Diphyllobothrium latum (the fish or broad tapeworm), the largest
human tapeworm. Several other Diphyllobothrium species have been
reported to infect humans, but less frequently; they include D.
pacificum, D. cordatum, D. ursi, D. dendriticum, D. lanceolatum, D.
dalliae, and D. yonagoensis.
Immature eggs are
passed in feces
Under appropriate conditions, the eggs mature (approximately 18 to 20
yield oncospheres which develop into a coracidia
After ingestion by a suitable freshwater crustacean (the copepod first
intermediate host) the coracidia develop into procercoid larvae
Following ingestion of the copepod by a suitable second intermediate
host, typically minnows and other small freshwater fish, the procercoid
larvae are released from the crustacean and migrate into the fish flesh
where they develop into a plerocercoid larvae (sparganum)
The plerocercoid larvae are the infective stage for humans. Because
humans do not generally eat undercooked minnows and similar small
freshwater fish, these do not represent an important source of
infection. Nevertheless, these small second intermediate hosts can be
eaten by larger predator species, e.g., trout, perch, walleyed pike
In this case, the sparganum can migrate to the musculature of the larger
predator fish and humans can acquire the disease by eating these later
intermediate infected host fish raw or undercooked
After ingestion of the infected fish, the plerocercoid develop into
immature adults and then into mature adult tapeworms which will reside
in the small intestine. The adults of D. latum attach to the
intestinal mucosa by means of the two bilateral groves (bothria) of
The adults can reach more than 10 m in length, with more than 3,000
proglottids. Immature eggs are discharged from the proglottids (up to
1,000,000 eggs per day per worm)
are passed in the feces
Eggs appear in the feces 5 to 6 weeks after infection. In addition to
humans, many other mammals can also serve as definitive hosts for D.
Diphyllobothriasis occurs in areas where lakes and rivers coexist with
human consumption of raw or undercooked freshwater fish. Such areas are
found in the Northern Hemisphere (Europe, newly independent states of
the former Soviet Union (NIS), North America, Asia), and in Uganda and
can be a long-lasting infection (decades). Most infections are
asymptomatic. Manifestations may include abdominal discomfort,
diarrhea, vomiting, and weight loss. Vitamin B12 deficiency
with pernicious anemia may occur. Massive infections may result in
intestinal obstruction. Migration of proglottids can cause
cholecystitis or cholangitis.
identification of eggs in the stool is the basis of specific diagnosis.
Eggs are usually numerous and can be demonstrated without concentration
techniques. Examination of proglottids passed in the stool is also of
comparison with other intestinal parasites
the drug of choice. Alternatively, Niclosamide can also be used to
* This drug is
approved by the FDA, but considered investigational for this purpose.