Seafood, roughly divided into the three categories of molluscs, crustaceans, and scaly fish, is one of the most common types of food allergy. While true food allergies only affect approximately 2% of the general population, seafood allergies are the third leading cause of food allergy, after eggs and milk. Not surprisingly, there is a higher incidence of seafood allergy in countries like Japan, Spain, and Scandinavian countries that feature seafood as a staple part of their diet. In these countries, about one person in every thousand people has fish allergy symptoms.

Once an individual becomes sensitised to a seafood allergen then the allergy will most likely persist for the rest of their lives. If fact, the allergy may become more pronounced with each subsequent exposure to the allergen. It has been suggested that atopic children should avoid seafood for the first three years of their life. Generally speaking, fish allergies seem more common in children than adults, while adults appear more prone to developing shellfish allergies than children.

The number of people sensitised to seafood allergies has increased correspondingly with the greater number of people now consuming seafood or working in the seafood industry. Workers who process fish, crabs, prawns or mussels are constantly exposed to the potentially allergenic components in seafood. Aerosolised particles of seafood and cooking fluids can create sensitisation through inhalation. Workers who handle seafood such as fish, crustaceans or molluscs without appropriate protection have occasionally developed occupational dermal allergies, being more likely to do so if the surface of the skin is broken. The frequency of occupational protein contact dermatitis is estimated to be around 10% of workers, while occupational asthma is believed to affect up to a third of workers in the seafood industry.

Seafood Allergy Symptoms

Seafood allergies commonly cause rashes, itching, rhinitis, urticaria (hives) and angioedema (swelling) of the face or throat, although nausea, dizziness, diarrhoea, and abdominal pains may also be symptoms. Conditions such as asthma and atopic eczema may worsen because of a seafood allergy. Handling seafood that you are allergic to can cause contact urticaria of the hands. Some people are so sensitive to an allergen that they cannot be exposed to traces of fish, even to the extent where the fumes of fish being cooked can trigger an allergic reaction.

The symptoms of seafood allergy can manifest themselves anywhere within a few minutes to two hours of ingesting or handling the seafood responsible for the reaction, although with molluscs the reaction can be delayed by up to six hours. The symptoms may persist for anything from a few hours to a day.

At its most dangerous, seafood allergy can trigger a life-threatening episode of anaphylaxis. This is a systemic reaction involving a dramatic drop in blood pressure and those susceptible to anaphylaxis should carry injectable epinephrine (adrenaline) in the form of a readily accessible EpiPen in case of an emergency.

Seafood Allergy Categories

The types of seafood that cause allergies can be divided into three groups. The mollusca group contains three different sub-classes of seafood covering species as diverse as oysters, abalone and squid. The crustacea group includes prawns, crabs, crayfish, and shrimps. The third group, the chordata, includes all common edible fish, including salmon, cod, tuna, and flounder. Cod is the most common type of fish allergy, followed by species like trout, salmon, mackerel, herring, haddock, and halibut. Sometimes people with a seafood allergy relating to one particular species are able to eat other species of seafood without any problems.

The following table categorises the most common edible seafood that can cause an allergic reaction into three groups.

Classification of the types of seafood that cause allergies

INVERTEBRATES
Group	Class	Species
Mollusca	Gastropoda Bivalvia (Shellfish) Cephalopoda	Abalone (Perlemoen), Snails (Escargot) Mussels, Oysters, Clams, Scallops, Cockles Squids (Calamari), Octopus, Cuttlefish
Arthropoda	Crustacea	Crabs, Lobsters, Shrimp, Prawn, Saltwater Crayfish (Rock Lobsters), Freshwater Crayfish (Yabbies)
VERTEBRATES
Group	Class	Species
Chordata	Condrichthyes Osteichthyes	Sharks, Rays Salmon, Trout, Pike, Cod, Haddock, Hake, Snapper, Mackerel, Tuna, Bonito, Grouper, Sole, Flounder, Halibut, Plaice

(Note that the degree of relation between each can be judged from the distance apart.)

Most seafood allergens belong to a group of muscle proteins. For example, parvalbumin is the allergen protein in codfish, while the proteins in crustaceans are called tropomyosin. While people with seafood allergy may only be sensitive to a few specific proteins, those proteins may be present in multiple species, which is why they may be at risk of allergic cross reactivity.

