Several months ago I promised to post some more information about parasites
in fish.

According to Robert S. Desowitz professor of tropical medicine at the
University of Hawaii and author of "New Guinea Tapeworms and Jewish
Grandmothers" (Norton Paperback, 1987) the only type of parasite from
saltwater fish that contaminate humans are a kind of nematodes called
anisakines. The following is cut and pasted from his book:

"Experimental studies tend to confirm that only the larvae of "marine
mammal" anisakines are capable of invading the tissues of an abnormal host,
such as humans.  When various kinds of larval anisakines were introduced
into the stomachs of rats, only those of "marine mammal" species were
subsequently found to have penetrated the stomach wall.
The developmental cycles of all "marine mammal" anisakines appear to be
similar. The adult female's eggs are shed, along with the host's feces,
into the sea.  The number of eggs produced daily is not known, but her
"human" cousin, Ascaris lumbricoides, is a fecund marvel laying 200,000
eggs each day of her life.  The eggs sink to the sea floor, and within a
few weeks a larva develops inside each one.  When embryonation is
completed, the lava emerges into the marine world.  If the developmental
journey is to continue, the larva must be eaten by the first intermediate
host, a shrimp.  The larva penetrates the shrimp's digestive tract and
comes to lie in the body cavity, where it molts, grows, and awaits the next
fateful event that will allow it to progress along the life cycle.  This
occurs when a fish or squid devours the infected shrimp.  The larva bores
through the stomach wall of the fish to become immured in the tissues.  A
few worms may penetrate to the viscera or musculature.  In the fish the
larvae molts again, to become the infective stage.  If a larger predatory
fish eats the infected fish, the larva merely transfers to the new host.
In this way the infective larva can be serially passed along the food chain
a seemingly indefinite number of times - from squid to fish, or fish to
squid, or fish to fish - without undergoing any further maturation.  The
final event in the cycle takes place when a marine mammal consumes the
infected fish or quid.  The larva escapes from the digested intermediate
host and attaches itself to the stomach wall of the marine mammal, where it
grows into a sexually mature adult."

He does not say but I conclude this maturation process takes place when
humans ingest the parasite as well.  The symptoms of an anisakine parasite
infestation are usually violent abdominal cramps that often require
emergency surgery to repair serious lesions in the area. Coughing up of
blood can also occur and the larvae are usually 3 inches long.

Desowitz goes on:

"Anisakid larvae have been found in fish from virtually all of the world's
briny waters - from tropical and arctic seas, from the Atlantic and the
Pacific.  A great variety of fish are parasitized, with the species of fish
and the species of infecting worm depending on the nature of the fish and
the mammal population in the marine habitat in question.  In coastal North
Atlantic waters, where seals are relatively plentiful, pollack and cod are
frequently infected with the "seal" worm, Phocanema.  In the waters
bordering Holland many herring are infected with the "porpoise" parasite,
Anisakis.  A recent outbreak of human anisakiasis in the Nantes region of
France is believed to have been caused by the regional gastronomic
specialty, raw sardines.  All sorts of fish caught off both American coasts
have been found to be infected - salmon, flounder, croaker, and the
mackerel, to name a few.  When Dr. George Jackson and his colleagues at the
Food and Drug Administration surveyed the fish sold in Washington, D.C.
markets, they examined 1,010 fish belonging to twenty genera and twenty
three species and recovered 6,547 parasitic nematodes".

"Those who dine on fish that has been baked, broiled, braised, or fried run
no risk of contracting anisakiasis.  Both cooking and freezing kill the
parasite.  The Dutch brought a rapid end to the outbreak of the infection
by enacting legislation that required all herring to be frozen before being
marketed.  Such legislation is impossible in Japan.  Sashimi cannot be
prepared from frozen fish, and the Japanese would not forgo their deeply
entrenched taste for raw fish in return for freedom from a disease that
affects "only" several hundred people each year.  At any rate, the variety
of fish consumed as sashimi is too great for any legislation to be
effective.  In the United States five cases of human anisakiasis have been
documented and for some unknown reason, these North American cases have
tended to be clinically much more benign than the infections experienced
elsewhere.  The variation in pathogenicity is perplexing.  Do the different
species of anisakine have different invasive potential, or is diversity
among human hosts involved?  The only such diversity that has been
suggested so far is the amount of gastric juice, with the Japanese tending
to have somewhat less than others.  It is believed that larval worms burrow
better in less acidic conditions".

"As far as can be determined, the two factors responsible for human
anisakiasis in Holland were the introduction of a new way of preparing
herring and a change in the traditional manner of processing the newly
caught fish.  Green herring was first placed on the market during the early
1950s.  My Dutch friends tell me that their countrymen took to this raw,
salted snack like Americans took to pizza.  At about the same time that
green herring was being introduced for public consumption, the traditional
practice of eviscerating the fish while still at sea gave away to that of
cleaning the fish ashore.  The fish passed before they were brought to port
and processed.  In some fish, the larvae of the potentially pathogenic
anisakid worms are naturally present in the flesh, but in herring, most of
the anisakid larvae are normally located in the tissues of the intestinal
tract, so the former custom of gutting the fresh herring immediately had
rendered them fit for consumption even in the raw state.  But now, as the
iced fish cooled, some of the larvae were prodded into a migratory movement
to seek the somewhat warmer protection of the  musculature.  Thus the green
herring was now more likely to contain a worm."

Please note that despite his intimate knowledge of parasites, Desowitz
continues to enjoy the consumption of raw saltwater fish. However,
freshwater fish which very often do contain parasites he does not advise to
eat raw.

Best, Peter
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