Shellfish allergy: symptoms, testing, avoidance and emergency treatment in the UK

A food allergy is caused by the immune system overreacting to a food. The NHS says it is not clear why this happens, but certain foods are more likely to cause a reaction, and shellfish such as prawns, crab and lobster are among the most common. Anaphylaxis UK explains that the immune system wrongly identifies the food as a threat and releases chemicals such as histamine, which cause the symptoms.

Shellfish is a broad term. Anaphylaxis UK divides it into crustaceans (crab, lobster, crayfish and prawns) and molluscs, which include bivalves such as mussels, oysters, razor shells, scallops and clams, gastropods such as limpets, periwinkles and snails found on land, and cephalopods such as squid, octopus and cuttlefish. Which types you react to varies from person to person.

It is also different from a food intolerance. The NHS notes that intolerance causes symptoms such as bloating and tummy pain, usually a few hours after eating, whereas allergy involves the immune system and can be serious.

Anaphylaxis UK says symptoms of shellfish allergy usually come on quickly, within minutes of eating the food. The NHS adds that symptoms can sometimes appear straight after eating or days later in some food allergies, and can affect different parts of the body at the same time.

• A red, raised, itchy rash (hives) anywhere on the body.

• Swelling of the face, lips or eyes.

• A tingling or itchy feeling in the mouth, or mild throat tightness.

• Stomach pain, vomiting or diarrhoea.

• More serious signs: swelling in the throat or tongue, a hoarse voice, difficulty swallowing, wheezing, breathing difficulty, noisy breathing or a persistent cough, dizziness, confusion, pale clammy skin or collapse.

The NHS says anaphylaxis is a life-threatening allergic reaction that happens very quickly. Call 999 if your lips, mouth, throat or tongue suddenly swell, you are breathing very fast or struggling to breathe, your throat feels tight or you cannot swallow, your skin, tongue or lips turn blue, grey or pale, you become very confused, drowsy or dizzy, or someone faints and cannot be woken.

If you have an adrenaline auto-injector or nasal spray, use it immediately, then call 999 even if you feel better. The NHS steps are to use the injector, call 999 and say you think it is anaphylaxis, lie down with legs raised, and use a second injector if there is no improvement after 5 minutes. Do not stand or walk at any time, even if you feel better.

The NHS advises seeing a GP if you think you or your child may have a food allergy, and a GP may refer you to a specialist for tests. Anaphylaxis UK says the consultant discusses your medical history and may suggest skin prick tests, blood tests and food challenge tests to diagnose the allergy and work out how serious it may be.

It also lists signs that suggest a higher risk of serious reactions: a previous serious reaction, asthma that is not well controlled, and reacting to a tiny amount of shellfish. The NHS adds that you may be asked to keep a food and symptom diary.

After diagnosis you will need to avoid the types of shellfish you are allergic to and foods containing them. Anaphylaxis UK says shellfish are on the list of major allergens, so they must be highlighted on ingredient labels, and you should read labels every time because recipes change. Restaurants, cafes, takeaways and other caterers are required to provide information on major allergens, and you should ask staff directly and mention that small amounts can cause a reaction.

Hidden sources it lists include scampi (which is a small type of lobster, a crustacean), oyster sauce, fish sauce (which can be made with shellfish), stocks and soups, and dishes such as paella, kedgeree, bouillabaisse, gumbo and jambalaya. Cross-contamination is a risk: chips may be fried in oil used for scampi, stir fries may contain traces of prawns, and shellfish often touch at fish counters.

The NHS suggests telling friends, family, nursery, school and work about the allergy, telling restaurant and airline staff, carrying 2 adrenaline auto-injectors or nasal sprays at all times if prescribed, and following the allergy action plan from your specialist. It also advises not cutting foods out of your diet without speaking to a GP, and wearing medical alert jewellery.

Anaphylaxis UK says that if you work in the seafood industry you are more at risk of developing an allergy because of regular exposure, with asthma, rashes and eye or nose symptoms among the signs to report to your GP.

The NHS route is a GP appointment, then referral to a specialist allergy clinic for tests and an action plan. Adrenaline is available on prescription.

Some people choose private allergy testing for speed. Anaphylaxis UK recommends referral to a specialist allergy clinic, so choose a clinician with allergy training, and avoid unvalidated tests. Make sure any results and action plan are shared with your GP.