ASDA Health Matters

This could get more complicated by separating molluscs into different groups, but that's not important in understanding seafood allergy - all you need to know is that if you are allergic (suffer an unpleasant or nasty reaction) to one group, it doesn't necessarily mean you'll be allergic to the other. Similarly, you can be allergic to crustaceans, but not to molluscs. Some people are allergic to both fish and shellfish, although some experts suggest this could be two separate allergies.

So, how is this so? In shellfish, a specific protein is the cause of allergic reactions, which is called an allergen, but the same protein is not present in fish - a different protein is responsible.

Food preparation and cooking methods can influence whether a reaction occurs. The allergens in fish can be destroyed when heated or canned, which means they don't react. Similarly, the allergens in shellfish can stabilise when heated, so you could find that you react to raw prawns but not to cooked ones. But remember - allergens are not always destroyed, so eating cooked or canned seafood is not a method to be relied upon for avoiding reactions.

Cross-reactivity

So, you now know fish and shellfish are different but there's an important word to remember - cross-reactivity. This simply means if you are allergic to one type of fish you may be allergic to other types as well, because the allergy-causing proteins are similar across many different fish. In just the same way, the proteins in shellfish are similar, so if you're allergic to prawns you could - but not necessarily - be allergic to mussels. Interestingly, the proteins in seafood are also present in insects, such as the cockroach and house-dust mite!

Prevalence

The prevalence of allergic reactions to fish and seafood in the UK is relatively unknown, although several surveys have found that reactions to fish and crustaceans are more common than reactions to molluscs, and that reactions to all shellfish are more commonly reported in adults - possibly because children generally only develop the taste for shellfish as they approach adulthood. In the USA though, it is estimated that prevalence is 2.3% for any seafood allergy and in parts of the world where seafood makes up a large part of the diet, such as Scandanavia and Singapore, prevalence is higher.

Causes of reactions and symptoms

When an allergen is eaten, it meets another substance in the body, called IgE (an antibody). Together, they produce lots of different chemicals which cause unpleasant and sometimes life-threatening symptoms. Those who are allergic to seafood produce higher levels of IgE in their bodies.

Mild symptoms include:

• Nausea and sickness
• Diarrhoea
• Stomach pain
• Wheezing
• Flushing and skin rashes

In more severe cases, an anaphylactic attack may occur. This affects the whole body and typically includes swelling and a drop in heart rate, as well as the symptoms above. If the swelling occurs around the throat and mouth, breathing will become difficult and medical attention will be required. Anaphylaxis usually occurs within minutes, but an attack can occur several hours after the allergenic food has been eaten.

If you've been diagnosed with anaphylaxis, you may have been given self-injectable adrenaline by your doctor to carry with you at all times. It is also wise to wear an identification bracelet/necklace stating the food to which you are sensitive.

Avoiding reactions

When eating out, ring the restaurant beforehand and explain the situation to the chef. Explain that your meal needs to be prepared using separate equipment and utensils, to avoid cross-contamination. For example, if oil has been used to fry prawns and you order a mushroom dish, are you confident that the same oil or frying pan will not be used?

Beware of stocks and soups too - you'd be surprised at what these contain! Those who are extremely sensitive to seafood may wish to ask their friends and family not to order fish or shellfish too, as there's a small risk that the allergens can be inhaled and cause a reaction.

Popular foods that commonly contain different kinds of fish are paella, bouillabaisse, kedgeree and many Oriental and Asian dishes.

When shopping, don't assume that the absence of an allergy advice symbol means the product is free of allergens. Allergy advice symbols or statements, are not a legal requirement. However, since 2005 all pre-packed fish and shellfish and products containing fish or shellfish must be labelled in their ingredients list, so always check this, especially if a product promotes its ‘New improved recipe' because the ingredients may have changed - don't be caught out!

In summary, the only way to avoid any risk of a reaction is to avoid all seafood, seafood products and food at risk of cross-contamination. This may seem unfair - but it's much better to be safe than sorry!