Infection with fish tapeworms of
the genus Diphyllobothrium is called diphyllobothriasis.
Humans become infected by eating raw or poorly cooked fish. The most
frequent species found in humans is the broad fish tapeworm, Diphyllobothrium
latum, geographically located in regions of North America,
especially Alaska and northern Canada, Europe, Japan, and Russia,
including Siberia. In Latin America, D pacificum has
also been found in humans on the Pacific coast of Peru, Chile, and
Ecuador (where raw marine fish are prepared with lemon as cebiche),
and Japan.1 Several other species of Diphyllobothrium also
infect humans, especially in Alaska. Usually, other definitive hosts,
such as bears, dogs, and cats, maintain the infections in nature,
and humans are incidentally involved.1,2
The adult tapeworm lives in the small intestine, where it may
attain a length greater than 10 m. The proglottids are wider than
they are long, hence the name broad fish tapeworm. Gravid
proglottids continuously expel eggs into the intestinal lumen through
a uterine pore. More than 1 million eggs may be passed in the feces
each day. The eggs measure approximately 60 μm
by 40 μm and have a lidlike structure called an
operculum. If the eggs reach water, a ciliated embryo develops within
the egg in about 2 weeks. This ciliated stage or coracidia then hatches
through the opened operculum and is ingested by one of several species
of copepod (water flea). In this minute crustacean, the embryo develops
into a first-stage, or procercoid, larva in 2 to 3 weeks. When a
freshwater fish eats the infected copepod, the larva penetrates
the fish’s intestinal wall and migrates to the muscles, where
it grows into a ribbonlike plerocercoid larva (also called a sparganum)
in approximately 1 month. Larger fish such as salmon, pike, perch,
and trout may eat the initial fish host, and the larva again invades
the muscle of the second fish. If the game fish is eaten raw or
inadequately cooked, the plerocercoid larva develops in the small intestine
of the definitive host into a mature adult after approximately 5 weeks
Life cycle of Diphyllobothrium latum (broad fish tapeworm).
Ingestion of raw or inadequately cooked fish containing plerocercoid
larvae is followed by development of a tapeworm in the small bowel
and passage of feces containing operculated eggs. Eggs deposited
in a freshwater pond or lake hatch and infect the first intermediate
host, a copepod (water flea). The infected copepod conveys the early
larval form to a small fish, which can in turn infect a number of
piscivorous fish until a human or other piscivorous mammal feeds
on the larger fish and acquires the infection. 1-6: The scolex (1)
of an adult worm attaches by sucking grooves to the wall of the
small intestine. Mature segments (2) deposit eggs in the gut lumen
that are ...
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