is small and round, nestled against it and hard to see.
- Vacuoles: two contractile vacuoles fill with water and empty it out of the cell. Food vacuoles hold what it has eaten: bacteria, starch grains and other particles, and red cells when it has damaged the colon's wall.
- Cytopyge: a small opening at the rounded back end, the cell's anus, where undigested remains leave.
The trophozoite multiplies by binary fission, a furrow across it dividing it into a front and a back cell of unequal size. Between divisions it may undergo conjugation, when two cells join at their mouths and exchange material from their micronuclei: a mixing of genes, not a way of multiplying.
The life cycle
Point at a name to light its pathway.
Balantidium coli passes to people as the cyst, swallowed in water or food contaminated with faeces, most often a pig's. In the large intestine the trophozoites multiply and form cysts again. Most infections cause no symptoms; in some the trophozoites invade the wall of the colon and make ulcers.
Simplified: a map, not anatomy: the patient and the pig drawn as icons and the parasites drawn small, nothing to scale; the steps within each stage left out: how the cyst forms and opens, and how conjugation exchanges the micronuclei; the places beyond the colon gathered into one, the appendix, liver, urinary and genital tract and lungs among them; only the pig drawn as a reservoir; people, rodents and monkeys can carry it too, and it can pass from person to person.
Pigs are the main reservoir: they usually carry Balantidium without harm and pass its cysts in their faeces. People are infected by swallowing cysts in water, which carries most infections, or food contaminated with faeces. The cyst opens in the small intestine and the trophozoite moves on to the large intestine, where it lives in the lumen of the colon and the appendix, multiplies, and forms new cysts, which are passed in the faeces. Trophozoites do not survive outside the colon or the stomach's acid, which is why the cyst is the infective form. People can carry it too and pass it to each other, which is how outbreaks start in crowded institutions where there are no pigs. Rodents and monkeys can also be hosts.
From here the infection goes one of three ways. In most people the trophozoites stay in the lumen and cause no symptoms, and these carriers keep passing cysts. In some, the trophozoites invade the colon's wall and multiply there. Rarely, they go through it.
Balantidiasis
Balantidiasis is found worldwide, but it is uncommon: its prevalence is estimated at 0.02 to 1 per cent. It is most common where pigs and people live close together, water is contaminated with their faeces and the climate is warm, as in Latin America, the Philippines, Papua New Guinea and parts of the Middle East. Surveys in New Guinea have found up to 28 per cent of pig farmers infected.
The illness
- Most infections cause no symptoms. The carriers are a reservoir for the people around them.
- Chronic infection: diarrhoea without blood that comes and goes, cramping abdominal pain and bad breath.
- Dysentery: in a fulminant infection the trophozoites dig flask-shaped ulcers into the colon's wall, much like those of Entamoeba histolytica, with diarrhoea that can be explosive, blood and mucus in the stool, tenesmus and weight loss. The colon can bleed heavily or perforate.
- Fulminant balantidiasis has a case fatality of about 30 per cent. It is most likely in people who are malnourished or alcoholic, have little stomach acid or another chronic disease, or whose immunity is weak.
Infection beyond the colon is rare but serious, and usually follows disease of the colon. A perforated colon leads to peritonitis, and a liver abscess can follow. The urinary and genital tract can be reached from the anus or through a fistula, and the appendix can be infected. Rarely, Balantidium reaches the lungs, mostly in people who are old or whose immunity is weak, as in two women reported from Greece, one with anal cancer and one with leukaemia; how it gets there, through the blood or breathed in, is not certain.
Diagnosis
- Microscopy of a fresh stool: the trophozoites, large and spinning, are easy to see in a wet mount even at low power. Trophozoites are found mostly in diarrhoea, where cysts can appear too; formed stools usually hold only cysts.
- It is passed only now and then and dies quickly outside the colon, so several samples are examined over several days, each at once or preserved. A negative sample does not rule it out.
- Stains such as iodine can hide its inner parts, and a heavily stained cyst can be mistaken for a helminth egg.
- In invasive disease, trophozoites are found in biopsies taken at endoscopy.
- Not to be confused with Entamoeba histolytica: Balantidium is far larger, covered in cilia rather than moving by pseudopodia, and has a bean-shaped macronucleus and a micronucleus rather than one to four nuclei alike; its cysts are 50 to 70 µm, an amoeba's 10 to 20 µm.
Treatment and prevention
- First choice: tetracycline, for 10 days. It is not given in pregnancy or to young children, whose teeth it stains.
- Alternatives: metronidazole, for 5 days, the drug for pregnancy and young children; iodoquinol, for 20 days; doxycycline; nitazoxanide. Resistance to these drugs has not been reported.
- A perforated colon needs surgery.
Prevention rests on clean drinking water, since chlorine at the levels used to treat water does not kill the cysts, and on keeping pigs and their faeces away from the streams and wells people drink from. Washing hands after handling pigs, washing raw produce, and safe disposal of human and pig faeces matter too. There is no vaccine.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1Where do most human infections with Balantidium coli come from?
2A fresh, liquid stool shows a large oval cell, about 100 µm long, covered in cilia and spinning as it swims, with a large bean-shaped nucleus. What is it?
3What does the micronucleus of Balantidium do that the macronucleus does not?
4Why may a single stool sample miss Balantidium in a patient with diarrhoea?
5Which drug is a first choice for balantidiasis?