Colorado State University Extension
SafeFood Rapid Response Network
SafeFood Newsletter - Winter 1996/1997 - Vol. 1, No. 2
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Fishing for Safe Seafood
Every day in the U.S., hundreds--maybe thousands-- of people get sick as a result of eating seafood. For most, it's an inconvenience, maybe an unpleasant day or two of diarrhea and stomach cramps. Others are disabled for weeks . . . or even years. A small number, mostly those who eat raw shellfish, die.
"The biggest seafood hazard by far is raw or undercooked shellfish," says Morris Potter of the CDC. According to the Food and Drug Administration (FDA), about 20 million Americans consume raw shellfish, which accounts for over 90% of seafood poisoning cases.
When you eat raw shellfish, you eat the whole organism, including its GI tract. Furthermore, shellfish feed by filtering two to three gallons of water every hour. That means they take in whatever is floating by--plankton and other food along with viruses, bacteria, mercury, etc.
Awaiting you in your next oyster, mussel, or clam could be:
- Norwalk Virus. The most common cause of
shellfish poisoning comes from human sewage. It
causes nausea, vomiting, or diarrhea. Most of the
100,000 or so attacked each year recover within a
day or two.
You can't see or smell Norwalk. Stewing or steaming doesn't necessarily solve the problem. Melvin Kohn of the Louisiana Department of Health and Hospitals says, "We know that high degrees of heat clearly can kill it and low degrees of heat don't. But how many degrees it has to be has not been determined."
- Vibrio. The worst strain, Vibrio vulnificus, kills
about 10 to 15 people each year and thrives in the
Gulf of Mexico. Every case traced by the CDC
since 1990 came from raw shellfish harvested in the
Gulf.
Only a small percentage of the population is susceptible to Vibrio vulnificus infections, but about half of those who get it die. The most vulnerable are those suffering from liver disease, liver damage caused by excess alcohol or iron-overload disease (hemochromatosis), and people with diabetes, AIDS, or cancer.
Heat does destroy Vibrio. As long as one avoids raw or undercooked Gulf Coast shellfish, especially from May through November when waters are warm, he or she is probably safe.
- Ciguatera. This is the most common finfish
poisoning in the U.S. Some 8,000 Americans are
made ill each year by the ciguatera toxin, which is
produced by plankton that grow on tropical reefs.
Small fish that eat the plankton also eat the toxin,
and large predatory fish that eat the small fish end
up carrying even more ciguatera. In the U.S.,
ciguatera poisoning cases are concentrated in
Florida, Hawaii, Puerto Rico, the Virgin Islands,
and Guam.
"The early symptoms can mimic any kind of foodborne illness, typically nausea, abdominal cramps, and diarrhea," says W. Robert Lange of the Johns Hopkins School of Medicine. "Later, a 'pins- and-needles' sensation occurs around and inside the mouth and in the hands and feet." Victims often experience "temperature reversals," where hot things feel cold and more likely, cold things feel hot.
Effects of ciguatera usually last about two weeks, but "as many as 20% of all victims will still have some symptoms at six months and 5% at two years," says Lange. Because of ciguatera's neurological symptoms, doctors have been known to misdiagnose it as multiple sclerosis.
To avoid ciguatera, "don't eat barracuda, no matter where you are," warns Lange. If you're visiting the Bahamas or the Caribbean, pass up grouper, red snapper, or any fish dish like bouillabaisse where you don't know the ingredients. "In the States, if you want grouper or red snapper, stick to smaller, younger specimens, which haven't accumulated as much toxin."
- Scombroid Poisoning. Less common and less
serious than ciguatera, scombroid poisoning occurs
when certain fish aren't kept cool enough. Bacteria
on the fish create large amounts of histamine, which
can cause tingling and burning sensations around
the mouth, as well as diarrhea, cramping, and an
itching rash. Symptoms seldom last more than a
few hours.
In the U.S., "most cases involve scombroid fish like mahi mahi or fresh tuna," says Lee Weddig of the National Fisheries Institute.
- Chemical Contaminants. Harmful metals,
industrial chemicals, and pesticides like mercury,
PCBs, dioxin, and chlordane often wash into rivers,
lakes, and oceans. Forty-seven states currently have
fish consumption advisories that warn about eating
certain species. They cover 1,740 rivers and lakes
(including all of the Great Lakes) and large sections
of coastal areas.
The most common reason for consumption advisories: mercury. In large amounts, the metal can cause brain and nerve damage, especially to fetuses and young children.
Large fish, like fresh tuna and swordfish, have the highest levels. A new study from the Seychelles Islands off East Africa's coast concluded that amounts in those fish don't appear to cause problems. "The children of mothers who ate a lot of these fish during their pregnancies do not show any signs of harmful effects, at least up to age two and a half," says Researcher Thomas Clarkson of the University of Rochester. His team is continuing to monitor the children.
While there may be little evidence that people get enough mercury or other contaminants from fish to suffer short-term harm, "there might be subtle, adverse neurological problems over the long term that we aren't noticing," says Tom Sinks of the CDC's Center for Environmental Health.
A recent study supports his point. Researchers tested children of women who ate at least four pounds a year of PCB-contaminated Lake Michigan fish for six years before they gave birth. Those children, now 11 years old, are more likely to have lower IQs and to lag behind their peers in reading comprehension.
Source: Nutrition Action Health Letter, Nov. 1996.
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