Shellfish Allergy: Symptoms, Causes, Risks, Treatment, Complications and Prevention



In recent years, the prevalence of shellfish allergy has risen. Data on the prevalence of shellfish allergy is limited, partly due to the lack of controlled studies in the population in which oral challenges can be performed. A recent review found that, among children, the prevalence of shellfish allergy is lower than 0.5%. In 2004, a telephone survey of 14, 984 people conducted in the United States showed that 2% of the population have shellfish allergies. In Singapore, 39% of 227 children with food allergy were shown to have a shellfish allergy. In Spain, epidemiological studies have shown that shellfish allergy is the 3rd leading cause of food allergies and that its prevalence has significantly increased.
In other studies in Australia, 85% of reactions to food were caused by shellfish.
Other studies show that 87% of food allergies were caused by shellfish. Shellfish allergy is more common in adults, however, around 5% of children have it.


Allergic reactions to shellfish can be mediated by non-immunologic or immunologic mechanisms. It occurs as a result of exposure to the components of the ingested product or to the shellfish itself. Shellfish allergies can be caused by many substances, such as protochordate, parasites, viruses, bacteria, toxins, preservatives, flavorings, and colorings.

There are studies that suggest that crustaceans are the most common causes of allergies to food and that the reaction to shellfish is more severe compared to other types of food.

After shellfish are ingested, symptoms develop and they are similar to those that occur with other food allergies. Reactions are rapid and occur within a few minutes after eating. Reactions occur almost always within 2 hours. The clinical presentation of shellfish includes skin involvement, mouth or oral allergy syndromes, digestive symptoms, asthma, anaphylaxis, and rhinitis.

Allergic symptoms occur not just after ingestion. They can occur after exposure to vapors in the house or elsewhere. This exposure triggers skin, respiratory, and systemic reactions, but the latter is rare.


The pathogenesis, or cause, of shellfish allergy, is similar to those which occur with other food allergies. Most of the allergic reactions to shellfish are immediate and occur within 2 hours after ingestion. However, late reactions have been reported up to 8 hours after exposure.

The cause of shellfish allergy is an immunologic reaction to proteins found in foods.
These immunologic reactions are mediated by the immune system. These disorders are induced by two main mechanisms: non-IgE-mediated or IgE-mediated. However, the most efficient way to classify immune disorders is IgE-mediate, non-IgE-mediated, and mixed disorders.

IgE-mediate disorders occur when allergens specific to a food found on the surface of basophils and mast cells bind to the food allergens. This activates cells to release cytokines as well as other potent mediators such as histamine.

The symptoms typically occur right away after food ingestion, which results in swelling, urticaria, and a wheezing cough. In some cases, hypotension occurs. Most IgE-mediated, food induced allergic reactions occur within a few minutes or hours after the food was ingested. IgE-mediated food allergy is not considered when symptoms occur more than 4 hours after the food was ingested.

On the other hand, non-IgE-mediated food allergies occur as a result of the activation of T cells, which produces cytokines, such as IL-4, IL-5, and IL-3. These latter three further the allergic response. A cascade of events also results in the infiltration of eosinophils.
These reactions, however, are generally slower in ones and are usually gastrointestinal in nature. In mixed disorders, the IgE and non-IgE-mediated mechanisms work together to further the disease process.

4Risk Factors

The risk factors for shellfish allergy is divided into environmental and genetic factors. Environmental factors include exposure to aerosols via the respiratory tract or the skin.
In children with atopic dermatitis, the risk for shellfish allergies is increased. A major area of study includes the high incidence of food allergies among children with eczema.
A potentially protective factor is the increased number of siblings, which decreases sensitization in the home.

A potentially protective factor is the increased number of siblings, which decreases sensitization in the home.

The lack of exposure to shellfish is another risk factor. Studies have shown that improved outcomes occur if oral exposure is started early on.

The prevalence of Clostridium species in the gut is another major source of food allergies. There are several factors that may influence this, such as breastfeeding, Caesarean section, and early child or maternal antibiotic exposure. It has been shown that children who are sensitized to food have decreased microbiota variety at age 3 months.

