Seafood as a term includes fish and shellfish. Shellfish usually refers both to crustaceans and molluscs, even though some molluscs don’t have shells (octopus/squid, for example), so not the best term.

But allergy to one does not imply allergy to the other! In fact, shellfish allergy is linked strongly to house dust mite allergy rather than fish, probably since they are all invetebrates with the same sort of Tropomysins.  Co-sensitivity is relatively common (20-40% of fish allergic also allergic to shellfish) so must simply be atopic disposition!

Chordata (finned fish) subdivide roughly into bottom feeders, mackerel/tuna [perciformes] group, salmon/trout [salmoniformes] group and the rest (so called “bony fish”) but this doesn’t mean much in allergy terms.  The best studied allergen Gad c 1, found in cod, is a parvalbumin.  These are found in muscle, esp slow twitch white flesh (cf dark muscled, fast fish eg tuna, swordfish).    At least 50% of those with allergy to one type of  fish will be allergic to another; there are no good predictors for this.

The rate of cross reactivity between different kinds of shellfish/molluscs is high since there is less variation in tropomysins.

Shellfish (crustaceans) are related to molluscs including abalone, clam, mussel, squid, octopus.  Allergy to these is more common in countries where these are commonly eaten viz Spain, Japan – which is not surprising when you think about it, although typically surprising to the people who live there.

Sneaky places you find seafood:

• Soups eg bouillabaisse
• Pate
• Seafood “crab” sticks – usually fish, not crab!
• Worcestershire sauce (anchovy)
• Pizza (anchovy)
• Caesar salad (anchovy)

Reactions to seafood may not be allergic:

• Anisakis is fish parasite, worldwide distribution, with a range of different allergens.  Larvae can cause immediate allergic response, but infection can also produce inflammatory symptoms of varying kinds, depending on where in the digestive tract the larvae are deposited.
• Scombroid toxicity esp associated with salmon, tuna, mackerel.  Besides flushing, vomiting and wheezing, there can be severe headache and dizziness.  Onset is minutes to hours.
• Ciguatoxin poisoning associated with reef fish eg sea bass, snapper.  Onset is slower – 30 minutes to hours, besides cramps, D+V there can be myalgia and paraesthesiae.
• Shellfish (mostly bivalves) can be the source of a range of toxins with effects as diverse as paraesthesia, myalgia, ataxia, even seizures.   Mostly D+V though.

Cod IgE >20 gives 95% PPV, other cut offs lacking.  IgE>8 (>5 for salmon) predicts objective symptoms and non tolerance (cf partial tolerance).

Given importance of omega 3, unnecessary restriction should be avoided – Canning fish reduces immunogenicity, challenge?  Interestingly, many fish allergens seem to get MORE allergenic with heating, not less – Gad c 1 is known to become airborne in steam without denaturing! Various cases of fatal anaphylaxis simply to inhaling vapour of frying fish (aerosolised proteins, NOT smell, that causes reaction).

[JACI 2017 letter – small numbers though]