by Paul Carnes, M.D.
FISH PARASITES THAT INFECT HUMANS
AND FISH CONSUMPTION RELATED ILLNESSES
For the most part almost any
cooked fished is safe to consume as long as it has been properly handled and prepared.
Parasitic infections are most problematic when fish and shellfish are consumed in the raw
state. This is not to say that eating either as "sashimi" isn't safe. If you do
consume raw fish/shellfish make sure it's not a freshwater species. Believe it or not
parasitic infections from freshwater fish/shellfish are very problematic in some parts of
the world (e.g. Southeast Asia.)
There are over 50 helminthic infections from fish/shellfish (i.e. parasites that
parasitizes the human gut) that can infect humans. How do we avoid becoming a
"victim". Well for most the answer is just cook your fish (however other
problems may still occur-see below). However for others, especially those who love sashimi
(like me!) there are a couple of precautions you can take. I will discuss these a little
bit later. Luckily most parasitic infections from ocean fish are rare. Pelagic fishes,
such as tuna probably have the least amount of parasitic load. This is mainly due to their
wide roaming migrations. This is because tuna, who are near the top of
the food chain, may consume prey that have parasites, but are not in an area long enough
to ingest many prey that might have a high parasitic load. What this does is decrease the
likelihood of becoming infected. There are however case reports of humans becoming
infected with roundworms after ingesting raw Yellowfin tuna. Homeguard fish are more
likely to have parasites simply because they don't move around much. Thus they continue to
consume prey that may have parasites. Juvenile or smaller fish also have less time to have
become infected with parasites. Also fishes that grow quickly also have less time to pick
There are some fish that can VERY problematic when it comes to parasites. These include
fish that spend some of their time in brackish or freshwater, like salmon. A study a few
years ago out of Seattle showed that 100% of wild caught salmon had roundworm larvae in
their flesh (the type that might infect humans). The study also showed that farm raised
salmon did not have any roundworm larvae in their flesh....something to consider the next
time you order salmon at the sushi bar.
Fortunately there may be some saving grace. Besides being killed by heat (i.e. cooking)
most parasites are killed by freezing the flesh. Another study out of Seattle a few years
ago showed that despite quite a few parasites being found in samples from Sushi bars
(again most were found in Salmon) the majority were already dead. Freezing the raw fish
prior to preparation was thought to be a major reason the parasites were dead.
What are some of the symptoms of parasitic infections? For the most part any unusual
gastrointestinal problem such as bloating, cramping, diarrhea or unusual bowel movements.
What makes the diagnosis difficult is that the symptoms may occur many hours or days after
the consumption of the fish/shellfish. Also many people may "write" off the
symptoms simply as a case of the "stomach flu" or other short-lived stomach
Besides parasites there are a couple of other illnesses that should be consider when
consuming raw fish/shellfish. One is hepatitis A. This virus, which infects the liver, is
transmitted because of unsanitary conditions, either because of where the fish/shellfish
were captured (like in a bay or polluted area) or during preparation. Other infections
that's possible are bacterial (e.g. Staphylococcus) and amoebic. Again these can occur
because of unsanitary conditions.
Now that I've scared most of you sushi and sashimi lovers, what about other problems
associated with consuming COOKED fish/shellfish. There are two types of fish/shellfish
poisonings that people should be aware of: Scombroid fish and Ciguatera poisonings.
Scombroid poisoning occurs secondary to the bacterial transformation of histidine (an
amino acid) to histamine in fish flesh. This can occur in inadequately refrigerated dark
fleshed fish such as the tunas and mackerels. Typical symptoms are like an allergic
reaction: skin flushing, headache, oral burning, cramps, nausea, diarrhea, and itching. Of
note hives and low
blood pressure (hypotension) are rare. Symptoms usually occur 10 to 30 minutes after
ingestion and may last 6 to 10 hours. Immediate treatment in a medical facility might be
necessary. Antihistamines, such as Benadryl can be beneficial or live saving. Although I
cannot prove it, I feel that a lot of reactions people have to fish are not because they
are allergic to iodine, but in fact is because of Scombroid fish poisoning.
Another type of poisoning is called Ciguatera poisoning. This occurs when a fish/shellfish
consumes prey that have in themselves consumed an algae that has toxins (e.g. red tide
like algae). These toxins can be found in large semitropical or tropical homeguard or reef
fishes such as yellowtail, barracuda, grouper and seabass. The toxin is a neurotoxin and
can cause gastroenteritis and symptoms such as numbness, hot/cold reversal, itching and
weakness. Rare cardiovascular symptoms such as very low heart rate and blood pressure are
possible. Symptoms usually occur 3-6 hours after consumption and may last from days to
months. Therapy is usually supportive and may require hospitalization. If you do catch a
large homeguard fish you might want to check to see if Ciguatera toxin is present.
Fortunately there now is a commercial test kit available to check for Ciguatera toxin.
So what are some rules of the road to avoid some of these infections/conditions? Try
1) Make sure your fish has been handled correctly (prevents Scombroid poisoning). If
possible freeze the raw flesh to decrease the likelihood of ingesting viable parasites.
Stay away from raw salmon unless it has been frozen, smoked or farm raised (I rarely eat
raw salmon). Don't experiment with consuming new types of raw fish, especially freshwater
regret it later.
2) Be careful with homeguard or reef fishes, especially the larger individuals. If you do
consume these fishes remember the symptoms of Ciguatera poisoning. A kit to check for
Ciguatera is commercially available.
3) Consuming fast growing fish species or smaller individuals might be safer overall. If
you have to consume bigger fishes just be aware of the symptoms.
4) Remember symptoms may occur many hours or days after consumption. If you do seek
medical attention it's always advisable to tell the health care provider about your
consumption of fish/shellfish.
5) Remember most of these conditions are unusual. But remember this knowledge may help
you, a family member or a friend in the future.
6) Does knowing all this stop
me from eating sashimi/sushi or other cooked fish? In a word: NOPE.
Hope this info helps. Again this info is not necessarily to be considered complete. Be
careful out there and consult your own physician if you have any questions.
Paul P. Carnes, M.D., AKA Pablo
met Paul via the Internet nearly as soon as I opened up the tackle sales end of things.
Paul lives in Oklahoma, and was my first customer from that State. He
surprised me with his knowledge of fishing (purchasing Newell reels no less), and takes
several long range trips annually. He didn't always live out in Oklahoma, but is
another transplanted Californian originating from San Diego. You'll catch Paul
popping up on message boards every now and then. If you see a post from Pablo, let
him know you appreciate his articles - lots of good info! Maybe if we're nice to
him, he'll zap over a picture of some nematode for this article. Okay, now, lets all
have some sashimi!]