FLAGELLATES : Subphylum Mastigophora, class Zoomastigophora

flagellate classification
the flagellates belong to the subphylum mastigophora,
class zoomastigophora. like the
amebas, the flagellates may be separated into two
categories, intestinal and extraintestinal. figure
4-1 identifies the species that fall under each
category.
giardia intestinalis
(gee’are-dee’uh/in-tes-ti-nal-is)
common associated disease or condition names:
giardiasis, traveler’s diarrhea.
initially known as cercomonas intestinalis,
this important flagellate was first discovered in
1859 by french scientist dr. f. lambl. in honor
of the significant contributions of both dr. lambl
and czechoslovakian scientist dr. giard to the
field of parasitology, stiles coined the term
giardia lamblia (pronounced lamb-bleé uh) in
1915 (see the notes of interest and new trends
section for additional historical information).
since the term giardia intestinalis is gaining
figure 4-1 parasite classification, the flagellates.
subphylum
mastigophora
class
zoomastigophora
intestinal species
giardia intestinalis
chilomastix mesnili
dientamoeba fragilis
trichomonas hominis
enteromonas hominis
retortamonas intestinalis
extraintestinal species
trichomonas tenax
trichomonas vaginalis
quick quiz! 4-3
the presence of nonpathogenic flagellates is important
because it suggests that: (objective 4-5a)
a. the patient will develop clinical signs and
symptoms.
b. only cyst forms will be recovered in corresponding
patient samples.
c. the parasites will invade multiple organ systems
in the body.
d. contaminated food or drink was consumed by the
patient.
chapter 4 the flagellates 81
popularity (some also consider giardia duodenale
as a synonym), its formal name is currently
under review by the international commission
on zoological nomenclature. for the purposes
of this text, this parasite will be referred to as
giardia intestinalis.
morphology
trophozoites. the typical g. intestinalis trophozoite
ranges from 8 to 20 ?m in length by 5
to 16 ?m in width (figs. 4-2 and 4-3 table 4-1).
the average g. intestinalis trophozoite, however,
measures 10 to 15 ?m long. the g. intestinalis
trophozoite is described as pear or teardroping
shaped. the broad anterior end of the organism
tapers off at the posterior end. the g. intestinalis
trophozoite characteristically exhibits motility
that resembles a falling leaf. the trophozoite is
bilaterally symmetrical, containing two ovoid to
spherical nuclei, each with a large karyosome,
figure 4-2 a, giardia intestinalis trophozoite. b, giardia intestinalis trophozoite. (b from forbes ba, sahm df,
weissfeld as: bailey & scott’s diagnostic microbiology, ed 12, st louis, 2007, mosby.)
median
(parabasal) bodies
flagella
axostyle
axonemes
nuclei
size range: 8-20 m by 5-16 m
average length: 10-15 m
a b
figure 4-3 giardia intestinalis trophozoite. note redstaining
nuclei (trichrome stain, ×1000).
parameter description
size range 8-20 ?m long
5-16 ?m wide
shape pear-shaped, teardroping
motility falling leaf
appearance bilaterally symmetrical
nuclei two ovoid-shaped, each with
a large karyosome
no peripheral chromatin
flagella four pairs, origination of each:
one pair, anterior end
one pair, posterior end
two pair, central, extending
laterally
other structures two median bodies
two axonemes
sucking disk
table 4-1 giardia intestinalis
trophozoite: typical
characteristics at
a glance
82 chapter 4 the flagellates
usually centrally located. peripheral chromatin is
absent. these nuclei are best detected on permanently
stained specimens. the trophozoite is supported
by an axostyle made up of two axonemes,
defined as the interior portions of the flagella.
two slightly curved rodlike structures, known as
median bodies, sit on the axonemes posterior to
the nuclei.
it is important to note that there is some confusion
regarding the proper name of the median
bodies. some texts refer to these structures as
parabasal bodies rather than median bodies,
suggesting that the two structures are different.
other texts consider median bodies and parabasal
bodies as two names for the same structure.
for the purposes of this text, the term median
body is used to define structures believed to be
associated with energy, metabolism, or support.
their exact function is unclear. although they
are sometimes difficult to detect, the typical g.
intestinalis trophozoite has four pairs of flagella.
one pair of flagella originates from the anterior
end and one pair extends from the posterior end.
the remaining two pairs of flagella are located
laterally, extending from the axonemes in the
center of the body. the g. intestinalis trophozoite
is equipped with a sucking disc. covering
50% to 75% of the ventral surface, the sucking
disk serves as the nourishment point of entry by
attaching to the intestinal villi of an infected
human.
figure 4-4 a, giardia intestinalis cyst. b, giardia intestinalis cyst. (b from forbes ba, sahm df, weissfeld as: bailey &
scott’s diagnostic microbiology, ed 12, st louis, 2007, mosby.)
nuclei
cyst wall
cytoplasm
beginning to
retract from
cyst wall
median (parabasal) bodies
size range: 8-17 m by 6-10 m
average length: 10-12 m
a b
figure 4-5 giardia intestinalis cyst. note red-staining
nuclei (trichrome stain, ×1000).
cysts. the typical ovoid g. intestinalis cyst
ranges in size from 8 to 17 ?m long by 6 to
10 ?m wide, with an average length of 10 to
12 ?m (figs. 4-4 and 4-5 table 4-2). the colorless
and smooth cyst wall is prominent and distinct
from the interior of the organism. the
cytoplasm is often retracted away from the cyst
wall, creating a clearing zone. this phenomenon
is especially possible after being preserved in formalin.
the immature cyst contains two nuclei
and two median bodies. four nuclei, which may
be seen in iodine wet preparations as well as
on permanent stains, and four median bodies
are present in the fully mature cysts. mature
chapter 4 the flagellates 83
cysts contain twice as many interior flagellar
structures.
laboratory diagnosis
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 (rtpcr).
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.