Mollusks are an important shellfish group along with crustaceans that can cause food allergy.  Scallop is a commercially important bivalve mollusk, globally found in polar to tropical countries. Bivalve allergy is reported from various countries like France, Spain, South Africa, Japan, Canada, Italy, Germany, Sweden, Denmark, Estonia, Lithuania, and Russia. The primary route of scallop allergen exposure is oral (ingestion), while the secondary route is through skin contact (handling, or cutting scallop) or inhalation of aerosols (cleaning, cooking, drying, and scrubbing) at scallop processing industries. Exposure of scallop allergen in sensitized individuals is reported to induce oral allergy syndrome, anaphylaxis, and gastrointestinal symptoms. Occupational exposure can induce asthma or contact urticaria among workers in scallop-processing plants. The major allergenic protein identified in scallop (Mimachlamys nobilis) is Mim n 1 (tropomyosin), which is similar to tropomyosin from other shellfish. High sequence homology in tropomyosin protein leads to cross-reactivity among mollusks (oysters, squid, mussels, and scallops) and crustaceans (shrimp, lobster, crab). Also, Mim n 1 is reported to cross-react with house dust mite, cockroach, and another insect tropomyosin. The basic step to avoid scallop allergic reaction in a sensitized individual is to avoid ingestion of all the bivalve mollusk species (oysters, clams, mussels, and clams), cephalopods (calamari), and crustacean species.

Worldwide distribution

Mollusca is an important shellfish group that causes food allergy; however, they are mostly combined with crustaceans under the term shellfish. As a result, the mollusk allergy remains clinically underreported (8). A wide variation in the allergy prevalence of bivalve mollusks has been observed worldwide depending on the consumption pattern in different countries. IgE-mediated bivalve allergy has been reported in countries such as France, Spain, South Africa, Japan, Canada, Italy, Germany, Sweden, Denmark, Estonia, Lithuania and Russia(9).

A study by Khan et al. (2011) evaluated characteristics of patient-reported seafood allergy in 5162 patients. The results reported 7.2% mollusk allergy and among these mollusk allergy patients, 2.9% had scallop allergy (10). In the United States (US), a nationwide survey by Sicherer et al. (2004) evaluated the prevalence of seafood allergy (fish and shellfish) among 5529 household participants and detected a 2% prevalence of shellfish allergy. Moreover, among total shellfish allergy patients, 41 (14%) reported an allergy to both one or more crustaceans and one or more mollusks or bivalves (11).

In Japan, scallop hypersensitivity is reported frequently due to scallop allergen tropomyosin (12).

Worldwide distribution

Scallops are distributed along the eastern coast of the North Atlantic, Northern part of south Norway to the Iberian Peninsula, Africa (west region), Canary Islands and Madiera (3). In Japan, Canada and Chile, scallops species belonging to the family Pectinidae are popular due to their meat quality, large biomass and economically important species (13).  In Europe, two species of scallops (genus Pecten) are sold and consumed i.e. king scallop which is Atlantic species, and other great Atlantic scallops are found in Mediterranean waters (14). Asian countries are known to farm scallop (Pecten yesoensis) (2). In the UK, scallops are 3^rd most valuable species, and the market is dominated by king scallops. The king scallop fisheries are distributed around the English Channel, Cardigan Bay (Wales), around the Isle of Man, south-east coast of Ireland, around the Channel Islands, Scotland (west and east coasts) (15). Great scallop is naturally found in coastal bays, Scotland, and Ireland (4). Though scallops require specific conditions to grow which are only available in few parts of the world, they are found to be distributed worldwide as seafood.

Main

The primary route of scallop allergen exposure is the oral route (ingestion) (1).

Secondary

Bivalve mollusks are reported to induce allergic reactions at the workplace (seafood processing plant, restaurant) through contact during handling or cutting  (5, 16). Another route of exposure is the inhalation of aerosols produced during cleaning, cooking, drying, and scrubbing. (9). Occupational exposure is reported among restaurant workers or workers handling bivalves (9).

Oral food allergy symptoms and anaphylaxis

Bivalve allergy symptoms are quite similar to other shellfish allergies. Common clinical manifestations include mild oral allergy such as itching of the lips, mouth, and throat and swelling of lips, tongue, throat and palate; cutaneous reactions such as urticaria, eczema; respiratory symptoms such as asthma and rhinitis; gastrointestinal (GI) symptoms such as diarrhea, vomiting; and systemic reactions such as anaphylactic shock (9).

A retrospective study by Thong et al. (2018) evaluating the pattern of food allergy in 120 adults in  Singapore found that, 6 out of 20 (30%) patients who had scallop positive skin prick test (SPT) showed oral allergy syndrome (OAS), while 11.1% patients had anaphylaxis. (17).

In another retrospective study that determined the characteristics of patient-reported seafood allergy in a large allergy referral population, the most commonly reported allergic reaction to shellfish included skin (77.7%), respiratory (50.5%), and GI symptoms. Vascular symptoms were less common and reported in 10-33% shellfish allergy and scallop allergic patients (10).

Asthma

Shellfish are known to induce occupational asthma among workers, working in seafood processing plants. Also, restaurant workers are reported to be sensitized to mollusks such as scallops (5). A case study by Goetz and colleagues (2000) found cross-reactivity of shrimp and scallop in a restaurant worker handling seafood. He developed IgE-mediated occupational asthma and contact urticaria. The allergen identified was a heat-stable protein with molecular masses ranging between 35 to 39 kDa which may be tropomyosin. Also, boiling shellfish in an open containers may be a potential source of allergen which can induce respiratory allergic symptoms in cooks (16). 

Prevention strategies

Avoidance

Individuals sensitized to scallop should avoid ingestion of any mollusk species such as bivalves (oysters, clams, mussels and clams), cephalopods (calamari), and also crustacean species (such as shrimp, lobster, and crab) (8, 9).  

Allergenic molecules

In all the edible crustacean and mollusk species, tropomyosin is a major allergen and more than 60% of shellfish allergic patients are found to be sensitive to tropomyosin. Crustacean and mollusk tropomyosin share 55 to 70 % identical amino acid sequence. Tropomyosin is a heat-stable protein with a molecular weight ranging from 33 to 38 kDa (7).

The allergenic molecules of mollusks are not well-defined (8). In a scallop species Mimachlamys nobilis (other name-Pecten nobilis, belonging to the family Pectinidae), one allergenic molecule has been identified, which is Mim n 1 (1, 18). This is a tropomyosin protein and a major allergen in scallops just like other shellfish species  (12).

Cross-reactivity

Individuals allergic to crustacean (shrimp, lobster, crawfish, or crab) may also be allergic to mollusks (oysters, squid, mussels, and scallops) due to similar allergenic protein content and people are expected to react similarly to both crustaceans and mollusks. A patient with a history of allergy to shellfish and clams may have a 5% probability of positive skin test to scallops. Patients allergic to bivalves (clams, mussels, oysters, and scallops) may also cross-react to other species of bivalves (5). Tropomyosin is considered a pan-allergen and is the principal cause of cross-reactivity between crustaceans and mollusks as well as with other invertebrates such as cockroaches, dust mites, and other insects (7).  

A study by Zhang et al. (2006) reported that 61.6% and 61.7% homology exist between shrimp and scallop and crab and scallop tropomyosin indicating a high level of cross-reactivity between shrimp, crab and scallops  (19).

A study by Rolland reported that scallop showed a high level of cross-reactivity with calamari (cephalopod) and with oyster (8).

House dust mite allergen tropomyosin cross-reacts with scallop Mim n 1 (tropomyosin) allergenic protein (18).

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: November 2020