Fish allergy: symptoms, diagnosis and avoiding hidden fish

A food allergy is the immune system wrongly identifying a food as a threat. The NHS describes it as the immune system overreacting to certain foods, and Anaphylaxis UK explains that chemicals such as histamine are released and cause symptoms. NICE CKS lists fish among the common allergens alongside cows' milk, eggs, peanuts and other legumes, tree nuts, shellfish and wheat.

Anaphylaxis UK says fish allergy often begins in childhood and may be lifelong, although some children outgrow it. It also says that it is not clear how likely outgrowing is or at what age, and that fish allergy can begin in adulthood too. A separate guide on this site covers shellfish allergy, which involves different proteins.

Fish allergy should not be confused with a food intolerance. The NHS notes that intolerance causes symptoms such as bloating and tummy pain, usually a few hours after eating, and is not an immune reaction.

Anaphylaxis UK lists mild to moderate symptoms as a red raised itchy rash (hives), swelling of the face, lips or eyes, a tingling or itchy mouth, mild throat tightness, and stomach pain, vomiting or diarrhoea. More serious symptoms, called anaphylaxis, are grouped as airway (swelling of the throat or tongue, hoarse voice, difficulty swallowing), breathing (wheeze, difficulty breathing, noisy breathing, persistent cough) and circulation (dizziness, faintness, sudden sleepiness, confusion, pale clammy skin, collapse).

The NHS adds that symptoms may start straight after eating or, for some food reactions, days later, although NICE CKS says classical IgE-mediated food allergy produces symptoms within seconds or minutes to 1 to 2 hours. Anaphylaxis UK says some clues that someone is at higher risk of a serious reaction are a previous serious reaction, asthma (especially if not well controlled) and having reacted to a tiny amount of fish.

Anaphylaxis UK says that people with fish allergy may react to vapours released while fish is being cooked nearby, because cooking can release the proteins into the air. Proteins have also been found in the air where there is a lot of unpackaged raw fish, such as fish markets, and may cause hay fever-like symptoms or asthma in some people. It notes that fish in ready-to-eat foods such as sandwiches does not create the same risk, and that not everyone reacts to airborne exposure.

Touching fish may cause skin irritation, itching or redness in some people, and the risk may increase with open cuts. The same source says touching fish is unlikely to cause anaphylaxis and advises washing hands with soap and water. People who work in the seafood industry are more at risk of developing the allergy because they are breathing in or touching fish proteins regularly, with symptoms that include asthma, rashes, rhinitis and conjunctivitis.

The NHS says a GP who thinks a food allergy is likely may refer to a specialist for tests, which may include a skin-prick test, blood tests, or a special diet in which the suspected food is avoided to see if symptoms improve, and a food and symptoms diary can help. Anaphylaxis UK says a GP can refer to a specialist allergy clinic and that if symptoms were serious a referral is important, while for mild symptoms the GP may arrange a blood test. An allergy consultant may suggest skin prick tests, blood tests and, if needed, food challenge tests.

NICE CKS says assessment should cover the suspected foods, symptoms, timing, whether exposures have been uneventful, how reproducible symptoms are, co-factors and other atopic conditions. It recommends referral to A&E for systemic symptoms or suspected anaphylaxis, and to an allergy specialist for a history of systemic symptoms, uncertain diagnosis, multiple suspected allergies or when testing and interpretation are needed. Self-testing or private kits are discussed in the allergy testing guides on this site.

Anaphylaxis UK also mentions two conditions that can mimic fish allergy: the herring worm Anisakis simplex, which can cause infection and allergic reactions in some people, and histamine (scombroid) poisoning from decomposing fish such as tuna and mackerel, which can affect anyone. A previous problem-free fish meal followed by a reaction is worth mentioning to a GP for this reason.

Once fish allergy is diagnosed, Anaphylaxis UK says you will probably be advised to avoid all fish, raw and cooked, and foods that contain it, because all fish contain similar proteins and there is a high chance of cross-contamination at fish counters and markets. It advises reading ingredient lists on packets every time, since recipes change, and asking staff directly when eating out, mentioning cross-contamination and that small amounts can cause a reaction.

Hidden sources it lists include fish sauce in many cuisines, Caesar dressing and Worcestershire sauce (which contain anchovy), scampi made with white fish, imitation crab or seafood sticks, kimchi made with fish sauce, sushi, and dishes such as kedgeree, paella, bouillabaisse and jambalaya. It warns that "seafood" can mean shellfish or fish, so always check.

Fish gelatin, sometimes used in marshmallows and nougat, can cause anaphylaxis in some people although others tolerate small amounts, so it should be discussed with the allergy specialist. Omega 3 supplements made from fish may be a problem; the source suggests choosing a non-fish source. Fish collagen, oil and scales may appear in cosmetics, and ingredient names such as "Pisces" or "Piscum" are worth knowing. The NHS adds that you should not cut foods out of your diet without speaking to a GP.

The NHS says people with a food allergy are given medicines to manage symptoms or use in an emergency: antihistamines for mild reactions and adrenaline auto-injectors such as EpiPen, or a nasal spray, for severe reactions. A specialist will provide an allergy action plan. Anaphylaxis UK says if you are prescribed adrenaline you must carry two in-date devices at all times, because a second dose may be needed if symptoms do not improve after five minutes or get worse. Medicines are mentioned here for information only; prescribing decisions belong to the clinician.

In an emergency the NHS says to use the adrenaline device immediately if prescribed, then call 999 for an ambulance even if you feel better. People with asthma should keep it well managed, as poorly controlled asthma can make a reaction worse. NICE CKS adds that people with food allergy should have a written allergy management plan and be reviewed at least annually.

On the NHS the route starts with the GP, who can arrange a blood test for milder cases or refer to an allergy clinic. Waiting times for NHS allergy clinics vary by area, and the NICE CKS referral criteria prioritise serious or uncertain cases. A dietitian may be involved where there are nutritional concerns or advice on avoiding or reintroducing allergens is needed.

Private allergy consultants can often offer a quicker appointment and the same range of tests, such as skin prick and specific IgE testing, and sometimes supervised food challenges where a clinic is equipped for them. The important point under either route is that tests should be interpreted by a clinician in light of the history, because a positive result alone does not always mean a person will react.