Understanding Fish Allergy

How It Differs From Other Types of Food Allergy

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Fish
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A fish allergy, as opposed to a shellfish allergy, is one in which your immune system reacts abnormally to a finned fish such as tuna, halibut, or salmon. It is a somewhat less common form of food allergy, affecting women more than men and adults more than children. Fish allergies often develop during early childhood but, in contrast to a milk or egg allergy, usually persist well beyond school age. Symptoms can range from mild to severe and may include skin rashes, respiratory symptoms, gastrointestinal distress, and anaphylaxis (which requires emergency attention).

The allergy is more common in areas where fish is a predominant part of the local diet, such as Scandinavia and parts of Asia. An allergy may not only be triggered by eating fish but by touching fish or consuming foods in which fish byproducts are used.

Causes

Allergies are caused by an abnormal immune response to an otherwise harmless allergy trigger, known as an allergen. When this happens, the immune system will release a substance known as immunoglobulin E (IgE), which causes mast cells and other blood cells to break open and release histamine into the bloodstream.

The normal role of histamine is to dilate blood vessels so that larger immune cells can access the site of an injury or infection. In the absence of an injury or infection, histamines can trigger the skin, respiratory, and gastrointestinal symptoms we recognize as allergic reactions.

The primary allergen responsible for a fish allergy is a protein known as parvalbumin. Parvalbumins vary little between different fish species, meaning that an allergy to one fish will usually result in an allergy to other fishes (a condition known as polysensitization).

Parvalbumins are extremely heat-tolerant and do not break down easily even after hours of cooking. As such, you may be just as sensitive to a piece of baked salmon as you are to raw sashimi.

Interestingly, while you may be allergic to different types of fish, having a fish allergy does not predispose you to a seafood allergy. A seafood allergy involves an entirely different allergen known as tropomyosin, which is found in crustaceans and mollusks.

Fish Associated With Allergy

Among the more than 20,000 known fish species, there are several for which the risk of allergy is especially high. According to research published in the Frontiers of Immunity, they include:

  • Carp
  • Cod
  • Flounder
  • Halibut
  • Herring
  • Mackerel
  • Pilchard
  • Redfish
  • Salmon
  • Sea bass
  • Swordfish
  • Tilapia
  • Trout
  • Tuna

Other Foods

Beyond the fish itself, fish or fish byproducts are often used to make such popular foods or condiments as Worcestershire sauce, Caesar salad dressing, caponata (Sicilian eggplant relish), taramasalata, and nuoc châm sauce.

Fish gelatin, derived from the bones and cartilage of fish, is often used as a setting agent in pepperoni, hot dogs, and other types of processed meats. Similarly, fish meal is commonly used in certain organic fertilizers.

Symptoms

Fish allergy symptoms are similar to those of other food allergies, and most occur within an hour of eating. They include:

  • Hives (urticaria) or a skin rash
  • Generalized itching
  • Stuffy nose and sneezing (allergic rhinitis)
  • Headaches
  • Breathing difficulty (asthma)
  • Indigestion and stomach pain
  • Belching, bloating, or flatulence
  • Diarrhea
  • Nausea or vomiting

Symptoms may also develop by simply inhaling cooked fish or coming into contact with surfaces or utensils used to prepare fish. In some cases, a skin reaction called contact dermatitis may develop by simply touching fish or fish residue.

In some people, a fish allergy may turn severe and lead the rapid development of anaphylaxis. This dangerous whole-body reaction is characterized by widespread rash, facial and tongue swelling, wheezing, shortness of breath, rapid heart rate, delirium, and a feeling of impending doom.

If not treated immediately with an injection of epinephrine and other emergency interventions, anaphylaxis can lead to shock, coma, cardiac or respiratory failure, and even death.

Diagnosis

A fish allergy can often be recognized by the appearance of symptoms soon after you have eaten or been exposed to fish. To confirm that fish is the cause, an allergist may recommend two minimally invasive tests:

  • Skin-prick tests involve the introduction of a small amount of a suspected allergen beneath your skin. If you are allergic to one of several of the test samples, you will develop an inflamed bump (called a wheal) within 15 to 60 minutes.
  • Blood antibody tests are used to check for the presence of an immune protein called an anti-parvalbumin antibody, which your body produces in response to a fish allergen.

If the tests aren’t conclusive, your allergist may recommend an oral food challenge. This is a procedure in which you eat a small amount of fish to see if you have a reaction. Because the response may be severe, it is only performed in the presence and under the direction of a medical professional who can deliver emergency treatment if needed.

An oral test should never be performed as an in-home experiment.

Differential Diagnoses

To ensure that fish is the source of your symptoms and not some other condition, your doctor may want to explore other possible causes. One such example is scrombroiosis, a type of food poisoning in which high levels of histamine are produced as a fish begins to spoil.

Other reactions may be caused by naturally occurring fish toxins that can cause poisoning in humans (including ciguatera found is sea bass and mackerel, which causes rash and respiratory symptoms in 50,000 people each year).

Less commonly, some people may experience cross-reactivity to chicken and fish in which a true allergy to chicken parvalbumins can sometimes trigger a cross-reactive response to fish.

Treatment

Since parvalbumins are found to varying degrees in different fish species, you may be well served to exclude all fish from your diet if you are allergic. This includes any sauces, condiments, or prepared foods that contain fish or fish byproducts.

Fish is one of eight allergens that must be listed on all food ingredient labels under the Food Allergen Labeling and Consumer Protection Act of 2004. Under the law, the manufacturer must identify the type of fish used and ensure that is it clearly indicated in easy-to-understand language.

If your allergy symptoms are mild, you can often treat them with an over-the-counter oral antihistamine. If you are at risk of a severe reaction, you may be advised to carry a single-use, pre-filled epinephrine injector, such as an EpiPen, which you would inject into your thigh in case of an emergency.

Unlike other forms of allergy, food allergies cannot be treated with allergy shots (immunotherapy).

Coping

While a fish allergy may be less common than a seafood or nut allergy, it can be every bit as serious. To this end, you should take steps to protect yourself if you have been diagnosed with a fish allergy.

Among some of the more useful tips:

  • Always read product labels. If you don't know if an ingredient on a label is fish, look it up on your smartphone.
  • Let someone else do the fish shopping. Even in the cleanest of seafood departments, surfaces, containers, and wrappings are likely to be exposed to fish residue.
  • Avoid fish restaurants. Even if you choose the beef, the deep fat fryers and griddles will likely have been used to cook fish. There are simply far too many opportunities for cross-contamination to take the risk.
  • Be careful in Asian restaurants. Many traditional dishes, including spring rolls and curries, regularly combine meat and seafood. Many soups are also made with fish-based stocks or contain bonito flakes (dried fermented tuna). If you are unsure what an ingredient is on a menu, ask.

A Word From Verywell

While some allergies can be annoying, others can be downright deadly. If you have had anaphylaxis in the past or are at risk, consider wearing an emergency medical bracelet outlining your condition, recommended treatments, and drugs to avoid.

In this way, if you are unconscious or unable to reply, the emergency medical team can take appropriate action without losing precious time.

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