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Dientamoeba fragilis is
a nonflagellate trichomonad parasite that inhabits the human colon
and has been associated with acute and chronic gastrointestinal symptoms.
Humans are likely the natural host for D fragilis.
Unlike most other intestinal protozoa, D fragilis has
no known cyst form and has not been found to invade tissues.1 The
organism is usually 7 to 12 μm in diameter and
demonstrates pointed or leaf-shaped pseudopodia (see Fig.
347-1). While moving actively in fresh fecal specimens, the
organism quickly becomes rounded and granular and is therefore less
identifiable in stored specimens. When suitably stained, most D
fragilis reveal two characteristic nuclei that each contains
a large karysome with granules. Some large uninucleate forms may
also be found. The organism feeds on bacteria and will ingest human
red blood cells.1 Advances in culturing techniques have
improved the ability to detect the organism2-4 but
are limited, since cultures cannot be done on fixed fecal samples. Because
of its small size, this organism may be overlooked by inexperienced laboratory
personnel or can be lost if not preserved properly.
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The mode of transmission of D fragilis is unknown;
however, several investigators have noted a high frequency of concomitant
infection with D fragilis and the pinworm Enterobius
vermicularis.1,4 It is thought that this
organism infects the human host by entering and passing in pinworm
eggs. Amoeboid bodies resembling D fragilis have
been described in eggs of E vermicularis.5 However,
attempts to culture D fragilis from pinworm eggs
or larvae have been consistently unsuccessful, and polymerase chain reaction
of nucleic acid from E vermicularis eggs did not
identify Dientamoeba DNA in coinfected individuals.6 Other
investigators have postulated that transmission occurs by the fecal-oral
route with other organisms.1
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D fragilis has been found in most parts of the
world. Prevalence varies widely from 1.4% to 38% in
selected populations.1In Manitoba, Canada, D
fragilis was second only to Blastocystis hominis on
stool examinations for parasites and was far more common than Giardia,Cryptosporidium, and
so on.5 Increased prevalence is seen in persons
residing in crowded living conditions, such as those living in institutions
and communes and those traveling outside the United States. A serological
survey has indicated high prevalence of antibodies to D
fragilis in healthy children, suggesting infection occurs
in most children, many during infancy.7
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Organisms infect mucosal crypts of the large intestine, from
the cecum to the rectum.1The parasite is not invasive
and does not cause cellular damage. Colonization may occur without disease
development, especially in adults. Symptoms come from irritation
of the colonic mucosa similar to other parasites. Like Cryptosporidium
parvum, D fragilis can cause disease ...