Fish is in the top 8 most common food allergens that cause 90% of all food-allergic reactions.  We are talking about “finned” fish, not shellfish.   Fish allergy is more common in adults than children and typically a long lasting allergy.

Edible fish are typically in the Osteichthyes class in which there are hundreds of species.  Frequently there is cross-reactivity between fish species.  In fact 95% of fish-allergic patients are sensitized to the major fish protein allergen called parvalbumin.  The major allergens between cod (Gad c 1) and carp (Cyp c 1) are 68% identical.  However, patients don’t always have clinical reactions to multiple fish depending on processing (see below).

Is Parvalbumin the same for all fish?  No, it varies!

• The amount of parvalbumin varies considerably among fish species which may explain the intensity and frequency of clinical allergic reactions to different fish.  In a study http://www.ncbi.nlm.nih.gov/pubmed/20559001 evaluating salmon, trout, cod, carp, mackerel, herring, redfish and tuna there was up to a 100-fold difference in the amount of parvalbumin in the meat.  Tuna had the lowest amount of parvalbumin at <0.05 mg per gram of meat followed by mackerel at 0.3-0.7 mg.  Trout, cod and salmon had 1-2.5 mg per gram of fish meat compared to carp, redfish, cod and herring which had the highest amount of parvalbumin at >2.5 mg.  The amount of parvalbumin in cooked samples decreased by 20-60%.  This means that some proteins are considered “heat stable” and

Does smoking or pickling fish affect the allergic potential?  Yes.

• In Norway, a study evaluated how different processing affects the allergenicity of fish proteins.  They studied fish that had been smoked, salted-sugar cured, canned, lye-treated (yummy!) and fermented.  They found that smoked fish had increased IgE (allergic antibody) binding while chemically processed or pickled herring had much lower or no IgE binding.   It turns out that the processing type was more important than the fish species, although individual responses varied.   Most patients allergic to cooked fresh salmon or tuna can ingest canned salmon or tuna without difficulty.  Some patients can experience allergic reactions (asthma) to inhaling airborne fish allergen while fish is cooking!

Fish Testing

• Allergy skin testing:   A negative skin test to fish is very likely to predict that a person is not allergic; however, a positive skin test requires clinical correlation.
• Allergy blood test:  At a serum specific IgE level of 20 KU_A/L, or higher, there is a 99% chance that the patient will experience an allergic reaction if he/she ingests fish.
• Even though patients with fish allergy may have multiple positive skin tests to various fish, when they underwent a blinded food challenge and ate each of these fish separately, 2/3 of patients reacted to 1 fish, 20% of patients reacted to 2 or 3 different fish.
• If you are offered a blood IgG or IgG₄ tests for specific foods including fish, something is “fishy” as controlled scientific trials do not support these tests to correlate with true allergy.  An IgG level is evidence of previous exposure (such as eating the fish), not necessarily a true allergy.

At Family Allergy Asthma & Sinus Care, if you have experienced an allergic reaction to fish, we can help.   Fish allergy is tricky; just like trying to catch them!