Studies also suggest that early mucosal exposure to shellfish may hinder the development of allergies.

Genetics may also play a role, as it has been shown that shellfish allergies are increased in children of parents who are atopic. Having a sibling with a food allergy is also correlated with having a shellfish allergy. 


Once a diagnosis of food allergy has been established, the only proven treatment is to avoid the allergen. Patients and caregivers should be educated about reading labels and avoiding high-risk situations such as buffets. Early management of allergic reactions should also be taught. Strict avoidance is the only proven therapy. However, this is not always possible, especially with the availability of highly processed food.

As a result of this, new approaches to treatment are being investigated. Immunotherapy for food allergies was described in the 1930s. Standard rush immunotherapy was used to desensitize a young girl. After the rush therapy, the child was able to tolerate odors of cooked fish and shellfish without any reactions. Rush immunotherapy is the administration of multiple injections on a daily basis in order to rapidly reach a maintenance dose.
The risk, however, of adverse reactions is higher compared to standard immunotherapy protocols. Thus, these patients are often given prophylactic doses of corticosteroids or antihistamines.

Children are at a greater risk for adverse reactions associated with rush immunotherapy. Many new considerations are being experimented. These include plasmid DNA vaccines, anti-IgE with immunotherapy, and mutated recombinant food proteins.

These novel approaches may help desensitize children and adults with fewer risks for systemic reactions.


The most serious complication of shellfish allergy is anaphylaxis and even anaphylactic shock. Food allergy is the leading cause of anaphylaxis, and many emergency room visits are due to this. It is the most severe form of a reaction to food that is mediated by IgE. Anaphylaxis due to food, including shellfish, is an important cause of death in the United States, which accounts for 150 deaths per year. It is estimated that about 300,000 cases of anaphylaxis occur each year, and 2,000 patients need hospitalization.

Anaphylaxis is a rapid IgE-mediated allergic reaction that involves many organ systems.
It can be fatal if not treated right away. The onset of anaphylaxis is only a few seconds after ingesting shellfish or other food products to which the person is allergic to.
The complications of anaphylaxis include pruritus, or itching, angioedema, or swelling around the eyes, and urticaria. These symptoms occur in about 80% of cases.

Cardiovascular symptoms may also occur due to anaphylaxis. These include arrhythmia, shock, hypotension, and syncope. In some cases, hypotension was identified as the primary symptom that a person is going into anaphylaxis.

Vasodilation, or increase in the diameter of blood vessels, can decrease the circulation blood volume by about 35% within about 10 minutes after ingestion. Therefore, fluid resuscitation and epinephrine administration is important.


If a person has shellfish allergies, precautions should be taken in order to avoid allergic reactions. Avoiding shellfish is not as easy as it seems. When dining out, it is important to be cautious about what is being eaten. The pain, oil, and utensils that were used for cooking shellfish should not be used for preparing food.
This may create cross-contamination. It may also be necessary to avoid eating at seafood restaurants, as this carries a high risk of cross-contamination with shellfish.

Labels should always be read and checked for shellfish components. Cross-contamination can also occur in stores where shellfish is processed or when food is displayed near shellfish. Therefore, labels should be carefully read.

Rarely, shellfish may be a hidden ingratiate in seafood flavoring or fish stock.
Companies are required to label the food that they produce according to the contents.
This is especially true for shellfish and other foods that may cause allergic reactions. However, these regulations do not apply to oysters, clams, scallops, and mollusks.

It may be necessary to avoid places where there are shellfish or where it is being processed. Some people react after simply touching shellfish or inhaling the steam from a pot with shellfish.

It is also important to always carry an epinephrine pen, since this may save lives if an anaphylactic reaction occurs. It is also prudent to wear a medical alert bracelet or necklace. This will let other people know of the shellfish allergy. Preventing shellfish allergic reactions is not simple. There are many things, places, and foods to avoid.