Scombroid Poisoning

Scombroid poisoning is histamine toxicity from eating spoiled fish. It is the commonest seafood-borne illness in many regions.

The fish at risk have a high free histidine content in their muscle: tuna, mackerel, mahi-mahi, bonito, sardines, anchovies, bluefish and amberjack.

When fish are held warmer than about 15 degrees Celsius after capture, bacteria (Morganella morganii, Klebsiella, Raoultella and Photobacterium species) decarboxylate histidine into histamine. Other amines such as cadaverine and putrescine are formed at the same time and block histamine breakdown in the gut.

Histamine is heat stable, so cooking, smoking, canning and freezing do not remove it. Spoiled fish may look and smell normal, and a peppery or metallic taste is a clue. Illness usually needs a histamine content over 50 mg per 100 g of fish. Because contamination is uneven within a fish, only some diners at a table may be affected. Onset is 10 to 60 minutes after the meal, and most cases resolve in 4 to 24 hours. Isoniazid and other monoamine oxidase inhibitors, asthma and alcohol increase severity.

Complications are bronchospasm, hypotension, arrhythmia and, rarely, death.

• Flushing of the face, neck and upper torso (the commonest sign), throbbing headache, burning sensation in the mouth, and a peppery taste
• Pruritus, urticaria, palpitations, dizziness, nausea, vomiting, abdominal cramps and watery diarrhea
• Severe cases: wheeze, stridor, hypotension and cardiac arrhythmia
• Several people ill after the same meal, with no fever
• Red flags: wheeze, hypotension, chest pain, a patient taking isoniazid, and symptoms lasting over 24 hours

1. Clinical diagnosis from the exposure and the rapid time course. Fish allergy is distinguished by an IgE-mediated reaction that recurs with every exposure in the same person, whereas scombroid affects anyone eating that fish.
2. Histamine content of the remaining fish: over 50 mg per 100 g confirms the cause. Report the case to public health and keep the fish.
3. Serum histamine within 4 hours or urinary N-methylhistamine, if available. Serum tryptase is normal, which separates it from true anaphylaxis.
4. ECG for palpitations, and oxygen saturation and chest examination for wheeze.

Differentials: fish or shellfish allergy, ciguatera poisoning, monosodium glutamate reaction, alcohol flushing, mastocytosis, carcinoid syndrome.

1. Antihistamines (first line):
   • Chlorphenamine 10 mg IV slowly (children 0.2 mg/kg, maximum 10 mg), or diphenhydramine 25 to 50 mg orally or IV every 6 hours.
   • Famotidine 20 mg IV (children 0.5 mg/kg, maximum 20 mg), then 20 mg orally twice daily.
   • Cetirizine 10 mg orally once daily for 2 to 3 days for persistent flushing and itch.
2. Bronchospasm: salbutamol 2.5 to 5 mg nebulized every 20 minutes for up to 3 doses (children 2.5 mg) with oxygen.
3. Hypotension: IV crystalloid 20 mL/kg. For severe bronchospasm or refractory hypotension, epinephrine 0.5 mg IM into the anterolateral thigh (children 0.01 mg/kg, maximum 0.5 mg), repeated every 5 minutes.
4. Decontamination: activated charcoal 50 g (children 1 g/kg) only if the patient presents within 1 hour of a large ingestion. Do not induce vomiting.
5. Observation: 4 to 6 hours, longer in asthmatic patients, those with cardiac disease, and those taking isoniazid, in whom symptoms last longer.

Prevention: fish should be chilled to under 4 degrees Celsius immediately after capture, and never refrozen after thawing in warm conditions. Counsel patients to avoid eating the same fish again if it tasted peppery.