Some shellfish reactions are not true allergies but are caused by marine toxins from contaminated shellfish.
| Feature | Shellfish Allergy | Shellfish Poisoning (Toxic Reaction) |
|---|---|---|
| Cause | IgE immune reaction to shellfish proteins | Toxins from algae-contaminated shellfish |
| Symptoms | Hives, anaphylaxis, swelling | Nausea, vomiting, diarrhea; tingling or numbness of the mouth and limbs; in paralytic poisoning, muscle weakness and breathing failure; in amnesic poisoning, confusion and memory loss |
| Onset | Minutes to hours | Minutes to hours, depending on the toxin; neurological signs of amnesic poisoning can appear up to 48–72 hours after eating |
| Triggers | Any shellfish species (allergy-dependent) | Bivalves (clams, mussels, oysters, scallops) |
| Treatment | Epinephrine, antihistamines, strict avoidance | No antidote. Call 911 — paralytic shellfish poisoning can stop breathing within hours and may need a ventilator. Cooking or freezing does not destroy the toxins. |
Shellfish poisoning can mimic an allergic reaction but is caused by toxins rather than an immune response.
The Four Shellfish Poisoning Syndromes
- Paralytic shellfish poisoning (PSP) — saxitoxins. Tingling and numbness of the lips and face, then weakness and possible respiratory paralysis. A medical emergency: call 911.
- Diarrhetic shellfish poisoning (DSP) — okadaic acid. Mainly diarrhea, nausea, vomiting and cramps.
- Amnesic shellfish poisoning (ASP) — domoic acid. Vomiting and diarrhea, then confusion and memory loss; neurological signs can appear up to 48–72 hours after eating.
- Neurotoxic shellfish poisoning (NSP) — brevetoxins. Nausea plus tingling, dizziness and reversal of hot and cold sensations.
These marine toxins are heat-stable: cooking, steaming or freezing does not destroy them, and contaminated shellfish usually look, smell and taste normal. Follow local shellfish-harvesting advisories.
Raw or undercooked shellfish, especially oysters, can also cause infections with Vibrio bacteria or norovirus. These cause vomiting and diarrhea, and Vibrio vulnificus can be life-threatening for people with liver disease or a weakened immune system. They are infections, not allergies.
Anaphylaxis: act fast. After an exposure, any breathing symptom (wheeze, repeated cough, throat tightness, hoarse voice), fainting or dizziness, or symptoms in two or more body systems (for example hives plus vomiting) is anaphylaxis — even without hives.
- Give epinephrine right away (auto-injector or nasal spray). Do not wait to see if symptoms get better.
- Call 911, even if symptoms improve.
- Lie flat with legs raised (sit up if breathing is hard). Give a second dose after 5–15 minutes if there is no improvement.
Antihistamines and steroids do not treat anaphylaxis and must never delay epinephrine. Carry two doses and follow the written emergency action plan from your allergist.