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48 CHAPTER 3

trophozoites feed on bacteria primarily in the cecal por- GIARDIA LAMBLIA
tion of the intestine, but encystation is not known to
occur. Giardiasis in humans is caused by the infection of the
small intestine by a single-celled flagellate called Giardia
lamblia. Giardiasis occurs worldwide and may infect up
Disease Transmission to a third of the population in developing countries. The

These two intestinal organisms, T. hominis and C. mesnili, organism is also found in other mammals, which serves
are considered to be nonpathogenic flagellates that must to make the disease difficult to eradicate. The Centers
be distinguished from pathogenic flagellates. It is often for Disease Control and Prevention (CDCP) estimates as
quite common for T. hominis to be found in the urine many as 2.5 million people annually are infected in the
of females due to the close proximity of the anus and the Unites States alone.
urethra. It is poorly defined as to the transmission but is Giardia infections are frequently referred to as giar-
generally conceded to gain entrance to the human bowel diasis, and is the most common of all intestinal flagellates.
by fecal-oral means. An infection by C. mesnili results in This organism is often found in the upper areas of the
a lack of symptoms and the parasite is more prevalent in small intestine, and is commonly found in isolated areas
tropical climates with warm and humid weather. Trans- throughout the world. Because Giardia is likely the most
mission occurs by ingesting infective cysts from food and common organism isolated from human stool specimens,
water where poor sanitary conditions are found (C. mesnili it is likely that van Leeuwenhoek observed the organ-
cysts and trophozoites are similar in appearance). ism in stool specimens after developing the first known
microscope, but he never published any description or
drawings of the organism. The first known description
Laboratory Diagnosis of G. lamblia occurred in the stool of a pediatric patient
The identification of C. mesnili and T. hominis are in 1859. Giardia infections do not lead to invasive dam-
age to the gastrointestinal tract as is true of some other
performed by microscopic examination. Both stained
and unstained specimens should be prepared for study. pathogens, and the patient may be asymptomatic, but is
able to transmit the organism by the fecal-oral route.
The C. mesnili organism is slightly larger than that of
T. hominis. C. mesnili has a pear-shaped appearance and
the nucleus is surrounded by fibrils that curl around the Morphology
cytostome, often seen near the nucleus, giving a charac-
teristic shepherd’s crook appearance. In the opinion of Both trophozoites and cysts may be found in the stool
most parasitologists and medical professions, neither sample, with cysts found most often in formed stools and
the flagellates T. hominis nor C. mesnili are considered trophozoites in liquid or loose stools. Cysts are the most
pathogenic, but they are included for comparison as they diagnostic stage found in a laboratory sample, perhaps
may be found and cause confusion as to their respective because most stools are formed that reach the laboratory
identities. The nucleus, cytostome, and curved fibrils are for examination are formed.
readily visible on a stained smear. The trophozoites of Trophozoites of G. lamblia are either oval or pear-
C. mesnili are also pear-shaped and measure from 6 to shaped and range from 9 to 21 μm in length and 5 to
24 μm in length and 4 to 8 μm wide. The single nucleus 15 μm in width. The trophozoite form of this organism
usually has a prominent karyosome, whereas the anterior has been likened to a “monkey face” with two nuclei as
flagella are difficult to see. eyes that contain central karyosomes which lack periph-
eral chromatin. The trophozoite stage is bilaterally sym-
Treatment and Prevention metrical with an axostyle evenly dividing the cell down
the middle. Two curved structures, called median bodies,
No treatment is indicated for either C. mesnili or lie parallel to each other and perhaps contribute to the
T. hominis. Because both are transmitted in a similar metabolism of the parasite. These morphological features
fashion and both are considered to be nonpathogenic, cross the axoneme at slight angles, giving the appearance
overall hygiene suffices to prevent the majority of infec- of a smiling mouth. Four lateral flagella along with two
tions by the two organisms. caudal and two lateral flagella extend from the central