Vol. XIX · Fish and Seafood Plate 7 of 9seafood hazards, scombroid, histamine, tuna, mackerel, shellfish poisoning, PSP, norovirus, Vibrio, raw oysters, shellfish allergy, mercury, pregnancyS

Seafood Hazards.

A tuna steak that sat in the sun on the dock can make you flushed, itchy and sick an hour after eating it perfectly cooked. The poison is already in the fish.

Scombroid poisoning is the commonest illness from fish. Tuna, mackerel, sardines, mahi-mahi and their relatives have flesh rich in histidine; if the fish is left warm after catch, bacteria such as Morganella turn it into histamine. Eaten, histamine causes flushing, headache, a peppery taste, itching and nausea within an hour — like an allergic reaction. Histamine is not destroyed by cooking, freezing or canning (XII.9), and fish with harmful levels can look and smell normal. The only control is chilling fish fast and keeping it cold from the boat to the plate.

Shellfish bring three more hazards. Filter-feeding bivalves concentrate toxins from algal blooms — paralytic, amnesic and diarrhetic shellfish poisons — which heat does not destroy; harvesting areas are tested and closed, so buy from licensed sources and never gather wild shellfish without checking local warnings. Raw oysters and clams can carry norovirus and Vibrio from the water, which cooking kills. And fish and crustacean shellfish are among the major allergens (XII.10). Finally, large long-lived predators — swordfish, shark, king mackerel, bigeye tuna — accumulate methylmercury, so pregnant women and young children are advised to choose smaller, lower-mercury fish. The workshop leaves a tuna out.

Below: the growth of histamine-forming bacteria on fish left at the temperature you choose for the hours you choose (microbe bench, teaching parameters for Morganella), against the count at which dangerous histamine can form, and the hours to reach it at each temperature.

What you will learn

Explain scombroid poisoning.
Explain why histamine cannot be cooked away.
Know the shellfish toxin and raw-oyster risks.
Advise on mercury and allergies.

Workshop

Microbe bench: a histamine-forming bacterium with teaching cardinal values (minimum 4 °C, optimum 37 °C, doubling about 26 minutes at the optimum), starting at 1 000 per gram; 10⁷ per gram is taken as the level at which hazardous histamine forms. Shaped on the FDA scombrotoxin guidance, not a prediction for a particular fish.

Hazards that heat cannot fix

With these teaching values, fish left at 30 °C reaches the histamine level in about 10 hours, at 25 °C in about 14 and at 20 °C in about 22; at 10 °C it does not within three days. Commercial rules are stricter than this sketch, because histamine can accumulate over a fish's whole history — on deck, at the market, in a warm car — and the hours add up. Buy tuna and mackerel from a cold counter, keep them on ice, and never eat fish that tastes peppery or tingles on the tongue.

For shellfish: buy from licensed sellers who can show where they were harvested; follow local closures; cook mussels, clams and oysters if you are pregnant, elderly, immunocompromised or have liver disease, because Vibrio vulnificus can be fatal for them. For allergy: shellfish allergy is usually lifelong, and cooking oil and steam carry allergens, so a separate pan is needed (XII.10). For mercury: two or three servings a week of lower-mercury fish (salmon, sardines, cod, prawns, canned light tuna) are recommended even in pregnancy; it is the large predators to avoid.

At the stove

Next time you buy tuna or mackerel, notice how it is displayed: on ice and below 4 °C, or not? Ask how it was handled after landing.

Minamata disease, named after the bay in Kumamoto, Japan, where in the 1950s a chemical factory's mercury waste was concentrated by fish and shellfish, poisoned thousands of people; the global treaty limiting mercury, signed in 2013, carries its name.

Terms

Scombroid poisoning: Illness from histamine formed in fish left warm.
Histamine: A compound formed from histidine by bacteria; not destroyed by cooking.
Shellfish poisoning: Illness from algal toxins concentrated by bivalves.
Methylmercury: The form of mercury concentrated up the food chain in fish.

Histamine forms in tuna and mackerel left warm and survives cooking: chill fish at once. Shellfish toxins are heat-stable: buy from licensed sources. Raw oysters carry viruses and Vibrio; large predators carry mercury.

Sources

U.S. FDA, Fish and Fishery Products Hazards and Controls Guidance, 4th ed. (2020), ch. 7, Scombrotoxin (histamine) formation: histamine-forming bacteria such as Morganella morganii grow fastest at warm temperatures; 50 ppm histamine is the defect action level and 500 ppm the hazard action level; histamine is not destroyed by cooking, freezing or canning; fish should be chilled promptly after death.
Paralytic, amnesic, diarrhetic and neurotoxic shellfish poisons are made by algae and concentrated by filter-feeding bivalves; they are heat-stable and not destroyed by cooking, and harvesting areas are monitored and closed during blooms (U.S. FDA, Fish and Fishery Products Hazards and Controls Guidance, 4th ed. (2020), ch. 6).
U.S. FDA and EPA, Advice about Eating Fish (2021): pregnant and breastfeeding women and young children should eat 2–3 servings a week of fish lower in mercury and avoid the highest-mercury fish — king mackerel, marlin, orange roughy, shark, swordfish, tilefish (Gulf of Mexico) and bigeye tuna.
U.S. FDA, Bad Bug Book, 2nd ed. (2012): Bacillus cereus emetic toxin is heat-stable and is formed in starchy foods held warm; Staphylococcus aureus enterotoxin survives cooking; scombrotoxin (histamine) forms in temperature-abused fish and is not destroyed by cooking; phytohaemagglutinin in raw red kidney beans — a few raw beans can cause illness; boiling 10 minutes destroys it, while heating to only about 80 °C can increase toxicity.