[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 4 The Flagellates
83
TABLE 4-2
Parameter
Size range
Shape
Nuclei
Cytoplasm
Other structures
cysts contain twice as many interior flagellar
structures.
Laboratory Diagnosis
Giardia intestinalis Cyst:
Typical Characteristics
at a Glance
Description
8-17 µm long
6-10 µm wide
Ovoid
Immature cyst, two
Mature cyst, four
Central karyosomes
No peripheral chromatin
Retracted from cell wall
Median bodies: two in immature
cyst or four in fully mature cyst
Interior flagellar structures*
*Twice as many in mature cyst as compared with immature cyst.
The specimen of choice for the traditional recovery
technique of G. intestinalis trophozoites and
cysts is stool. It is important to note that Giardia
is often shed in the stool in showers, meaning
that many organisms may be passed and recovered
on one day’s sample and the following
day’s sample may reveal no parasites at all.
Thus, examination of multiple samples is recommended
prior to reporting that a patient is free
of Giardia. Duodenal contents obtained by aspiration,
as well as upper small intestine biopsies,
may also be collected for examination. Duodenal
contents can identify G. intestinalis using
the string test, also known as Enterotest.
Several other diagnostic techniques are available
for identifying G. intestinalis, including
fecal antigen detection by enzyme immunoassays
(EIA) and enzyme-linked immunosorbent assay
(ELISA). Direct Fluorescence detection of both
Giardia and Cryptosporidium (see Chapter 7), as
well as a Giardia Western immunoblotting (blot)
test have shown promising results in recent
studies.
The newest form of identifying Giardia is
using real-time polymerase chain reaction (RT-
PCR). This molecular method is sensitive enough
for environment monitoring because studies
suggest that a single Giardia cyst may be detected
using molecular methods.
Life Cycle Notes
On ingestion, the infective G. intestinalis cysts
enter the stomach. The digestive juices, particularly
gastric acid, stimulate the cysts to
excyst in the duodenum. The resulting trophozoites
become established and multiply approximately
every 8 hours via longitudinal binary
fission. The trophozoites feed by attaching their
sucking disks to the mucosa of the duodenum.
Trophozoites may also infect the common bile
duct and gallbladder. Changes that result in
an unacceptable environment for trophozoite
multiplication stimulate encystation, which
occurs as the trophozoites migrate into the
large bowel. The cysts enter the outside environment
via the feces and may remain viable
for as long as 3 months in water. Trophozoites
entering into the outside environment quickly
disintegrate.
Epidemiology
G. intestinalis may be found worldwide—in
lakes, streams, and other water sources—and are
considered to be one of the most common intestinal
parasites, especially among children. Ingestion
of water contaminated with G. intestinalis
is considered to be the major cause of parasitic
diarrheal outbreaks in the United States. It is
interesting to note that G. intestinalis cysts are
resistant to the routine chlorination procedures
carried out at most water plant facilities. Filtration
as well as chemical treatment of this water
is crucial to obtain adequate drinking water. In
addition to contaminated water, G. intestinalis
may be transmitted by eating contaminated fruits
or vegetables. Person-to-person contact through
oral-anal sexual practices or via the fecal-oral
route may also transfer G. intestinalis.