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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