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where researchers have linked them to eating raw
or undercooked freshwater fish.
Researchers have tied many infections, mostly
in Asia, to eating raw, pickled, or poorly cooked
freshwater crabs and crawfish (especially Chinese
“drunken crabs”) that are contaminated with lung
flukes, another major fluke group comprising eight
known species. These animals produce a serious
human disease called paragonimiasis, in which
immature worms infect the lungs and encapsulate
themselves in protective cysts, where they can
remain for decades.
Investigators have also linked more than 65
fluke species, primarily from Asia, to human
intestinal tract infections. One noteworthy
geographical exception is Nanophyetus salmincola,
an intestinal worm that is sometimes called the
“salmon-poisoning fluke,” which has been transmitted
to people in parts of the U.S. Pacific
Northwest, southwestern Canada, and eastern
Siberia. “Fish flu,” as infection with this fluke has
been dubbed, naturally infects skunks, raccoons,
and minks.
Health officials have implicated the practice of
eating raw, underprocessed, or smoked salmon
and steelhead trout in many cases of human
infection. Although exposure is often fatal to dogs
because of a secondary infection carried by the
fluke, the human disease generally leads to little
more than abdominal discomfort, diarrhea, and
nausea. Physicians can easily treat the malady
once they properly diagnose it.
Like flukes, tapeworms are relatively uncommon
in the United States and other developed
countries, but they can persist for months or years
inside travelers, immigrants, and others who have
dined on raw or undercooked pork, beef, or
freshwater fish that harbor the organisms. The
beef tapeworm, Taenia saginata, and the pork
tapeworm, T. solium, are the most prevalent of
these noninvasive infection-causing parasites.
Unlike most other pathogens, both live out most
of their lives inside human hosts, where they
reproduce and produce their eggs. Unfortunately,
tapeworms can survive for as many as 30 years
within human intestines, where they can grow to
astounding lengthsup to 9.1 m / 30 ft!
Once tapeworm eggs are shed through human
feces, the hardy capsules remain viable for months
while exposed, waiting until they are eaten by an
intermediate host. For T. saginata, cattle serve as
the primary intermediate host, whereas T. solium
relies on pigs for transmission.
A third tapeworm species, Diphyllobothrium
latum, exploits small freshwater crustaceans as
intermediates, which are in turn gobbled up by
larger fish. Inside their animal hosts, tapeworm
eggs hatch into tiny larvae that burrow into the
intestinal wall and hitch a ride through the
bloodstream to muscles and other tissues. Once
in place, the larvae form protective cysts that
can be transferred to humans who eat contaminated
beef, pork, or fish. The cysts then hatch in
the digestive system, where they develop into
flat, ribbon-like worms that use tiny suckers to
latch onto the slippery intestinal wall in much
the same way that mountain climbers stick to ice
walls with crampons and ice axes.
Most cases of tapeworm infection are asymptomatic,
although the parasites can cause abdominal
pain, weight loss, or even intestinal blockage in
their hosts. In some people, D. latum can produce
anemia by absorbing vitamin B12. Contamination
of food or water with the eggs of T. solium can
result in a far more serious disease called cysticercosis,
in which the hatched larvae migrate to
various body tissues and form cysts. The cysts can
prove fatal if they lodge in sensitive organs such as
the heart, brain, or spinal cord.
Smoking and drying foods does not kill tapeworms,
and freshwater fishincluding walleye
and northern pikethat is served as sushi can
contain pathogenic cysts. Fortunately, the cysts
are usually visible in infected flesh, and the larvae
of all three tapeworm species can be dispatched
by freezing for 48 h at −18 °C / −0.4 °F, by hotsmoking
for 5 min or more at 60 °C / 140 °F, or by
following the time-and-temperature cooking
recommendations given in chapter 3 on Food
Safety, page 162.
A Trichinella worm lives within a cyst in
pork muscle.
124 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 125