Genus Balantidium

All species of this genus are obligate parasites. Balantidiasis is a zoonosis acquired in man via water or food contaminated with cysts. Pigs are natural reservoirs of the infection but inter-human infection is also possible. The only ciliate species of human interest is Balantidium coli.

Balantidium coli

Cosmopolitan parasite, reported to be more common in areas where humans live in close contact with pigs. This is the only ciliate pathogenic to humans. Prevalence of infection with Balantidium coli varies according to the geographical area: it is a real problem in developing countries, where water is often contaminated by human or pig feces containing Balantidium coli cysts. In both swine and man, the infection may be latent and asymptomatic or invasive and symptomatic; in the latter case it may develop into dysentery similar to that caused by Entamoeba histolytica.


Vegetative forms that are present in stool can survive for up to 10 days at room temperature. Therefore, in fresh samples, it is easy to observe Balantidium coli at low magnification (10x objective), which is recognized for both a rotational movement and a rapid, erratic translational movement (videoclip).

The parasite lives in the lumen of the large intestine (colon, caecum) where it reproduces asexually by binary fission. Cell division begins with the formation of a transverse furrow in the mother cell and the subsequent formation of two asymmetrical daughter cells, one anterior (proter) and one posterior (opisthe). The cytostome of the mother cell is retained in the anterior cell, while in the posterior one a new cytostome is formed. There have been reports that Balantidium coli also reproduces sexually by conjugation, but details about the nuclear division are lacking.

Size: 30-200 μm long, 40-70 μm wide.

Morphology: oval. The whole surface of the trophozoite is dotted with countless cilia that, with their synchronous movement, ensure motility. In the tapered anterior end, a funnel-shaped hollow (cytostome) is visible in a sub-terminal position, lined with cilia; in the posterior end, a small groove can be seen which represents the cytopyge (anus). The cytoplasm contains a kidney-shaped macronucleus and a roundish micronucleus, positioned close to the concave part of the macronucleus and often hidden by it. The cytoplasm also contains numerous food vacuoles with granules of starch, cellular debris, bacteria, erythrocytes, etc. Osmotic pressure of the parasite is regulated by two contractile vacuoles through the cytopyge. The undigested food residues are eliminated through the cytopyge.


This is the infectious stage of the parasite.

Size: 50-70 μm diameter.

Morphology: roundish or slightly oval. Encystment is never seen in cultures. Precystic forms may be observed in fresh unpreserved specimens maintained at room temperature.

Precysts have a very thin wall and contain a trophozoite that has lost its typical shape and that quickly rotates on itself (videoclip). The parasite does not fill the cyst completely; a clear space containing granulations can be seen. The cytostome is visible only in some rotational positions, while the macronucleus and micronucleus are almost never seen. In fixed specimens, cysts have a thick wall that can appear with two layers and are completely filled by the parasite; in recently formed cysts, numerous cilia covering the organism can be seen by focusing up and down over the specimen. The macronucleus and especially the micronucleus are not always clearly visible, nor are the contractile vacuoles. Very old cysts may display a granular appearance.

Wet-mount examination by phase contrast microscopy is the most suitable method to visualize the inner structures of both cysts and trophozoites. Stains suitable for use on wet mounts should be used in small quantities or diluted, because they quickly accumulate in the cytoplasm, darkening the inner structures. The same problem is encountered with permanent stains even if, with hematoxylin, it is possible to monitor the degree of destaining by microscopic examination until an optimum result is achieved. Concentration methods by sedimentation or flotation can also be used in the search for Balantidium coli cysts.


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