The dominating allergen in fish is the allergenic molecule Gad c1 found in cod and its homologues, and previously referred to as allergen M. Some individuals with cod allergy are known to have cross-reactivity with other fish species, including herring, mackerel and plaice. Other people with an allergy to shrimp will also exhibit cross-reactivity with other crustaceans, including crab, lobster, and crayfish. There is some evidence to suggest that the protein tropomyosin is a cross-reactive allergen for both crustacean and mollusc allergy.

Seafood Allergy Cross-reactivity

It has been estimated that if someone has an allergy to fish then they have about a 50% chance of being allergic to at least one other species of fish. If they are allergic to shellfish then they will have approximately a 75% chance of having an allergy involving another shellfish. There appears to be little, if any, cross-reactivity between fish and shellfish but concomitant allergies are always possible.

Research has ascertained some common cross-reactivity clusters. Among these are:

Salmon, sardine, and mackerel
Cod and tune
Octopus and squid
Crab and shrimp

These groups give an indication of groups of allergens to test for IgE reactivity when screening for seafood sensitisation.

There are incidences where food allergies have been linked to what has been referred to as "house dust mite-crustaceans-molluscs-syndrome". It has been estimated that 80% of food allergies in adults are preceded by a clinical or sub-clinical sensation to inhaled allergens. This is because the food allergy is caused by cross-reactivity between ingested food and inhaled particles.

Non-allergic Reactions

There can be a number of complications in detecting whether or not someone actually has a seafood allergy. One of the difficulties of diagnosing a seafood allergy is the common occurrence of cases of non-allergic food poisoning from eating seafood. Seafood is believed to be the most common cause of food poisoning in the United States, responsible for at least one in six cases of food poisoning. A condition known as histamine fish poisoning (HFP), also referred to as scombrotoxism or scombroid fish poisoning, is caused by the toxins present in fish and shellfish. This is caused by a chemical reaction that occurs after eating bacterially contaminated fish with dark meat, producing symptoms similar to those found in allergic reactions. Most frequently the spoiled fish comes from the scombridae and scomberesocidae fish families, which include the mackerel and the tuna. Other non-scombroid fish, including sardines, bluefish, and mahi-mahi, which have darker meat can cause histamine poisoning if they are allowed to spoil.

Histamine fish poisoning usually only causes a mild form of food poisoning, which may be misdiagnosed as a food allergy. The fish themselves are non-toxic when they are caught but will increase in the amount of histamine they contain over time as their bacterial content increases, usually because they have received an inadequate level of refrigeration. This allows marine bacteria in the gut of this fish to convert the amino acid histadine into histamine. A fish with a large content of histamine may be hard to detect because the high level of histamine may not be apparent from the look or odour of the fish. When ingested, however, the fish may have an abnormal metallic or peppery taste. Antihistamines can be taken to effectively break down the histamine intoxication that causes histamine fish poisoning.

Precautions taken in cooking and preparing the fish will not remove the histamine because, as with most seafood, the toxins are very stable. Raw fish tends to be more allergenic than cooked fish because cooking the fish tends to alter the allergenic proteins it contains. On the other hand, the limpet, which is a mollusc, has been found in some studies to be more allergenic after it was cooked.

An adverse reaction may also result from eating seafood that has been contaminated with algae-derived neurotoxins, causing ciguatera poisoning or paralytic shellfish poisoning. Ciguatoxin occurs after reef fish that have consumed dinoflagellates are eaten by other fish and pass into the human food chain. On the West Coast of the United States, filter-feeding shellfish such as mussels and oysters may contain more toxins during the late summer due to the presence of toxins produced by the bloom of dinoflagellates in their tissues. This toxic algae causes a reddish discolouration of the coastal waters known as "red tide".

These algae-derived neurotoxins tend to be fat-soluble and heat-stable, meaning that they are unaffected by how the seafood is cooked. They can interfere with how a person's nerve endings function within a couple of hours of consuming seafood containing them.

Bacterial or viral contamination of seafood is also a problem. Salmonella and other bacterial contamination may occur if shellfish is harvested from waters that contain untreated sewage.

