Worldwide crabs are widely processed and commercially used shellfish species.Crab allergy prevalence is rising in Asian countries like China, Taiwan and Singapore. The main route of crab allergen exposure is ingestion of crab meat and the secondary route is occupational exposure (inhalation or contact). Allergens in crabs can induce oral food allergy symptoms and also reactions including urticaria, angioedema, eczema, gastrointestinal symptoms (diarrhea, vomiting), systemic reactions (anaphylactic shock), and respiratory symptoms. In Canada, snow crab allergen is found to be one of the major causes of respiratory allergic symptoms, such as asthma and rhinitis in fishermen and crab industry workers. Major allergens identified in crab are tropomyosin and arginine kinase. The most important pan-allergen in crab and other shellfish is tropomyosin, which causes cross-reactivity with other crustaceans, mollusks, mites, cockroaches, and parasites.  In sensitized individuals, it is advised to avoid crab meat intake and exposure to crab meat (handling) to prevent crab allergic reactions.

Worldwide distribution

Shellfish allergy is one of the common food allergies, which may continue for a long time, usually persisting till adulthood. Depending on the country and its dietary habits, the prevalence of shellfish allergy is reported to range from 1.3% to 5.2% (8). Shellfish allergy is prevalent among adults and children in Asian countries. It is also common in western countries like Europe, the USA, and even found in Australia (6). Crab allergy prevalence is increasing in Asian countries like China, Taipei, and Singapore (9). An increased prevalence of shellfish allergy (which includes crab) compared to other food allergies has been reported in some Asian patients with asthma and allergic rhinitis (AR) (9). In the Philippines and Singapore shellfish allergy in teenagers was reported to be 5.12% and 5.23% respectively (10).  A study conducted on children residing in Singapore showed more prevalence of shellfish allergy in native children as compared to non-native children. In the age group of 4-6years, the prevalence rate was 1.19% for native children and 0.55% for non-native children. Similarly, in the age group of 14-16 years, the prevalence rate was 5.23% for native children and 0.96% was for non-native children (11).

A retrospective study by Thong et al. (2018) evaluated the pattern of food allergy other than the only rash in 120 pre-enlistees in Singapore. Shellfish/crustaceans were the most commonly reported allergen (78%) and the crab was found to cause oral allergy syndrome (OAS) in 15.2% of sensitized individuals (12). A systemic review by Moonesinghe et al. (2016) reported that in southeast Asia, the prevalence of crab allergy among 3 to 7-year-old children is low and is about 0.7%. In Taiwan, crab allergy prevalence in adults was found to be 2.3% and in children only 0.4% (13).

A systematic review detected that in Europe, crab allergy prevalence (assessed by clinical history and positive skin prick test) was about 0.2% (13). One study in France with 580 patients having adverse reactions to food found the presence of specific IgE to crab in 34% of patients (6).

A telephonic survey of 14,948 individuals in the US reported that 2-3% of individuals claimed to have seafood allergy out of which 2.2 % was due to shellfish. Children (0.5%) estimated to have a lower allergy rate as compared to adults (2.5%). Shrimp was found to be the most common cause of allergy followed by crab, lobster, clam, oyster, and mussel in the same decreasing order (11). In line with this survey, another study in the USA detected adults have a higher incidence of shellfish allergy (2.8%) as compared to children (0.6%) and women had more incidence (3.6%) as compared to men (2%) (10).

In Atlantic Canada, the snow crab fishery is the largest snow crab fishery in the world. Snow crab meat is reported as an important cause of severe hypersensitivity reactions such as anaphylaxis and asthma in fishermen and crab industry workers in this area (7). Prevalence of occupational asthma due to snow crab was reported to be 15.6% among workers, while the prevalence of protein contact dermatitis ranged from 3 - 11% and asthma and allergy ranged from 7 – 36% in workers exposed to arthropods (14).

A pilot survey was conducted by Bønløkke et al. (2012) to evaluate snow crab sensitization, allergy and asthma among 20 workers at a snow crab industry in Greenland using skin prick tests (SPTs) with common aeroallergens and specific snow crab allergens. Results suggested that the frequency of specific sensitization was high (about 40%) among these 20 workers (15).

A study conducted by Gautrin et al. (2009) evaluated the prevalence of occupational asthma (OA) and occupational allergy (OAI) in workers from snow crab processing industries. The study included 215 workers from Newfoundland and Labrador, Canada. Results showed the prevalence of immunological sensitization to snow crab was 18.4% among the workers (16).

Risk factors

Individuals who are in the vicinity of cooking shellfish by boiling, steaming or frying are prone to develop allergic reactions by inhalation of vapors (11). Fishermen, processing workers, shell grinders, cooks and even restaurant workers are at risk of developing allergic reactions to shellfish (17).

Smoking may increase the risk of developing snow crab occupational asthma, whereas atopy may increase the risk of developing occupational crab allergy (1).

Pediatric issues

Children with seafood allergy have increased specific IgE antibody levels with vast epitope diversity compared to adults. Typically, there are two types of epitopes in food allergens: sequential and conformational. Sensitization to sequential epitopes has an association with persistent allergy and the individual may recognize a higher number of IgE sequential epitopes. It has been detected that in older patients with a persistent food allergy, sensitization to specific sequential IgE epitopes varies from those identified by the children who may outgrow their allergy  (18, 19). 

Worldwide distribution

Among different crab species, snow crab is distributed in the northwest Atlantic (Canada and West-Greenland), north Pacific and Japan Sea (20). In the USA, they are found in Alaska (the Arctic and sub-Arctic sea), north of Aleutian Islands, Bering, Chukchi and Beaufort Sea (21). Other types of crabs, such as swimming crabs and blue crabs are located at the Atlantic coast of North and South America, and mud crabs are found in Indo-West Pacific (22).

