Fish allergy occurs when the immune system overreacts to proteins in fish. Symptoms may appear quickly or take hours to days to develop. Rapid symptoms can include anaphylaxis, a serious condition that needs epinephrine treatment. Other symptoms may involve atopic dermatitis or inflammation of the esophagus. Fish is one of eight common food allergens responsible for 90% of food allergies: cow's milk, eggs, wheat, shellfish, peanuts, tree nuts, fish, and soy beans.

Unlike allergies to milk and eggs, which often decrease with age, fish allergy usually begins in school-age children and lasts into adulthood. Signs that the allergy may persist into adulthood include anaphylaxis, high levels of fish-specific serum immunoglobulin E (IgE), and a strong reaction to skin prick tests. It is not clear if introducing fish to babies' diets between 4 and 6 months old reduces the risk of developing a fish allergy later. Adults who work in fishing or processing fish are more likely to develop a fish allergy later in life.

Signs and symptoms

Food allergies often cause symptoms to begin within minutes to hours after eating something a person is allergic to. This type of reaction, called IgE-mediated, can be very serious and may lead to anaphylaxis. Common symptoms include a rash, hives, itching in the mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of the lips, tongue, eyelids, or face, trouble swallowing, a runny or stuffy nose, a hoarse voice, wheezing, difficulty breathing, diarrhea, stomach pain, feeling lightheaded, fainting, nausea, or vomiting. Reactions that are not IgE-mediated may take hours to days to develop after eating the allergen and are usually less severe. Symptoms can differ between people and even between different allergic reactions.

Anaphylaxis is a life-threatening allergic reaction. It often causes trouble breathing, such as wheezing or difficulty breathing, and may lead to blue skin color (cyanosis). It can also cause problems with blood flow, such as a weak pulse, pale skin, or fainting. This happens when special proteins in the immune system, called IgE antibodies, trigger symptoms in parts of the body not directly touched by the allergen. If not treated, the reaction can cause blood vessels to widen, leading to very low blood pressure, a condition known as anaphylactic shock.

Causes

The cause is usually eating fish or foods that contain fish. Once a person has an allergic reaction to fish, the sensitivity often lasts a lifetime. The immune system reacts strongly to certain proteins in fish, especially parvalbumin, but sometimes to other proteins like fish collagen. Allergic reactions to shellfish and crustaceans, such as lobster and shrimp, are caused by a different protein called tropomyosin. Because these proteins are different, people who are allergic to fish are not usually allergic to shellfish or crustaceans.

Cross-contact, also called cross-contamination, happens when foods are processed in factories, markets, or prepared in restaurants and homes. Allergenic proteins can move from one food to another during these processes.

The food-borne parasite Anisakis is a type of worm found in saltwater fish, certain fish that swim from oceans to rivers to breed, and squid. These worms can infect humans if raw or slightly processed infected fish or squid is eaten, causing a condition called anisakiasis. Symptoms include severe stomach pain, nausea, and vomiting. Allergic reactions to Anisakis proteins can also occur, even if the fish or squid is frozen or cooked, because some of the worm’s proteins are not destroyed by heat. These reactions may include hives, asthma, or severe allergic responses.

A review from 1990 estimated that 28.5 million people worldwide worked in some part of the seafood industry, including fishing, aquaculture, processing, and cooking. Men are more common in fishing jobs, while women are more common in processing plants. Exposure to fish allergens can happen through breathing in wet or dry air from fish handling or processing, or through skin contact from cuts or breaks. About 10% of adults who work with seafood may develop asthma, with higher rates for those handling crustaceans and lower rates for those handling fish. Skin reactions, such as itchy rashes (hives), occur in 3% to 11% of people. These health issues are mainly caused by the protein parvalbumin, which triggers an immune response.

Exercise can sometimes worsen an allergic reaction to food. A condition called food-dependent, exercise-induced anaphylaxis occurs when someone eats a food they are mildly allergic to and then exercises intensely within a few hours. This can cause a severe allergic reaction. Fish are specifically linked to this condition. One theory suggests that exercise causes the body to release substances like histamine from immune cells. Some studies say exercise is not the only factor that can worsen reactions, as eating the allergenic food with alcohol or aspirin may also cause severe allergic symptoms.

Mechanisms

Food allergies are grouped into three types based on how the immune system reacts:

• IgE-mediated (classic) – This is the most common type. It causes quick symptoms after eating, which can sometimes lead to a severe reaction called anaphylaxis.
• Non-IgE mediated – This type involves the immune system but not IgE antibodies. Symptoms may appear hours or days after eating, making it harder to diagnose.
• IgE and non-IgE-mediated – This combines features of the first two types.

Allergic reactions happen when the immune system overreacts to harmless substances, like proteins in food. Scientists are still studying why some proteins cause allergies while others do not. One idea is that proteins that survive digestion in the stomach and reach the small intestine are more likely to cause allergies. However, cooking and digestion can sometimes reduce, increase, or not change a protein’s ability to cause an allergy. Cooking can also change the structure of proteins, possibly making them less likely to cause allergies.

Allergic reactions have two main stages. The first is an acute response, which happens immediately after contact with an allergen. This can end quickly or lead to a late-phase reaction, which may last longer and cause more tissue damage. During the acute phase, immune cells called lymphocytes produce a type of antibody called secreted IgE (sIgE), which attaches to receptors on mast cells and basophils. These cells release chemicals like histamine, cytokines, interleukins, leukotrienes, and prostaglandins, causing symptoms such as runny nose, itching, trouble breathing, and potentially anaphylaxis. Symptoms can affect the whole body (like anaphylaxis) or specific areas, such as the skin (hives, eczema) or lungs (asthma). Allergies can also be triggered by breathing in or touching allergens if the skin is broken.

