Shellfish allergy
Shellfish allergy is among the most common food allergies. "Shellfish" is a colloquial and fisheries term for aquatic invertebrates used as food, including various species of molluscs such as clams, mussels, oysters and scallops, crustaceans such as shrimp, lobsters and crabs, and cephalopods such as squid and octopus. Biologically, not all of these groups are closely related to each other, and allergies to different groups of shellfish may have different mechanisms of action. Shellfish allergy is an immune hypersensitivity to proteins found in shellfish. Symptoms can be either rapid or gradual in onset. The latter can take hours to days to appear. The former may include anaphylaxis, a potentially life-threatening condition which requires treatment with epinephrine. Other presentations may include atopic dermatitis or inflammation of the esophagus. Shellfish is one of the eight common food allergens, responsible for 90% of allergic reactions to foods: cow's milk, eggs, wheat, shellfish, peanuts, tree nuts, fish, and soy beans.
Unlike early childhood allergic reactions to milk and eggs, which often lessen as the children age, shellfish allergy tends to first appear in school-age children and older, and persist in adulthood. Strong predictors for adult-persistence are anaphylaxis, high shellfish-specific serum immunoglobulin E and robust response to the skin prick test. Adult onset of shellfish allergy is common in workers in the shellfish catching and processing industry.
Signs and symptoms
Food allergy symptoms usually occur within minutes after exposure but can be delayed to hours depending on the nature of the immune system response. Symptoms may include rash, hives, itching of mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea, or vomiting. Symptoms of allergies vary from person to person and may vary from incident to incident. Serious danger regarding allergies can begin when the respiratory tract or blood circulation is affected. The former can be indicated by wheezing, a blocked airway and cyanosis, and the latter by weak pulse, pale skin, and fainting. When these symptoms occur the allergic reaction is called anaphylaxis. Anaphylaxis occurs when IgE antibodies are involved, and areas of the body that are not in direct contact with the food become affected and show severe symptoms. Untreated, this can proceed to vasodilation, a low blood pressure situation called anaphylactic shock.Causes
Eating shellfish
The cause is typically the eating of shellfish or foods that contain shellfish. The shellfish types causing clinical allergy are, in decreasing incidence:- Shrimp
- Crab
- Lobster
- Clam
- Oyster
- Mussel.
Cross-contact
Cross-contact, also referred to as cross-contamination, occurs when foods are being processed in factories or at food markets, or are being prepared for cooking in restaurants and home kitchens. The allergenic proteins are transferred from one food to another.Shellfish parasite
The food-borne parasite Anisakis is a genus of nematodes known to be present in squid. Anisakis are directly infective to humans whenever infected squid are consumed raw or slightly processed, causing a condition called anisakiasis. Symptoms from consuming live nematodes include severe abdominal pain, nausea, and vomiting. In addition, there can be an allergic reaction to Anisakis proteins, even if the food in question was frozen or cooked before being consumed, killing the nematodes, as some of the nematode proteins are resistant to cold and heat. Allergic reactions can include hives, asthma and true anaphylactic reactions.Occupational exposure
An industry review conducted in 1990 estimated that 28.5 million people worldwide were engaged in some aspect of the seafood industry: fishing, aquaculture, processing and industrial cooking. Men predominate in fishing, women in processing facilities. Exposure to shellfish allergenic proteins includes inhalation of wet aerosols from fresh shellfish handling, and dermal contact through skin breaks and cuts. Prevalence of seafood-induced adult asthma is reported as in the range of 7% to 36%. Prevalence of skin allergy reactions, often characterized by itchy rash, range from 3% to 11%. The shellfish-induced health outcomes are mainly due to the protein tropomyosin causing an IgE mediated immune system response.Cross-reactivity to non-shellfish
, the major allergen in shellfish allergy, is also found in dust mites and cockroaches. Exposure to inhaled tropomyosins from dust mites is thought to be the primary sensitizer for shellfish allergy, an example of inhalant-to-food cross-reactivity. Epidemiological surveys have confirmed correlation between shellfish and dust mite sensitizations. An additional confirmation was seen in Orthodox Jews with no history of shellfish consumption, in that skin tests confirming dust mite allergy were also positive for shellfish tropomyosin. In addition to tropomyosin, arginine kinase and hemocyanin seem to have a role in cross-reactivity to dust mites.Exercise as contributing factor
Exercise can be a contributing factor to an allergic food response. There is a condition called food-dependent, exercise-induced anaphylaxis. For people with this condition, exercise alone is not sufficient, nor is consumption of a food to which they are mildly allergic, but when the food in question is consumed within a few hours before high-intensity exercise, the result can be anaphylaxis. Shellfish are specifically mentioned as a causative food. One theory is that exercise is stimulating the release of mediators such as histamine from IgE-activated mast cells. Two of the reviews postulate that exercise is not essential for the development of symptoms, but rather that it is one of several augmentation factors, citing evidence that the culprit food in combination with alcohol or aspirin will result in a respiratory anaphylactic reaction.Mechanisms
Scientifically, the various animals known as "shellfish" are scattered across different phyla and classes. Clams, mussels, oysters, and scallops are in the phylum Mollusca and class Bivalvia, squid and octopus are in the class Cephalopoda. Crustaceans are in the phylum Arthropoda and class Crustacea. Their chemistry is different and to understand the underlying mechanisms of various allergies to these animals requires an understanding of the diversity of the different proteins found and a knowledge of their proper scientific name and classification.Allergic response
Conditions caused by food allergies are classified into three groups according to the mechanism of the allergic response:- IgE-mediated – the most common type, manifesting acute changes that occur shortly after eating, and may progress to anaphylaxis
- Non-IgE mediated – characterized by an immune response not involving immunoglobulin E; may occur hours to days after eating, complicating diagnosis
- IgE and non-IgE-mediated – a hybrid of the above two types
The pathophysiology of allergic responses can be divided into two time periods: The first is an acute response that occurs within minutes after exposure to an allergen. This phase can either subside or progress into a "late-phase reaction" which can substantially prolong the symptoms of a response, and result in more tissue damage hours later. In the early stages of acute allergic reaction, lymphocytes previously sensitized to a specific protein or protein fraction react by quickly producing a particular type of antibody known as secreted IgE, which circulates in the blood and binds to IgE-specific receptors on the surface of other kinds of immune cells called mast cells and basophils. Both of these are involved in the acute inflammatory response. Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators called into the surrounding tissue causing several systemic effects, such as vasodilation, mucous secretion, nerve stimulation, and smooth-muscle contraction. This results in runny nose, itchiness, shortness of breath, and potentially anaphylaxis. Depending on the individual, the allergen, and the mode of introduction, the symptoms can be system-wide, or localized to particular body systems; asthma is localized to the respiratory system while hives and eczema are localized to the skin. In addition to reacting to oral consumption, skin and asthma reactions can be triggered by inhallation or contact if there are skin abrasions or cuts.
After the chemical mediators of the acute response subside, late-phase responses can often occur due to the migration of other white blood cells such as neutrophils, lymphocytes, eosinophils, and macrophages to the initial reaction sites. This is usually seen 2–24 hours after the original reaction. Cytokines from mast cells may also play a role in the persistence of long-term effects. Late-phase responses seen in asthma are slightly different from those seen in other allergic responses, although they are still caused by release of mediators from eosinophils.
In addition to IgE-mediated responses, shellfish allergy can manifest as atopic dermatitis, especially in infants and young children. Some will display both, so that a child could react to an oral food challenge with allergic symptoms, followed a day or two later with a flare up of atopic dermatitis and/or gastrointestinal symptoms, including allergic eosinophilic esophagitis.