Main

Ingestion of crab mostly triggers allergic reactions immediately or sometime late phase in sensitized individuals (6, 9).

Secondary

Inhalation is another route of exposure. Workers working in the sea-food industry are exposed to air-borne shellfish particulate matter formed because of different processing activities such as filleting, freezing, cooking, smoking, drying which get suspended in the air as aerosols and can be inhaled (23).

Oral allergy symptoms and Anaphylaxis

On exposure to shellfish, IgE-mediated allergic reaction takes place that may involve single or multiple organs. These reactions seen may be immediate or late-phase with a delay of up to 8 hours after intake of food. Oral allergy symptoms (OAS) include mouth and throat itching and swelling of the lip. These reactions are followed by reactions on the skin like urticaria (red and raised welts), periorbital angioedema, and skin redness (24).

Shellfish is known to induce food-dependent exercise-induced anaphylaxis. It is important to consider if exercise was performed before an allergic response. Shellfish allergy has also proved to be the leading cause of food anaphylaxis in Hong Kong and Taiwan (10).

Ingestion of crab in sensitized individuals can lead to skin reactions including urticaria, angioedema, eczema, gastrointestinal (GI) symptoms (diarrhea, vomiting), systemic reactions (anaphylactic shock), and respiratory symptoms (9). Skin rash was observed in the majority of crab processing plant workers (about 50%) in a study in Canada (15). In children, crab allergy can trigger skin, respiratory, and GI tract allergic symptoms (19).

Thong et al. (2018) studied the pattern of food allergy in adults in Singapore and found that mostly OAS resulted from shellfish/crustaceans including crab (15.2%) and the common manifestations were itchy lips and throat with/without lip angioedema and even anaphylaxis was observed in 29.9% of patients (12).

Allergic rhinitis

Crab is identified as a major food allergen by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO). Crab can induce respiratory symptoms like allergic rhinitis (AR) and asthma (9). Rhinitis or conjunctivitis (50%), asthma (45%), and lower respiratory infection (39%) were commonly observed among crab processing plant workers (15). Occupational allergy and asthma are common problems of snow crab workers in Greenland, Canada as found in the pilot study by Bonlokke et al. (2012) (15).

Asthma

A study with workers in crab processing industries of Canada found 15.8% and 14.9% of workers (among a total of 215 workers) developing occupational asthma (OA) and occupational allergy (OAI) after getting exposed to snow crab allergen. Moreover, exposure to the snow crab allergens is found to be associated with OA and OAI in a dose-response manner. Major determinants of OA were atopy, female gender and smoking (16).

A cross-sectional survey by Beaudet et al. (2002) evaluated crab allergen concentration and respiratory symptoms among 82 workers from five crab-processing vessels near Dutch Harbor, Alaska. The results showed a high prevalence of respiratory symptoms among all the workers especially in the crab-processing season, which included cough (28%), phlegm (11%), wheeze, and other asthma-like symptoms (4%). The aerosolized crab allergen level ranged from 79 ng/m³ to 21,093 ng/m³ (25).

Another study by Cartier et al. (2004) evaluated the incidence of snow crab sensitization and OA in four crab plants in Canada. The study reported 18% (39 out of 215 participants) of the workers developed occupational crab asthma and indicated that crab asthma sensitization is a major health problem in snow crab plant workers (1).

Prevention strategies

Avoidance

Avoidance is one of the most effective means for prevention of an adverse reaction to food allergens. By avoiding direct ingestion of crabs or indirect contact through skin or products containing crabs, the allergic symptoms can be prevented. Patients that are allergic to shellfish like crab, shrimp, and lobster have to avoid the entire group of shellfish due to high cross-reactivity rates (26). Also, it is important to protect the consumer by accurate labeling of the food products to prevent a potentially life-threatening condition for allergic persons (27).

It is important to advise the allergic patients and their caregivers that crab is a shellfish, its allergenic protein during handling and cooking can trigger an allergic reaction in crab allergic patients. Also, children with a shellfish allergy may be educated not to accept food from other children (19). 

Allergenic molecules

Shellfish allergen proteins have low molecular weight, heat-stable, and water-soluble (6). Tropomyosin (molecular weight of 33 kDa), is a major allergen identified in crab. It is found in muscle and non-muscle cells and plays important role in muscle contraction among vertebrates and invertebrates. Another major allergen identified in crab is arginine kinase (40 kDa), a heat-sensitive enzyme, which catalyzes the transfer of phosphate between ATP and various phosphagens in invertebrates. Other potential allergens identified in crabs are sarcoplasmic calcium-binding protein (CBPs) (molecular weight of 20 kDa), troponin (molecular weight of 23 kDa), α-actin (molecular weight of 42 kDa), and smooth endoplasmic reticulum Ca²⁺ATPase (113 kDa) (7, 9).

Cross-reactivity

Tropomyosin has cross-reactivity among different species of the shellfish family (especially crustacean) such as shrimp, crab, and lobster due to identical amino acids (up to 95-100%) (6, 19). Also, tropomyosin, an invertebrate pan allergen is responsible for cross-reactivity among foods, and aero-allergens originated from animals (28).

In invertebrates (crustaceans, mollusks, mites, and cockroaches) tropomyosin and arginine kinase are cross-reactive pan-allergens, due to similar phylogenetic identity. Studies reported that tropomyosin and arginine kinase are cross-reactive with allergens of Portunus pelagicus (blue swimming crab) (9).

Subjects allergic to the Crustacean family, often react to mollusk group (such as squid or cuttlefish, abalone, limpet, oyster, mussel, scallop, clam) as well as with mites, cockroaches, and parasites (6, 29).

A limited amount of tropomyosin cross-reactivity is reported between fish and shellfish allergens to date (10).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: November 2020