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 (IgE) 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. King Crab Once an allergic reaction has occurred it usually remains a lifelong sensitivity. The immune system overreacts to proteins found in shellfish, most commonly to tropomyosin, but often to other proteins, such as arginine kinase, myosin light chain and sarcoplasmic calcium-binding protein. The allergic reaction to fish is to a different protein, parvalbumin; there is no cross-reactivity between fish and shellfish allergy. It is important to identify the specific type of animal causing the reaction, as not all "shellfish" are biologically related and allergenic proteins can differ significantly between species. King crab, while commonly called a crab, it is not a “true crab” (belonging to the anomuran group rather than Brachyura), and it contains a unique allergen (malate dehydrogenase, MDH) not found in true crabs.
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 (or marine fish) 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% (higher for crustaceans and lower for bony fish). Prevalence of skin allergy reactions, often characterized by itchy rash (hives), 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 Tropomyosin, 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.
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