Page 60 - parasitology for medical and clinical laboratoryprofessionals

40 CHAPTER 3

been documented to be as high as 25 percent or more of Trichomonas hominis, although there are others that may
the general population. This organism is thought to be be found in both cystic and trophozoitic forms. Also
primarily contracted from pigs which appear to be the remember that these organisms may be characterized by
natural host for B. coli along with perhaps a few other exhibiting an undulating membrane, as previously listed
species of animals, and humans become infected through in Table 2-1.
contact with these animals. But there has also been some
evidence of human-to-human transmission of the infec-
tion, which might allow for the 25 percent rate of infec- DIENTAMOEBA FRAGILIS
tion in areas where close contact with pigs is common. The organism Dientamoeba fragilis does not have
The organism affects primarily the large intestine, where external flagella that are prominent as do other flagel-
it is known to cause lesions in the mucosal membranes. lates. It is considered a flagellate because it retains flagel-
lar characteristics which could be considered vestigial
Laboratory Diagnosis morphology where the flagellae have been lost. D. fragilis
is considered to be more akin to the trichomonad, a
A wet mount of the organism B. coli, viewed micro- true flagellated organism, due to the type of motility it is
scopically, reveals cilia that may be seen where they affect capable of exhibiting. Although the organism primarily
rotary motion of the parasite. A permanent slide of a moves in amoeboid fashion by extending pseudopodia
stained fecal specimen or material obtained by sigmoido- with somewhat serrated (sharp-toothed) edges, D. fragilis
scopic examination (endoscope inserted into the bowel) also frequently exhibits active motility resembling that
enables identification by the large size and surrounding of other flagellates. Therefore it has been suggested that
cilia in the trophozoite stage. In the cyst stage, the two D. fragilis is an amoeba of flagellate ancestry, as the nuclear
wall layers are ciliated but are difficult to visualize. structure of the organism is more similar to that of flagel-
lated trichomonads than to other species of Entamoeba.
Treatment and Prevention As mentioned previously, there arguably appears to be
no cyst stage that provides protection for the organism
Consistency in performing personal hygiene and man- against the environment, so the trophozoite does not sur-
aging sanitary conditions are effective in preventing vive for long outside the body, preventing easy transmis-
infection with B. coli. Caution should be exercised when sion directly from one host to another (see “Life Cycle”).
handling monkeys or working with them in zoos and in
research laboratories where these animals may be uti- Morphology
lized, although it has been theorized that there may be
a cyst stage that is implicated in fecal-oral transmission. D. fragilis is a single-celled parasite that infects the gas-
But pigs and the raising and processing of them lead to trointestinal tract of humans. In the binucleate form of
the greatest number of infections. Antibiotics including the organism there is a spindle structure located between
metronidazole, iodoquinol, and tetracycline should be the two nuclei, which stems from certain polar configura-
effective in resolving the infection. tions adjacent to a nucleus. A complex Golgi apparatus,
a parallel series of flattened saclike structures, is seen best
with an electron microscope.
PATHOGENIC FLAGELLATES
OF THE UROGENITAL AND Symptoms
INTESTINAL SYSTEMS
The Dientamoeba fragilis organism, not always con-
Another major group of parasites with medical impor- sidered a pathogen, was originally considered to be an
tance that inhabit the intestinal tract are the flagellates. amoeba rather than a flagellate (Figure 3-3). However,
These organisms have a rather simple life cycle, and at gastrointestinal symptoms of pain, diarrhea, and tender-
least three of the following four species do not have a cyst ness are present in conjunction with the finding of this
stage but only a trophozoite stage, during which unlike organism in the stool. Some persons may exhibit mild
many protozoa they are infective. These are Dientamoeba diarrhea, abdominal pain, and flatulence, as well as nau-
fragilis, Trichomonas vaginalis, Trichomonas tenax, and sea and fatigue, whereas others may experience rather