Shellfish Allergy: Symptoms, Testing, and Treatment

Shellfish allergy is, by far, the most common adult onset food allergy. Approximately 2.3 percent of the general population is reported to have a shellfish allergy. This type of food allergy can develop at any time, and is often reported in those who have been able to safely consume shellfish in the past. Unlike a milk or egg allergy, when a child develops a shellfish allergy it is often persistent, and without intervention it can be lifelong.

What is a Shellfish Allergy?

A shellfish allergy reaction occurs when your body identifies specific proteins that it perceives as threats, and then tries to fight them off. Your immune system creates an antibody called Immunoglobulin E (IgE) that sets off an immune response. This response can result in a number of different allergic symptoms which range in severity. Although reactions like itching and swelling may seem to be small and just a nuisance, when multiple symptoms occur simultaneously that impact at least two bodily systems — or a single severe reaction such as respiratory distress or throat closing occurs — a shellfish allergy can lead to anaphylaxis and be life-threatening.

What Causes a Shellfish Allergy?

Shellfish allergy is typically caused by tropomyosin, a key protein found in the mollusk and crustacean families. Additional allergens in mollusks include amylase, arginine kinase, myosin heavy chain, and haemocyanin. Tropomyosin is also a shared protein in invertebrate shells.

As a result, insects (such as cockroaches) and arachnids (such as dust mites) can trigger a similar allergic reaction. This similarity creates a high correlation between dust mite allergies and shrimp allergies. Even vegetarians could develop a shrimp allergy through inadvertently breathing in dust mites! 

Can You Develop a Shellfish Allergy?

Even if you have never had any issue eating shellfish, it is possible to develop a shellfish allergy at some point in your life.  While shellfish allergies can impact anyone, it’s more common to have them develop in adults. In fact, it’s estimated that 60% of people who have shellfish allergies have their first allergic experience as an adult.

Are Fish and Shellfish Allergies the Same?

No. Someone who is allergic to shellfish may or may not be allergic to fin fish, and vice versa.

Can a Shellfish Allergy Go Away?

Shellfish allergies, like all food allergies, can change over time. Unfortunately, the majority of people who do develop a shellfish allergy will have it for their lifetime. Treatment options, including oral immunotherapy, do exist to help build up a tolerance to the allergen and reduce the risk of accidental exposure.

Shellfish Allergy Symptoms

Shellfish allergy reactions can range from mild to severe symptoms. The most common symptoms of a shellfish allergy reaction include: 

    • Hives or rash
    • Itching
    • Swelling in the lips, tongue, mouth, throat, or around the eyes
    • Nasal congestion
    • Wheezing or shortness of breath
    • Nausea or vomiting
    • Abdominal pain or discomfort
    • Diarrhea
    • Dizziness 

Severe shellfish allergies can result in anaphylaxis, a severe, life-threatening allergic reaction. In anaphylaxis, the immune system overreacts to proteins in shellfish and mollusks. This causes a rush of chemicals to fight off the invading proteins. Signs of anaphylaxis include:  

    • Pale or bluish tint on the skin 
    • Throat constriction
    • Shortness of breath
    • Difficulty breathing
    • Confusion and/or dizziness
    • Fainting or loss of consciousness
    • Weak, rapid pulse

Anaphylaxis is a medical emergency and must be treated with epinephrine immediately. With severe shellfish allergies, anaphylaxis can occur within minutes. Those with a shellfish allergy are advised to carry two epinephrine auto-injectors to ensure rapid treatment in case of accidental ingestion or contact. 

Shellfish Allergy Test

Shellfish allergy can be particularly challenging to diagnose. Although the signs of an allergic reaction tend to be more severe than other food allergies, the resulting symptoms can mirror those of other conditions. 

For example, histamine toxicity (scombroid poisoning, which is a form of food poisoning) can present in similar ways to shellfish allergy. Some fish contain high levels of histidine, which the body converts to histamine, creating a response that mimics an allergic reaction. Histamine toxicity most commonly occurs in fish that has not been refrigerated properly or in fish that has spoiled. Other forms of shellfish-related food poisoning that may seem like an allergy include paralytic shellfish poisoning, neurotoxic shellfish poisoning, and amnesic shellfish poisoning.

