Summary

Atlantic salmon (Salmo salar) is a commercially important fish, which is popularly consumed due to its good source of vitamins, high-content of proteins, and omega-3 polyunsaturated fatty acids. Salmon is native to subarctic regions of the North Atlantic Ocean, Canada, and Russia and salmon aquaculture is significant in some countries. Worldwide, fish allergy prevalence is 0.2%. Fish allergy prevalence is higher in countries where fish is consumed as a staple food and where a high number of fish-processing industries are found. Salmon allergy is primarily induced through oral route and secondarily through exposure to vapors while cooking and boiling fish or fish processing and through contact. Ingestion of salmon by sensitized individuals triggers allergic symptoms such as oral allergy syndrome, angioedema, urticaria, anaphylaxis, nausea, and gastrointestinal symptoms (nausea, vomiting and abdominal pain). Also, occupational exposure to an aerosolized salmon allergen or skin contact can lead to asthma, rhinitis, and skin rash. The major allergen identified is Sal s 1, a parvalbumin, which is a highly stable and heat resistant protein found in salmon muscle tissue. Fish parvalbumin is a panallergen, which is responsible for cross-reactivity among different fish species. About 50% of patients hypersensitive to cod are reported to react to salmon. Cross-reactivity between finned fish and shellfish such as crustaceans is also reported. Hence, individuals with fish allergies are advised to avoid fish in any form in the diet.

Epidemiology

Worldwide distribution

Prevalence of fish allergy is found to be higher in countries where there is high fish consumption and many fish-processing industries are found. Fish allergy prevalence is 0.2% worldwide, while in the United States (US) the prevalence was 0.4%, and among Asian countries, the Philippines (2.29%) has the highest fish allergy prevalence compared to Singapore (0.26%) and Thailand (0.29%). In European countries, the prevalence of fish allergy in the adult population is about 0.8% and is a little higher than the children. The countrywide prevalence of clinically confirmed fish allergy in the pediatric population in Europe was found to be 0.7 % in Sweden (4 years old), 0.6% in Germany (2-6 years old), and 5% in Finland (1-4 years old). In Norway, fish allergy prevalence is reported to be 0.1%. A nationwide, cross-sectional, random telephone survey performed in the US found that seafood allergy is reported by 2.3% of the general population and among them, 2% was for shellfish, 0.4% for fish and 0.2% for both types A study by Turner et al. (2011) in Australia reported IgE-mediated fish allergy in 95 out of total 167 children with a history of food or seafood allergy. Among these fish allergic patients, 38 children were found to be allergic to salmon or tuna.

In the USA, salmon along with tuna, catfish, and cod are the most important species reported for causing allergy, followed by flounder, halibut, trout, and bass. A retrospective study by Khan et al. (2011) in the US evaluated 5162 adult patients (≥18 years) with a seafood allergy and found a fish allergy prevalence of 13.8%. Among the fish allergy patients, the prevalence of salmon allergy was found to be 23.8%. A retrospective study from Australia showed 5.6% of children (among 2999 food-allergic children) were allergic to fish, particularly salmon, tuna, and white fish. A cohort study in Europe detected 12% of patients (among 62 patients) to be mono sensitized to salmonids (salmon, trout).

Route Of Exposure

Main

The primary route of exposure is ingestion of raw, cooked, or processed salmon when the allergen is exposed to the gastrointestinal (GI) tract.

Secondary

Inhalation of wet aerosols from the fish heading, degutting, and boiling at fish processing unit and also skin contact while handling fish.

Clinical Relevance

Oral allergy symptoms and Anaphylaxis

Sensitization to fish usually begins at the childhood stage and the patients mostly remain allergic in their entire life. Fish allergic patients on ingestion of fish or on coming in contact with fish or fish products may develop a single symptom or several clinical manifestations, including urticaria and eczema. Clinical manifestations of fish allergy typically include:

Ingestion of salmon (raw, cooked, or processed) in sensitized individuals may lead to angioedema, rhinitis, OAS, urticaria, anaphylaxis, nausea, and gastrointestinal symptoms.

