[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 11 The Trematodes
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appropriate conditions flips open to release its
contents for further development, called an operculum,
such as in Fasciolopsis and Fasciola.
Other members of the trematodes may be distinguished
by the presence and location of spines,
as seen in the Schistosoma spp. The various
larval stages typically occur outside the human
host and are rarely if ever encountered. The
rarely seen adult flukes are thin and nonsegmented,
resembling leaves in shape and thickness.
They typically range in length from 1 to
5 cm. Each adult fluke is equipped with two
muscular, cup-shaped suckers, one oral and the
other located ventrally, a simple digestive system,
and a genital tract. Like the typical cestode, the
average trematode uses its body surface as a
means for absorbing and releasing essential
nutrients and waste products.
Based on the organism’s life cycle, the trematodes
may be placed into two categories, those
that reside in the intestine, bile duct, or lung
(organ-dwelling) and those that reside in the
blood vessels around the intestine and bladder
(blood-dwelling). A brief general description of
each type of life cycle follows. Only the specifics
related to each parasite are discussed on an
individual basis.
The organ-dwelling flukes include all trematodes
except those belonging to the genus Schistosoma.
Human infection of such organ-dwelling
flukes occurs following the ingestion of water
plants (e.g., water chestnuts), fish, crab, or crayfish
contaminated with the encysted form of the
parasite known as metacercaria.
On entrance into the intestinal tract, the
encysted metacercaria excysts and migrates to
the intestine, bile duct, or lung. Development
into the adult stage occurs here. Following selffertilization
(all organ-dwelling flukes are hermaphroditic),
the resulting eggs exit the host via
the feces or sputum. On contact with fresh water,
the miracidium (contents of the egg) emerges
from each egg. Specific species of snails serve as
the first intermediate host. The miracidium penetrates
into the snail, where the development of
a larval form consisting of a saclike structure
(sporocyst) occurs. Numerous rediae (a larval
stage that forms in the sporocyst) result and
ultimately produce many cercariae (final-stage
larvae). The cercariae emerge from the snail and
encyst on water plants or enter a fish, crab, or
crayfish, which serves as the second intermediate
host. Human consumption of these contaminated
items initiates a new cycle.
The blood-dwelling flukes consist of the Schistosoma
spp. Human infection of these flukes
occurs following the penetration of cercariae into
the skin. This typically happens when an unsuspecting
human swims or wades in contaminated
water. Following penetration, the resulting schistosomule
(the morphologic form that emerges
from cercariae following human penetration)
takes up residence in the blood vessels around
the liver, intestinal tract, or urinary bladder,
where maturation into adulthood occurs. Because
sexes are separate, the presence of both an adult
male and an adult female is necessary for copulation
to take place. Completion of this mating
process results in numerous eggs. Passage of the
eggs may take place in the urine or stool, depending
on the species. The development of the
miracidium, sporocyst (daughter sporocysts are
produced in place of rediae in this cycle), and
cercariae occur in the same manner as those of
the organ-dwelling flukes. The cercariae emerge
from the snail. An additional host is not required
in this cycle. The cercariae, on penetrating the
skin of a new human host, initiate a new cycle.
Quick Quiz! 11-1
The first intermediate host for all the trematodes is
which of the following? (Objective 11-5)
A. Fish
B. Snail
C. Shrimp
D. Water plant
LABORATORY DIAGNOSIS
The specimen of choice for the recovery of trematode
organisms is species-dependent. Samples