Fishy proteins
Fish is one of the eight regulated food allergens that must be labelled in many countries. Individuals allergic to fish are most often sensitive to proteins called parvalbumins. These proteins are present in many isoforms (proteins with similar structure and function), and cross-reactivity between the different isoforms is high. They are usually found in the white muscle tissue of fish, and in much higher quantities compared to their presence in red muscle tissue (this may explain why some fish-allergic individuals can tolerate tuna, which contains mostly red muscle tissue). Although most people with a fish allergy are allergic to a range of species, some individuals may only be allergic to a specific species of fish (mono-allergy).
Fish allergy is a complicated allergy to live with; and similarly, fish allergens are complicated allergens to test for. Currently there are two test methods routinely used for the detection of fish: ELISA (enzyme-linked immunosorbent assay) and PCR (polymerase chain reaction).
A study conducted in Australia has shown that some ELISA kits are more suited to detecting fish species from the Northern Hemisphere, rather than those normally caught in the Asia-Pacific region. Since there are thousands of different fish species in our oceans, to create an ELISA test that can detect for all parvalbumin isoforms from all potentially allergenic fish is nearly impossible. In this study, bony, cartilaginous and shellfish were tested. Using ELISA, only some of the bony fish tested positive for fish, and none of the cartilaginous fish (sharks, skates and rays). None of the shellfish tested positive using the fish ELISAs; but this is to be expected, as shellfish contain yet another different allergenic protein (which may even differ between crustaceans and molluscs). Another drawback of ELISA, should the fish be heat-treated, is that this may cause the proteins to change shape or structure, preventing the test from detecting fish in a sample. Although parvalbumins are comparatively heat stable, studies have shown that parvalbumin recovery is much lower in cooked or processed samples compared to raw fish.
The alternative method to ELISA is PCR. However, this method involves indirect measurement – it does not measure the amount of allergenic protein present (which is necessary for completing a risk assessment); rather, it detects the presence or absence of fish DNA. Depending on the design of the test, this may be more inclusive of a wider variety of fish species from around the world, and processing or heat treatment of samples may not affect this method as much as it does ELISA.
A test method that has been around for some years but has only recently been applied to allergen testing is MS (mass spectrometry). This method (similarly to ELISA, but not to PCR) detects the actual allergenic substance. The difference between ELISA and MS is that ELISA looks for the whole or intact protein, and MS detects smaller protein fragments. ELISAs are limited to looking for only one protein at a time; MS can detect multiple peptides at the same time. This solves the problem of samples that have been heat treated, and may well be the solution to the problem of not being able to detect most or all different species of fish simultaneously.
FACTS can assist with fish allergen testing and fish species identification – please contact us for more information