Seafood is one of the most common causes of severe allergy reactions in adults. Approximately 12 million Americans suffer from food allergies, with 6.9 million allergic to fish and/or shellfish.
A seafood allergy occurs when the immune system mistakenly interprets the proteins contained fish or shellfish as a harmful substance. When a person with this allergy comes in contact with seafood, the body produces antibodies to fight the harmful substance, and this triggers an allergic reaction.
Allergic reactions to fish most commonly occur upon ingestion, but can also be triggered by the inhalation of fish protein while being cooked, or upon direct contact to the skin. The most common reactions include rash (atopic dermatitis), redness and swelling around the mouth, hives (urticaria), wheezing and difficulty breathing, stomachache, cramping, diarrhea or vomiting, asthma, and in extreme cases, anaphylaxis. Seafood allergy, if not treated promptly, can be fatal. Severe reactions generally occur within minutes after consuming the allergen.
Unlike most food allergies which appear in childhood and resolve later in life, seafood allergy commonly persists throughout adulthood. And, unlike many other foods, the onset of fish allergy is common during adulthood. Women are affected more commonly than men.
Classifications of seafood
Seafood causing allergic reactions can be classified into three groups: Fish, Shellfish, and Mollusks.
Fish: Included in this group are the bony fish which commonly cause allergic reactions such as cod, salmon, pollock, snapper, eel, and tilapia, to name just a few, and cartilaginous fish such as shark.
The most allergenic fish is thought to be cod. Some fish, specifically tuna and mackerel, are considered to be less allergenic than others. But, because of the high frequency of cross-reactivity, individuals with allergies to one type of fish are likely to have allergies to others. This cross-reactivity is caused by the protein parvalbumin that is present in many fish. For this reason, most people with an allergy to one fish are advised to avoid all fish, including eel and shark. Many fish allergic individuals can eat shellfish and/or mollusks, and vice versa, but one must be allergy tested before assuming that there is not an allergy to more than one seafood group.
Shellfish or Crustaceans: This category includes shrimp, lobster, crayfish, prawn, and crabs. Tropomyosin, the protein that most commonly causes shellfish allergies, is also found in dust mites and cockroaches, and there is some evidence of cross-reactivity between shellfish and some insects.
Mollusks: Clams, oysters, scallops, abalone, cockle, cuttlefish, escargot (snails), octopus, squid, and mussels are included in this category.
Less obvious sources of seafood allergens
Individuals with seafood allergy must be especially diligent about avoiding the ingestion of seafood proteins which may be in other foods. The list below, although not inclusive, demonstrates how widespread the presence of fish is in the foods we commonly eat.
Worcestershire sauce: Contains fish (anchovies).
Steak sauce: May contain seafood.
Caesar salad: Often contains anchovies either on the salad or in the dressing.
Caviar: Made of fish eggs.
Marinara sauce: Sometimes contains anchovies.
Roe: Unfertilized fish eggs.
Surimi (a type of imitation crab popular in some Asian restaurants): Contains fish proteins for added flavoring.
Hot dogs, bologna and ham: Can contain fish flavoring (surimi).
Pizza toppings: Sometimes contains anchovies.
Fried rice and spring rolls: May contain shrimp.
Caponata (a Sicilian relish): Can contain anchovies.
Gelatin-based foods (e.g., marshmallows): May contain seafood.
Pet fish food: This dried substance is made of brine shrimp and other seafood. It can easily become airborne and, if inhaled, may trigger symptoms in some people with seafood allergies.
Pet food: Dog and cat food may contain seafood.
Glucosamine, a dietary supplement sometimes recommended for patients with arthritis, is often made from the shells of crustaceans. The proteins that are most likely to cause food allergies are not found in the shell, and recent studies have indicated that glucosamine is safe for people with shellfish allergies; however, if you are concerned, you can try vegetarian glucosamine.
Another potential source of shellfish allergens is Omega-3 supplements, which are often made from seafood. The most common source used to manufacture these is fish (mostly cod liver), but check ingredients on the label before you take these.
Masqueraders of seafood allergy
There are conditions which can mimic an allergic reaction to seafood. The following two examples are illustrative.
Anisakis simplex allergy and infestation can masquerade as an allergic reaction. Anisakis simplex is a fish parasite which is found world-wide, and infests a variety of hosts along the food chain. Between 5 and 80 % of fish samples have been found to be infested in various studies. Cooking at high temperatures or storage in industrial freezers is required to kill the parasite. Anisakis can cause two major problems in humans: allergic reactions and infection with the parasite.
Anisakis simplex allergy resembles other allergic reactions to food. Allergic reactions to Anisakis simplex should be suspected when allergic-like reactions occur after eating seafood, yet the results of skin tests to seafood are negative. Reactions often occur intermittently after eating seafood, rather than on every occasion.
Anisakis simplex infection, or anisakiasis, results from eating raw or undercooked seafood such as sushi. Infection can cause nausea, vomiting, and abdominal pain. Diagnosis is based on seeing the parasite using a fiberoptic telescope during endoscopy.
While not all reactions to seafood are allergic in origin, some can resemble allergic reactions. Scrombroid fish poisoning is a classic example. Scombroid fish poisoning is an allergic-like reaction that occurs after eating foods with high histamine content.
Scombroid poisoning is caused by the ingestion of scombroid and scombroid-like marine fish species that have begun to spoil with the growth of particular types of bacteria, generally due to the lack of proper refrigeration. Fish most commonly involved are members of the scombridae family (tunas and mackerels) and a few non-scombridae relatives (bluefish, dolphin or mahi-mahi, and amberjack).
The suspect toxin is an elevated level of histamine generated by bacterial breakdown of substances in the muscle protein. This natural spoilage process is thought to release additional by-products, which cause the toxic effect. Unfortunately, freezing, cooking, smoking, curing or canning does not destroy the potential toxins.
Affected fish often have a metallic or peppery taste. Symptoms usually commence within 30 minutes of eating. Because the symptoms are caused by the chemical histamine, they can be identical to an allergic reaction. The most common symptoms include flushing, itching, urticaria or hives, nausea, vomiting, abdominal cramps, dizziness, palpitations and headache. Severe episodes may result in wheezing and a drop in blood pressure.
Reading labels
As one of the "big eight" most common food allergens, fish is covered under the Food Allergy Labeling and Consumer Protection Act (FALCPA). This requires that manufacturers label the presence of fish in clear language on food labels, either in the list of ingredients or following the word "contains" after the ingredient list. Fish is not a particularly common hidden ingredient and generally appears as its own species in ingredient lists. But people with fish allergies should learn the names of many different types of fish for maximum safety in reading labels.
Be careful!
There are recorded instances of inhalation reactions due to aerosolized fish proteins, so people with fish allergies should avoid hibachi-style communal grill restaurants if fish is on the menu. Seafood restaurants and sushi bars are high risks for cross-contamination due to the close proximity of fish and non-fish items. Another source of potential cross-contamination is frying oil; if fish has been fried in oil, people with fish allergies should avoid eating any other food fried in the same oil.
