[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 10 The Cestodes
255
TABLE 10-10
Parameter
Scolex
Number of sucking grooves
Shape of sucking groove
Gravid Proglottid
Shape
Location and appearance of
uterine structure
seen. Recovery of intact D. latum proglottids and
scolices from untreated patients is rare. Stool
samples from infected patients who have had
drug treatment should be examined. To ensure
that the treatment was successful, the expected
findings consist of the passage of the scolex and
absence of (new) proglottids.
Life Cycle Notes
Diphyllobothrium
Latum Adult: Typical
Characteristics at
a Glance
Description
Two
Almond
Wider than long
Central; rosette
Centrally located rosette
shaped uterine structure
FIGURE 10-16 Diphyllobothrium latum proglottid.
The life cycle of D. latum is relatively complex
in that it requires two intermediate hosts for its
completion. Human D. latum infection is initiated
when the pleurocercoid, defined as a precursor
larval stage, is ingested when consuming
contaminated freshwater fish that is raw or
undercooked. In the small intestine, the scolex
emerges from the pleurocercoid and attaches
to the intestinal mucosa. The adult worm selffertilizes
and the undeveloped eggs are passed
into the outside environment via the stool. If
these eggs were to come in contact with fresh
water, the coracidium, or free-swimming larva,
would hatch. This larva is ingested by the first
intermediate host, the Cyclops species copepod.
The next stage in D. latum development, the
larval stage known as the procercoid, occurs in
the copepod. The infected copepod is ingested by
the second intermediate host, a freshwater fish.
Once inside the fish, the procercoid develops
into a pleurocercoid larva in the muscle tissue.
Human ingestion of the contaminated fish initiates
a new cycle.
It is interesting to note that often the second
intermediate host, a freshwater fish, is small in
size. These fish have been known to become
meals to larger fish before directly infecting
humans. The larger fish harbor the parasite and
serve as a transport host. Human consumption
of these larger fish will result in the release of the
pleurocercoid larva, which will develop and continue
the life cycle.
Epidemiology
D. latum may be found in a variety of temperate
regions worldwide. In the United States, Alaska
and the Great Lakes region are known to harbor
the parasite. Other noted endemic areas include
parts of South America and Asia, Central Africa,
the Baltic region, and Finland. It is in these areas
that raw and/or freshwater fish are routinely consumed.
In addition to humans, a number of fisheating
animals may also become infected with D.
latum and serve as definitive hosts.
Clinical Symptoms
Asymptomatic. It is estimated that most
infected persons with adult D. latum worms
exhibit no clinical symptoms.
Diphyllobothriasis: Fish Tapeworm Infection,
Broad Fish Tapeworm Infection. Persons infected
with D. latum often experience symptoms consistent
with digestive discomfort. Overall weakness,
weight loss, and abdominal pain may also
develop. When the adult D. latum worm takes
up residence in and attaches itself to the