What Is Shellfish Allergy

Introduction

A shellfish allergy is an abnormal reaction of the immune system following ingestion of, or exposure to, shellfish. 

An allergy is a hypersensitive reaction of the immune system to something that is usually harmless. The reaction is caused by the immune system identifying a substance as a threat and creating antibodies (proteins that help eliminate unwanted substances from the body) as a protective measure. Antibody production starts a chain reaction involving the release of different chemicals in the body, which produce a range of symptoms.

Substances which create this immune response are called allergens. Many different substances, including foods, can be allergens. Shellfish is a common food allergen.

An allergy to shellfish is more common in adults than children, although it can be experienced at any age. According to the Mayo Clinic, shellfish are the most common adult food allergy. Shellfish allergies in children may be less common simply because children usually eat less of these foods. Once developed, a shellfish allergy usually lasts for life.¹ 

Shellfish is a colloquial term for aquatic animals, including crustaceans (such as crab, lobster, and prawns) and molluscs (such as oysters, scallops, mussels, clams, cockles, snails, squid and octopus). 

Some people have a general crustacean allergy or mollusc allergy, where they are allergic to the whole class. Others are allergic to certain species only. Allergy to crustaceans is more common than allergy to molluscs. (Food Allergy Research and Education website)

According to the American Academy of Allergy, Asthma & Immunology, there is a high likelihood of cross-reactivity of shellfish allergens. This is due to the presence of proteins that commonly cause reactions (such as Tropomyosin) in many species of the same class.² This means that someone with an allergy to one species has a high risk of also reacting to other species within the same class. For example, someone with an allergy to prawns may also have an allergy to other crustaceans, such as crabs. 

Types of shellfish allergies

All shellfish allergies involve an immune system reaction involving a complex series of different biological processes. These reactions can be classified as IgE-mediated, non-IgE-mediated or a mix of both, depending on whether the antibody, Immunoglobulin E (IgE) is produced.³

IgE-mediated reactions usually occur rapidly, with symptoms appearing within hours of exposure to an allergen, sometimes within minutes.³ Symptoms can be experienced in multiple organs, including the skin, respiratory system and gastrointestinal system. These reactions can also lead to the life-threatening state of anaphylaxis (an-a-fi-LAK-sis), further information on which is provided below.

Non IgE-mediated reactions usually produce symptoms less rapidly after ingestion. Symptoms are most common in the gastrointestinal system, although the skin and respiratory system can also be involved.⁴ 

More information about the two types of allergic response can be found on the World Allergy Organization website. 

Causes of shellfish allergy

The likelihood of having allergic reactions, in general, may be influenced by an individual’s genetic predisposition. However, according to the American College of Asthma, Allergies and Immunology, food allergies are not directly inherited by a child from parents. 

The reasons for the development of food allergies are complex and not fully understood. A combination of both genetic and environmental factors is thought to be involved.⁵ Research in this area is ongoing.

Shellfish allergy symptoms 

Shellfish allergies can produce symptoms in the respiratory system, cardiovascular system, gastrointestinal tract and skin. Symptoms can range in severity, from mild to life-threatening. 

In most cases, symptoms will appear within two hours of exposure and are often experienced within minutes.⁶ However, a delay of several hours before onset is possible.⁷

While most people will experience symptoms after eating shellfish, a reaction can be triggered in some people, even after touching it or coming into contact with the steam from cooking it.

Symptoms include: 

• Weak pulse
• Dizziness, fainting
• Skin appearing pale or blue
• Shortness of breath, wheezing, cough
• Nasal/sinus congestion
• Tightness in the throat
•  Difficulty swallowing, breathing, talking
• Swelling of tongue, lips, throat, face
• Tingling/itching of mouth, tongue, lips
• Itching/irritation of the skin
• Hives (also called urticaria) - an itchy skin rash
• Stomach cramps, nausea, vomiting, diarrhoea
• Anaphylaxis - a severe, rapid and potentially life-threatening reaction

Anaphylaxis and shellfish allergy

An anaphylactic reaction is a sudden, severe allergic reaction involving multiple systems of the body. This type of reaction is treated as a medical emergency. Without urgent treatment, it can be life-threatening. 

