Chapter 343

Balantidium coli is the largest protozoan parasite and the only ciliate known to cause infection in humans. B. coli is a zoonosis and has been found in pigs, rodents, cattle, reptiles, birds, fishes, annelids, arthropods, and many simian hosts. Pigs are typically the source of human infections, although species-to-species transmission requires adaptation of the parasite. The host’s health can be a factor, since individuals who are malnourished or suffering from concurrent infections are at greater risk of developing balantidiasis. Once a porcine strain becomes established in the human intestine, the infection may spread from human-to-human, and this may account for the occasional reported epidemics.

The genus Balantidium belongs to the phylum Ciliophora, class Litostomatea, order Vestibuliferida, and family Balantidiidae.1 Many species have been described on the basis of morphological features or the host in which they were identified; however, the validity of these species is unconfirmed.The parasite has two stages in its life cycle: the trophozoite and the cyst. The cyst is the resting, resistant stage and is essential for transmission, since the trophozoite cannot survive passage through the stomach’s low pH. The cysts are spherical or ovoid with a diameter of 40 to 60 μm and remain viable at room temperature for at least 2 weeks, particularly if kept moist and away from direct sunlight.2

Cysts are formed in the lumen of the colon or in freshly evacuated stools. Nuclear division does not occur in the cyst; therefore, only one parasite is obtained on excystation. The motile trophozoite is the form for division. Its shape and size vary with the amount of ingested food, from 30 to 300 μm in length and 30 to 100 μm in width. The surface is covered by longitudinal rows of cilia that function as the organs of locomotion and give the trophozoite its characteristic morphology.2 The anterior end contains the cell mouth (cytostome) through which debris, bacteria, and other particulate material are ingested and pass into food vacuoles. The cytoplasm contains a large macronucleus, a micronucleus, several digestive vacuoles, and two contractile vacuoles that serve as osmoregulatory organelles. The macronucleus is bean shaped, and the micronucleus is spherical and generally lies in the concavity of the macronucleus. The posterior end is rounded and contains the anal pore (cytopyge), through which the residual contents of food vacuoles empty. Reproduction occurs by transverse binary fission or, less commonly, by budding.2 In addition, a sexual event (ie, conjugation) has been observed in cultures. The parasite can be grown in vitro in xenic cultures, at reference or research laboratories.1

Balantidiasis is a disease of tropical and subtropical regions and is a reflection of poor sanitation and inadequate protection of the water supply from sewage contamination. Domestic and wild swine represent a reservoir for human infections. Several studies report high incidence rates (47%) in intensive pig farms3 and up to 100% in research farms.4 Infections occur when fecal ...

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