A retrospective study by Turner et al. (2011) analyzed 167 seafood (shellfish and fish) allergic children and reported that tuna along with salmon, prawn, and whitefish were the most frequent seafood allergens. About 21% of children had reported anaphylaxis. Other symptoms reported were angioedema and urticaria. Gastrointestinal symptoms such as nausea, vomiting, and abdominal pain were reported in 12% of children. Ingestion of salmon (including canned) reported inducing lip angioedema, itchy throat, and perioral hives. Also, among all the tuna and/or salmon allergic children, about 21% were able to tolerate a canned form of fish.

In one US study with 5162 seafood allergic patients, skin manifestation was most commonly reported by the fish allergic patients, followed by respiratory manifestations. Out of 5 salmon allergic patients, all showed symptoms related to skin, and 1 showed respiratory symptoms.

Allergic rhinitis

Occupational exposure to wet aerosols from fish heading and degutting, and boiling fish in sensitized workers can lead to respiratory symptoms such as asthma, rhinitis, dyspnea, wheezing, tightness of throat as well as skin rash. Ocular or upper respiratory symptoms such as rhinorrhea and nasal pruritus induced by vapor from seafood were reported in 16% of children in a study with 167 seafood allergic children.

Asthma

Inhalation of vapor during cooking and processing seafood can induce upper and lower airway symptoms. A study by Dahlman-Hoglund et al. (2012) analyzed exposure of aeroallergens and respiratory symptoms among 26 workers in the salmon processing industry. The study found that workers were exposed to a high level of salmon allergen at the filleting machines and tables. Asthma was reported in 3 workers, 26 workers were atopic, 17 workers reported respiratory symptoms among atopic or smokers. 13 workers reported onset of respiratory symptoms after they were employed. Upper and lower respiratory tract symptoms were reported in 58% and 38% of workers respectively. Nine workers had a cough, 7 had dyspnea, 11 had a frequent cough with phlegm, and 9 had wheeze anytime.

A survey by Douglas et al. (1995) on 291 employees working in an automated salmon processing plant in the UK reported 24 (8.2%) individuals with occupational asthma. Aerosolized salmon-serum proteins were found near the machines. The onset of symptoms was within 2 weeks to 3 months on commencing the job. The risk factor for developing asthma and IgE antibody production was found to be smoking.

Atopic Dermatitis

Fish aeroallergens may trigger atopic dermatitis in some individuals.

Prevention And Therapy

Prevention strategies

Avoidance

If an individual is suffering from a fish allergy, it is advisable to avoid fish in any form in the diet.

Cross-Reactivity

Patients allergic to finned fish, such as tuna may show 50% cross-reactivity to other finned fishes such as salmon and cod. Currently, an association between clinical monosensitivity to salmonid fishes (such as salmon, trout, etc.) and salmonid parvalbumin-specific IgE antibodies has been detected. This indicated that cross-reactivity may occur in these closely related fishes. Fish parvalbumin is a panallergen, which shows cross-reactivity among closely related fishes. Cod allergic patients (approximately 50%) also cross-react with salmon, due to 58% homology of amino acid sequence between cod allergen Gad c 1 and salmon allergen Sal s 1. A study by Vando et al. (2005) evaluated cross-reactivity among 9 commonly edible fish: cod, salmon, pollack, mackerel, tuna, herring, wolffish, halibut, and flounder. Results showed nine (out of 10 patients) had positive skin prick test for cod, salmon, and pollack. Tuna allergen and pollack allergen (The c 1) are highly cross-reactive to the salmon allergen. The allergens Gad c 1 (Cod), Sal s 1 (Salmon), and The c 1 (pollack), herring, and wolffish are the most potent cross-reacting allergens.

Minor allergens β-enolases and the aldolases of fish are also reported to cross-react, but reactivity differs from individual to individual.

Moreover, shellfish (shrimp and lobster) or finned fish (salmon, tuna, and cod) allergic patients are recommended to avoid all the species of shellfish or finned fish due to the high rate of cross-reactivity.