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Diner, beware! Scombroid, or histamine, poisoning poses a risk if certain types of fish haven’t been properly preserved. Though cooking can kill the causative bacteria, it can’t touch the histamine produced from them.

This afternoon, you’re seeing your middle-age patient Stanley* on an emergency basis. He has symptoms of an allergic reaction — he’s covered with a bright red, itchy rash — though there are no visible wheals. You ask what he may have eaten recently, and Stanley says he just had lunch with his wife at a local seafood restaurant, ordering grilled mahi-mahi. But before he finished the meal, he felt flushed, and his heart was racing.

For several minutes, you’re stumped. Stanley has no allergy to anything that you’re aware of. In fact, he’s the least likely of your patients to have a food reaction: Stanley’s diet rarely strays from the straight and narrow — basically, as he once said, he’s a meat-and-potatoes man.

You ask Stanley about the mahi-mahi, and he replies that his wife talked him into trying the dolphin, “just for something different.” Different it was — the fish had an unpleasant, rather acrid taste. “Next time,” he says, “I’m getting the steak!”

That’s when you realize what ails him — scombroid poisoning — and you order the antihistamine medication standard for an allergic reaction.

The real culprit

But though treatment resembles what practitioners administer to counteract an allergy, scombroid poisoning (or histamine poisoning) is just that: a toxic reaction that can occur after eating certain inadequately preserved fish. Originally, it was associated with the suborder Scom­ broidea, which includes such fish as tuna, mackerel, and butterfly kingfish. Nowadays, however, nonscombroid fish like dolphin and amberjack are thought to be the major vectors.

According to the Centers for Disease Control and Prevention, scombroid poisoning is the leading food-borne disease in the United States that’s caused by a chemical agent. It accounts for about 5% of food-borne outbreaks reported each year, but since health care providers are not required to report these illnesses, the actual number of poisonings may be far higher.¹

Scombroid fish contains a chemical called histadine. Bacteria multiply in fish stored in a warm environment and convert histadine into scombrotoxin. The main component of scombrotoxin is histamine, which is the cause of symptoms associated with scombroid poisoning.² Cooking, smoking, curing, canning, or freezing the fish does not reduce histamine levels, even though the causative bacteria may have been destroyed.³

It’s important to note that the amount of histamine can vary with the type or size of the fish, so not everyone eating the same food may be affected by histamine poisoning.² Although there’s no test for this toxin that’s 100% reliable, many who’ve ingested the sus­ pect fish say it has a
distinctive peppery, bitter taste.⁴

That said, the diagnosis is usually made clinically based on presenting symptoms, and treatment consists of both H1 and H2 histamine blockers, in addition to supportive care — acetaminophen for headache, for example, or antiemetics for nausea.

When ingested in significant amounts, histamine causes a classic allergic response, usually within 30 minutes. Flushing, palpitation, headache, nausea, and diarrhea are the most common symptoms, but the patient may also experience tachycardia, wheezing, and fluctuation in blood pressure. A distinctive rash — described as bright red, hot, and itchy — develops on the face, neck, chest, and back, but unlike an allergic reaction, there is no whealing.² Symptoms usually persist for about three hours, but the patient may take longer to recover in severe cases — up to eight hours.

Treatment options include the following:

• Administration of diphenhydramine (Benadryl) 50mg IV/IM/PO every six hours as directed⁵
• Possible administration of an H2 receptor blocker, such as ranitidine or cimetidine, twice daily to reduce stomach acidity
• Possible emesis, especially in severe cases⁴
• Administration of supportive care (ECGs for tachycardiac patients, chest x-rays for related respiratory discomfort, and supplemental oxygen for any associated shortness of breath)

In severe cases, it may be necessary to continue antihistamines for several days. Options to consider include cetirizine, loratadine, and fexofenadine.²

Also, activated charcoal may be beneficial if given early or if the patient has eaten a great deal of fish.

Oral antihistamines are quite effective in most scombroid poisoning cases since symptoms tend to resolve quickly — within 10 to 15 minutes of taking a quick-acting antihistamine tablet.²

The importance of obtaining a thorough patient history cannot be overstated. While most scombroid poisoning occurs within minutes after ingestion, some reactions have been documented as far out as two hours following the meal. Obviously, it’s important to find out the patients’ food intake over the course of several hours, as well as the source of the food — local fisheries, restaurants, or the patient’s own freezer or refrigerator.