Oral allergy syndrome (OAS) can also mimic shellfish allergy. This type of food allergy is generally considered to be mild, and occurs when the mouth and throat come into contact with raw vegetables or fruits. OAS shares symptoms with shellfish allergy, such as itchiness of the mouth and throat and swelling in the mouth (in rare cases swelling of the throat), which can make it difficult to separate the two.

Complicating things further, some people who are severely allergic to shellfish don’t have to actually ingest it to have a reaction – they may react if they are in close proximity to shellfish being cooked or if their food touched shellfish.

An allergist can help diagnose a shellfish allergy and the gold standard of food allergy testing encompasses three ways to clinically determine whether someone has a true shellfish allergy or is suffering from another ailment.

Skin Prick Test

A safe and low-risk test that involves lightly pricking the skin with a suspected allergen. Because there is a risk of cross-reactivity with dust mites, results from a skin prick test when evaluating for shellfish allergies must be carefully analyzed and protected from cross-contamination. 

Blood Test

A small sample of blood can be tested to measure the amount of IgE antibodies that the immune system has deployed as an allergic response to shellfish.

Oral Food Challenge

A step-by-step process whereby the patient gradually consumes small amounts of the suspected allergen over several hours. Because severe allergic reactions to shellfish can become life-threatening, this is strictly performed under the supervision of a trained clinician. Additionally, the clinician must have determined that the patient is likely to safely – and successfully – pass the food challenge.

Types of Shellfish

The crustacean and mollusk families encompass a wide range of individual species. A patient can be allergic to the mollusk family or the crustacean family or to both shellfish families. 

Types of crustaceans:

    • Crab
    • Lobster
    • Shrimp
    • Crawfish
    • Krill
    • Prawns

Types of Mollusks:

    • Oysters
    • Snails
    • Clams
    • Scallops
    • Squid
    • Octopus
    • Mussels

Foods to Avoid with a Shellfish Allergy

While some foods clearly have shellfish in them, others may be more subtle. Some common foods that may contain shellfish include:

    • Soy sauce
    • Worcestershire sauce
    • Imitation crab
    • Ceviche
    • Fish stock
    • Fish sauce
    • Gumbo
    • Jambalaya
    • Paella
    • Clamato
    • Seafood flavorings

Those with a shellfish allergy must be vigilant about avoiding cross-contamination, particularly when dining out. Dishes, silverware, cooking pans, and oils that have come into contact with shellfish may induce an allergic reaction. As a result, certain types of foods that tend to rely heavily on shellfish may need to be avoided entirely. Chinese, Thai, Japanese, Vietnamese, Malaysian, Cajun or Creole restaurants are all examples of cuisine that use shellfish heavily. 

Shellfish can also be found in unlikely places such as pet food, vitamins and nutritional supplements, and even certain types of fertilizers. 

Shellfish Allergy Treatment with Oral Immunotherapy

At Latitude Food Allergy Care, we provide oral immunotherapy (OIT) to help patients improve their quality of life. OIT can be an effective treatment option for single or multiple food allergies and is safe for all ages. 92% of Latitude OIT patients successfully reach maintenance and are desensitized to their allergens. 

The goal of oral immunotherapy is to continually expose the patient to increasing amounts of an allergen. As OIT progresses, the body is gradually desensitized and learns to adapt. This allows for fewer dietary restrictions, and protects against severe or life-threatening reactions due to accidental exposure. If clinically relevant, patients can choose to combine their oral immunotherapy with Xolair, an anti-IgE monoclonal antibody that helps inhibit allergic reactions.

Is OIT Safe for Treating Shellfish Allergies?

Long-term treatment of shellfish allergy (as opposed to strict avoidance) is an ongoing field of study, but positive results can be seen from OIT. To be a successful candidate for oral immunotherapy treatment, patients must consume small amounts of shellfish on a daily basis.

To determine whether a patient may be a candidate for OIT, our expert clinical team will evaluate existing health conditions, collect a full medical history, and discuss lifestyle. 

Are you or a loved one suffering from a suspected shellfish allergy? Take the next step toward better health, safety, and quality of life with Latitude Food Allergy Care. Our expert clinicians are available to discuss your testing and treatment options. Contact us today to schedule a no-obligation conversation with one of our Patient Care Coordinators.

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