Symptoms of anaphylaxis include:

• Skin that is flushed, blue or pale in appearance
• Hives, rash, itching of skin
• Wheezing, coughing or difficulty breathing
• Swelling of mouth, throat, tongue
• Swollen eyes, lips, hands, feet
• Low blood pressure
• Weak, rapid pulse, heart palpitations
• Dizziness, lightheadedness, fainting
• Abdominal pain, nausea, vomiting, diarrhoea
• Anxiety, confusion, slurred speech
• Collapse or unconsciousness

Anaphylaxis usually occurs rapidly, within minutes of exposure to an allergen, but onset is occasionally more delayed. It is generally caused by an IgE-mediated allergic response. 

Shellfish allergy diagnosis

• Skin prick testing - small amounts of allergen are placed on the skin and the skin is penetrated by a needle prick. The area is monitored for signs of skin reaction, such as redness and swelling. Only reliable for IgE-mediated allergies.
• Blood tests - to check levels of the antibody Immunoglobulin E (IgE)
• Oral food challenge testing - suspected allergenic food is deliberately eaten under supervised conditions in a healthcare setting. The individual is monitored for symptoms of an allergic response.⁸

Skin prick testing and blood testing can only be used to confirm IgE-mediated allergies.⁷ IgE-mediated, non-IgE-mediated and mixed shellfish allergies can be diagnosed by Oral Food Challenge testing (OFC). 

Shellfish allergy testing is not used in all cases. Many people with milder symptoms do not receive a formal diagnosis. 

Shellfish allergy management

Many allergic reactions to shellfish are mild and do not require any treatment. However, options are available to address symptoms if needed. Severe reactions require urgent medical treatment.

Treatment of shellfish allergy mild reactions 

Mild allergic reactions can be treated with antihistamines or steroids (also called corticosteroids). Treatment to restore fluid loss may also be needed when dehydration occurs due to severe gastrointestinal symptoms.

Treatment for shellfish allergy anaphylaxis 

An anaphylactic reaction is a medical emergency that is potentially life-threatening and requires immediate treatment. Emergency Services should be contacted (in the UK, dial 999).

Adrenaline (also known as epinephrine) is usually the first treatment for anaphylaxis. People who know that they have a severe allergy often carry an emergency adrenaline auto-injection device (such as an EpiPen). Even after administration of adrenaline, individuals should still be assessed at an Accident & Emergency department (A&E).

Immunotherapy treatment for shellfish allergy

Some food allergies are now being treated with immunotherapy to desensitise the immune response. These treatments progressively expose the immune system to the known allergen so that tolerance is developed. However, according to a 2020 review of this area, immunotherapy is not yet established as a treatment for shellfish allergy.¹ Research in this area is ongoing. 

Living with a shellfish allergy

Shellfish allergy prevention requires avoidance of the known allergen. This involves being vigilant with checking ingredients in food and understanding how it is prepared. 

Shellfish cross-contamination can occur when other foods come into contact with shellfish. This is possible during food storage, processing or preparation. For example, use of the same cooking oil or equipment or direct contact of different items whilst on display at a fish counter. 

In the UK, businesses selling food are required by law to tell consumers when allergens such as shellfish are included in their products. Further information on food labelling in the UK can be found here: www.food.gov.uk/business-guidance/allergen-guidance-for-food-businesses. 

FAQs

How common is shellfish allergy?

A 2014 study of the prevalence of food allergies in Europe showed that 1.3% of the population self-reported as having a lifelong shellfish allergy.²

Rates of shellfish allergy vary across different countries and are significantly higher in locations with higher shellfish consumption.⁶

Can I be allergic to one type of shellfish and not another?

It is possible to be allergic to one species of shellfish and not to others. However, the main protein allergens found in shellfish are common to many species within the same class. If you are allergic to one species, you, therefore, have a high likelihood of being allergic to others in the same class.¹⁰

When should you seek medical help for shellfish allergy

A severe allergic reaction to shellfish can result in anaphylaxis, a sudden and potentially life-threatening condition which requires immediate medical treatment. Emergency medical help should be sought for anyone with suspected anaphylaxis.

If you experience milder symptoms of a shellfish allergy, you can consult your GP for further evaluation and advice. 

Summary

Shellfish allergy is a food allergy which arises from a hypersensitive response of the body’s immune system. Symptoms can occur immediately or can be delayed by several hours and may be experienced in the skin, respiratory system, gastrointestinal system, or a combination of these.

In severe reactions, someone with a shellfish allergy can develop anaphylaxis, a sudden and potentially life-threatening reaction which requires immediate medical attention.

Shellfish allergies are usually managed through avoidance of known allergens.