Another complication regarding seafood allergy comes from the possible presence of the anisakis parasitic worm in some species of fish. The nematode anisakis simplex is an intestinal parasite known to induce IgE-mediated reactions and is found worldwide in hosts all along the food chain. An anisakis infection, also known as "herring worm disease" or anisakiasis, is particularly a problem in countries where fish is a dietary staple and is often eaten raw or undercooked. Such an infection can cause nausea, vomiting, and stomach pain. The anisakis parasite is quite hardy and temperatures as hot as 60 degrees Celsius or as cold as an industrial freezer are often required to kill it. Such an infection can cause nausea, vomiting, and stomach pain.

Anisakis can result in symptoms similar to seafood allergy because of a reaction to the parasite, even if the worm is already dead. These reactions often occur intermittently rather than on every occasion someone consumes seafood because of the fact that anisakis is not always present. A skin or blood test will test negative for seafood allergy because it is the anisakis itself that causes the reaction.

Identifying Seafood Allergens

Another complication with identifying a cause of seafood allergy is that patients often identify seafood by their common names, such as "lobster", rather than by the much more accurate species name. The application of names like "lobster" and "crayfish" may vary from place to place, depending on whether confusion or marketing acumen has taken advantage of the fact that these two animals have a similar appearance. The term "tuna", for example, is used to describe several different tuna species but, fortunately, canned tuna is considered the least allergenic form of fish.

Someone with seafood allergy must be aware of hidden seafood allergens. An example of this is the anchovies contained in Worcestershire sauce, Caesar salad dressing and the "Bloody Mary" cocktail. Surimi, an imitation crabmeat made using Pacific whiting or pollock, contains fish and egg white. Both surimi and codfish incorporate a common allergen known as 63-kDa protein. Pollock has also been used as a component of beef and pork substitutes present in some hot dogs, pizza toppings, and ham. Some adhesives used on the back of envelopes or stamps contain fish proteins. Fish extracts like isinglass can also be used in the fining process for wine.

Sometimes the risk comes from other food being in close proximity to seafood. For example, some fish and chip shops cook the chips in the same oil and restaurants with a large selection of seafood should be alerted of your allergy before you dine there. Prawns and other seafood may be barbecued and seafood allergy sufferers should cook their food in foil to avoid contamination. Many Asian foods feature shellfish as common ingredients like, for example, the use of prawns in soups and rice dishes.

So-called "aquarium allergy" relates to an allergy to the components used in the food used to feed pet fish and is more likely if an individual has a pre-existing respiratory condition like asthma. This pet food typically contains the protein of a variety of arthropod species, in both adult and larvae forms, including chironomus, daphnia, and brine shrimp, and fresh water worms like tubifex and enchytraea. Some other pet foods designed for cats and dogs may also contain fish products and could put a seafood allergy sufferer at risk if they were come into close contact with the animal.

Although seafood is a rich source of natural iodine, individuals with seafood allergies are not at greater risk of iodine allergy because of the different way that the iodine used in antiseptic medicines and x-ray contrast agents work.

It is possible that an individual may only have intolerance towards seafood rather than an allergy that involves the immune system. Seafood intolerance can result from the pharmacological properties of the food consumed. Canned or pickled fish may contain large amounts of tyramine or histamine, which can cause headaches. Additional ingredients such as the food dye tartrazine or food additives like monosodium glutamite (MSG) can commonly cause adverse reactions, including one colloquially referred to as "Chinese restaurant syndrome".

Diagnosis of Seafood Allergy

There are a number of techniques that can be used to diagnose seafood allergy. No matter which method is used, a detailed discussion of your medical history will be needed to assist the doctor or allergist in finding the allergens that trigger your condition. This will help to ascertain whether the reaction is caused by an allergy or by a toxin in the food. Skin prick tests, Radioallergosorbent test (RAST) and ImmunoCAP Specific IgE blood tests may be used. A positive reaction to a skin test needs to be followed by detailed testing with individual seafood species. A double-blind placebo-controlled food challenge (DBPCFC) is a method that eliminates both patient and administrator bias. It involves gradually feeding increasing doses of food suspected of causing a reaction at predetermined time intervals to see whether it is tolerated or whether it results in allergy symptoms. Patients with a confirmed allergy to a particular type of seafood should also be tested for allergies to other species, because of the possibility of concomitant allergies.