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74 CHAPTER 3

SUMMARY

Protozoa are among the most prevalent of parasites, but frequently leads to a heavy infestation only days after
unfortunately are also among the most difficult to find infection.
microscopically. Protozoa are typically spread by the Intestinal amoebae may lead to serious infection,
fecal-oral route, meaning that food contaminated by feces although some cases cause chronic infection with no
may find its way onto the kitchen counter and table, and damage occurring. However, some victims progress to
may result in an infection. amoebic colitis or fulminant colitis. Entamoeba histoly-
Two major groups of protozoa occur as human tica is pathogenic, and may invade the intestinal walls,
pathogens. These are called ciliates and flagellates, and whereas Entamoeba dispar, as well as Entamoeba coli,
their chief difference lies in their locomotion structures. are considered nonpathogenic. To diagnose E. histoly-
Balantidium coli is an example of a ciliate that is trans- tica, a fresh fecal sample or a swab from a rectal ulcer is
mitted from monkeys and pigs, for the most part, to examined under the microscope. The movements of the
human hosts. Primarily pigs, the natural host for Balan- motile forms become perceptibly slower as the specimen
tidium coli, are responsible for the majority of the cases cools, making it more difficult to identify the parasites.
in humans of this disease, but there have been at least Therefore, careful differentiation is required to distin-
a few instances of transmission from human to human. guish between E. histolytica and the other nonpatho-
These organisms are quite large and have short hairy genic forms of intestinal amoebae.
cilia surrounding the body, making the organism appear Blastocystis hominis was formerly thought to be a
to have a rotary motion. yeast, and is prevalent in many animals, birds, and insects,
Four major organisms of the flagellate type have but now is known to be neither a yeast nor a protozoan.
several long hairlike projections that are used by tails Constipation, diarrhea, and abdominal distress are the
for swimming. They also exhibit a phenomenon called predominant symptoms and signs of the infections by
an undulating membrane, which makes an organism’s most strains. Almost all humans and animals have had
exterior morphological structures appear to be rising contact with Blastocystis at some point. Four different
and falling rhythmically. These organisms are Trichomo- morphological forms—vacuolar, granular, cyst, and amoe-
nas vaginalis, T. tenax, T. hominis, and Dientamoeba boid—challenge the testing personnel when examining a
fragilis. At least three of these have no cyst stage, so these stool specimen. It is thought that all of these forms may
organisms have an extremely simple life cycle. D. fragilis, be present in the host simultaneously The cyst forms of
originally not considered a pathogen, is now, as symptoms this organism are able to survive in harsh environmental
subside upon treatment. conditions due to its thick, multi-layered cyst wall.
T. vaginalis is a common sexually transmitted Other considerable numbers of amoebae are pres-
organism, and may be found in both males and females. ent in human stools, and are capable of causing pain and
This organism is often confused with white blood cells if other symptoms and signs of distress. Most of these are
movement is not present. T. tenax, found in the mouth, identified chiefly by size and the numbers of nuclei they
and T. hominis, found in the intestine, are considered possess. Some of these miscellaneous amoebae are harm-
harmless as a parasite of man. Chilomastix mesnili is a less when colonizing the intestine, whereas others cause
pear-shaped parasite that like T. hominis is considered mild to considerable abdominal discomfort. E. histoly-
nonpathogenic. tica is the primary disease-causing species of this sort,
Infections with Giardia lamblia are common and often other species are confused with E. histolytica.
throughout the world and are perhaps the most fre- Another challenge for the microbiologist or parasitologist
quently identified intestinal parasite. G. lamblia is trans- is that of differentiating between bacterial and amoebic
mitted mainly through the fecal-oral route and is often dysentery. Symptoms may be similar, but the treatment is
found in contaminated well water and streams. Often the completely different. Diagnosis is mostly made through
infection is accompanied by diarrhea and intestinal pain, clinical signs and symptoms, but dysentery must be
but may be asymptomatic. Sanitary life styles will largely confirmed by microscopic examination. Other medical
prevent infection with G. lamblia. Rapid replication that conditions must also be ruled out to avoid misdiagnosis
occurs due to the binucleate morphology of G. lamblia and wrongful treatment.