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Immature eggs are
passed in feces
.
Under appropriate conditions, the eggs mature (approximately 18 to 20 days)
and
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 their scolex
.
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)
and 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.
latum.
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