After the acute response, late-phase reactions may occur 2–24 hours later. Other immune cells, like neutrophils, lymphocytes, eosinophils, and macrophages, may move to the affected area. Cytokines from mast cells might also contribute to long-term effects. In asthma, late-phase reactions involve eosinophils releasing mediators.

Fish allergies can sometimes cause atopic dermatitis, especially in young children. Some children may have both an immediate allergic reaction and later skin or digestive issues, such as allergic eosinophilic esophagitis.

A protein called parvalbumin is a major cause of fish allergies but not shellfish allergies (which are caused by tropomyosin). Parvalbumin is not broken down by heat or digestion, so cooking does not reduce its ability to cause allergies. Most bony fish (like tuna) have β-parvalbumin as a major allergen, while cartilaginous fish (like sharks) have α-parvalbumin. Allergies to bony fish rarely cross-react with cartilaginous fish or chicken.

Bony fish have between 7 and 22 genes for parvalbumin, including alpha, beta-1 (oncomodulin), and beta-2. In most bony fish, beta-2 parvalbumin is the main allergen, but in some, alpha parvalbumin is more common. Allergen naming depends on the order of discovery, so the same number may refer to different genes in different fish.

Other fish proteins, such as enolase, aldolase, and collagen, can also cause allergies. Fish collagen is used in foods like gummy candies and may be in supplements or medicines. Standard allergy tests using parvalbumin may not detect collagen allergies. People may be allergic to parvalbumin, collagen, or both.

Scombroid food poisoning (also called scrombroid) is a reaction that resembles an allergy but is not caused by the immune system. It happens when spoiled fish high in histidine (a protein) are eaten. Bacteria in the fish convert histidine into histamine, which causes symptoms like flushed skin, headache, itching, blurred vision, stomach pain, and diarrhea. These symptoms usually start 10–30 minutes after eating. Common fish linked to scombroid include tuna, mackerel, sardines, and herring. Cooking, smoking, or freezing does not remove histamine from spoiled fish.

Diagnosis

Doctors diagnose a fish allergy by looking at a person's past allergic reactions, doing a skin prick test, and checking for fish-specific IgE in the blood. To be sure of the diagnosis, a special test called a double-blind, placebo-controlled food challenge is used. Sometimes, people who say they are allergic to fish may not actually be allergic when tested this way.

Prevention

When fish is added to a baby's diet, it may influence the chance of developing allergies, but different advice exists. Studies on allergies in general suggest that starting solid foods between 4 and 6 months may lower the risk of allergies later. Studies focused on when fish is introduced show that eating fish during the first year of life may reduce the risk of eczema and allergic rhinitis later. However, eating fish during pregnancy does not seem to have the same effect on these conditions.

Treatment

Treatment for accidental eating of fish products by people with allergies depends on how sensitive the person is. A medicine called diphenhydramine, which is an antihistamine, may be given by a doctor. Sometimes, prednisone may be prescribed to help prevent a delayed allergic reaction. Severe allergic reactions, called anaphylaxis, may need treatment with an epinephrine pen, which is a device that gives a shot to help during emergencies. Unlike with egg allergies, where research is being done on oral immunotherapy (a treatment that involves eating small amounts of the allergen over time), a 2015 review found no published studies testing this method for fish allergies.

Epidemiology

Incidence and prevalence are terms used to describe how diseases spread in a population. Incidence refers to the number of new cases diagnosed in a specific time period, often measured as new cases per year for every million people. Prevalence refers to the total number of people living with a disease at a given time, measured as existing cases per million people during a specific period. Studies report that self-reported fish allergy rates in the general population range from 0% to 2.5%. Self-reported allergy rates are always higher than confirmed allergy rates found through food challenges. Two studies found confirmed allergy rates to be 0.1% and 0.3%, respectively.

Regulation

Whether the number of people with food allergies is increasing or not, awareness of food allergies has grown, affecting the quality of life for children, their parents, and their caregivers. In the United States, the Food Allergen Labeling and Consumer Protection Act (FALCPA), passed in August 2004 and effective January 1, 2006, reminds people about allergies every time they read food labels. Many restaurants now include allergen warnings on their menus. The Culinary Institute of America, a top school for chef training, offers classes on cooking without allergens and has a special kitchen for this purpose. Schools also have rules about which foods can be brought into school. Even with these steps, people with severe allergies know that accidental exposure can still happen at homes, schools, or restaurants.

To address the risks that certain foods pose to people with allergies, some countries have created laws requiring food labels to clearly state if products contain major allergens or parts of major allergens added intentionally. However, there are no laws that require labels to show small amounts of allergens that may be present due to cross-contamination.

FALCPA requires companies to list on food labels if a packaged product contains any of eight major allergens added intentionally: cow's milk, peanuts, eggs, shellfish, fish, tree nuts, soy, or wheat. This list was created in 1999 by the World Health Organization's Codex Alimentarius Commission. To follow FALCPA rules, if an ingredient comes from one of these allergens, the label must either include the "food sourced name" in parentheses, such as "Casein (milk)," or include a separate statement next to the ingredient list, such as "Contains milk" (and any other listed allergens).