This was previously posted to my old blog, Itch, Swell, Ooze, Wheeze in September 2014. This is an updated version for 2019.
I have always felt that a diagnosis of fish allergy can be quite vague, ‘fish’ is such a general term and encompasses many thousands of species of fish, so it is a more complex allergy to deal with than its name suggests.
A diagnosis of fish allergy may follow as a result of a reaction from eating fish.
An IgE allergic reaction to fish is an immune response triggered by a response to proteins in the fish, the symptoms of which would be throat swelling, facial swelling, anaphylaxis, hives etc. Allergy UK estimate that 15% of people suffering from a fish allergy also reacted to vapours released when cooking. Once medication is taken these effects should reduce quickly (except in the case of anaphylaxis, this can take longer to recover from).
A non-IgE allergic reaction to fish would be delayed up to 72 hours after ingestion and generally affects the gastrointestinal system, the symptoms are varied and many. After antihistamines are taken it still might take several doses and days to reduce the symptoms associated with the reaction.
When fish is tinned or smoked the histamine level increases, so may also affect those who have a general histamine intolerance. In these type of reactions the amount of histamine in the body builds up from various foods and environmental factors to cause an allergic reaction.
A vomiting/diarrhoea reaction to fish can also be caused by food poisoning. It is more likely in the UK that food poisoning from fish will be caused by bacteria called Eschericia coli (E coli). The onset of is unlikely be immediate, but may be confused with a non-IgE reaction, especially for those who already have multiple non-IgE allergies.
Another possible misdiagnosis of fish allergy might be scrombotoxicity. This occurs as histamine is released from the fish flesh in some types of fish and causes an allergic like reaction with intense itching, fast pulse and skin flushing. This reaction is due to the direct toxic effect of spoiled food so is referred to as food toxicity rather than a true allergic reaction.
For fish allergies a well taken history and food diary (challenges in mild cases) is the key to discriminating between an allergy to one species of fish, groups of fish or all fish. It is thought that about 40% of people can tolerate more than one species of fish and in the case of those with multiple allergies it is important to avoid unnecessary diet restrictions.
Some smoked or canned fish contain high levels of histamine causing reactions. Other commercially processed fish are lower in allergenic proteins due to loss of IgE binding so cause less reactions.
Like any allergy avoidance is key, if anaphylactic to fish avoid any products which ‘may contain’ fish. The hidden places for fish allergens are:
- Certain salad dressings
- Worcester Sauce
- Pet foods
- Prawn crackers
- Thai foods
- Stock Cubes
- Relish (caponata)
- Fish oil and Glucosamine Supplements
- Lip Gloss/Lip balm
Key Allergens/Proteins in Fish Allergy
The key protein associated with fish allergy was always thought to be Parvalbumins, which are a group of proteins found in most species of fish. It is found in different concentrations in different species of fish, which would be an indicator that this is the offending protein if certain fish can be tolerated and others cannot.
Parvalbumin is a very stable protein, so able to cause reactions when cooked or as vapour during cooking (but food is thought to be 20-60% lower in parvalbumins when cooked). It is found in high concentrations in the light muscle of fish rather than the dark muscle, so fish like cod and carp are higher in parvalbumin levels compared to swordfish and tuna which have lower levels as they have more dark muscle tissue (tuna parvalbumin is structurally different which is an important factor in allergenicity). This would mean the size, age, health and species of the fish affects the protein levels, which is why it sometimes seems a mystery that you are reacting to fish and other times you can tolerate it.
Parvalbumin is the protein used for skin prick testing and RAST tests, so you/your child may show as negative to these tests if their allergy is to other proteins found in fish, namely endolases, aldolases, collagen and most importantly gelatin.
Gelatin is a protein found in both mammalian and fish meat/skin and is a common allergen. It is possible that some fish derived gelatin contains parvalbumins and vice versa. Fish gelatin is used less often than mammalian gelatin in food products, but its use is on the increase in pharmaceuticals and health supplements.
Manufacturers are more likely to declare in food products where gelatin is derived from fish, but medicines in the UK and US do not yet have to declare their allergens.
This is a blog and should not be used for advice on diagnosis or treatments.
If you think you may have a food allergy please contact your GP in the first instance to discuss treatment options.
References and Further Readings
Fish allergens at a glance: variable allergenicity of parvalbumins, the major fish allergens, http://journal.frontiersin.org/Journal/10.3389/fimmu.2014.00179/full
Specific IgE to fish extracts does not predict allergy to specific species within an adult fish allergic population, http://www.ctajournal.com/content/pdf/2045-7022-4-27.pdf
IgE antibody to fish gelatin (type I collagen) in patients with fish allergy, http://www.ncbi.nlm.nih.gov/pubmed/10984381
Important variations in parvalbumin content in common fish species: a factor possibly contributing to variable allergenicity, http://www.ncbi.nlm.nih.gov/pubmed/20559001
Cross-reactivity in fish allergy: A double-blind, placebo-controlled food-challenge trial, https://www.jacionline.org/article/S0091-6749(17)30741-8/abstract
Fish collagen is an important panallergen in the Japanese population, https://onlinelibrary.wiley.com/doi/abs/10.1111/all.12836
Allergy to fish collagen: Thermostability of collagen and IgE reactivity of patients’ sera with extracts of 11 species of bony and cartilaginous fish, https://www.jstage.jst.go.jp/article/allergolint/65/4/65_450/_article/-char/ja/