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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.
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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
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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
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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 ...