Mussels are marine, bivalve mollusks distributed worldwide and consumed in various parts of the world. Blue mussels are commercially important bivalve mollusks used as food for many years and found in temperate seas around the globe. They can tolerate a wide range of salinity, temperature, and wave exposure. Ingestion of mussel (raw, cooked, or processed) is the primary route of exposure to mussel allergen. Secondary routes of exposure are inhalation of aeroallergens generated during cleaning, cooking, drying, and scrubbing of mussels as well as skin contact at the workplace. Ingestion of mussels induces oral allergic syndrome, urticaria, angioedema, anaphylaxis, gastrointestinal symptoms such as nausea, vomiting, and diarrhea in sensitized individuals. Occupational exposure to mussels in restaurant workers or food handlers or workers in seafood processing plants has been reported to induce respiratory symptoms such as asthma and rhinitis, and contact dermatitis. The major allergen identified in blue mussel is Myt e 1, tropomyosin. High sequence homology in tropomyosin protein leads to cross-reactivity among mollusks (oysters, squid, mussels, and scallops) and crustaceans (shrimp, lobster, crab), and even with house dust mites, cockroaches, and parasites. The primary approach to prevent mussel allergic reactions for sensitized individuals is to avoid ingestion of molluscan 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 (7).

The prevalence of bivalve mollusk allergy varies around the world based on their consumption pattern. IgE-mediated bivalve allergy has been reported in several countries such as France, Spain, South Africa, Japan, Canada, Italy, Germany, Sweden, Denmark, Estonia, Lithuania, and Russia (8). In the United States (US), a nationwide survey by Sicherer et al. (2004) evaluated the prevalence of seafood (fish and shellfish) allergy 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 (9).

Among bivalves, mussel allergy has been more commonly reported than either scallop or clam allergy (10). A cross-sectional, descriptive, questionnaire-based survey by Rance et al. (2005) estimated the prevalence of food allergies among 3500 school children in France. The result showed a 0.8% prevalence of mussel allergy (11). A representative cross-sectional survey by Zuberbier et al. (2004) among 13,300 people from Berlin, Germany has reported mussel allergy prevalence of 0.1% as observed in skin prick test (SPT) (12). A retrospective chart review 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, 1.4% had mussel allergy (13). A prospective multicenter study by Azofra et al. (2017) evaluated the clinical characteristics of 45 mollusk-allergic patients in Spain. Results showed a total of 31% of patients were allergic to bivalve mollusks and among them, 9% were allergic to mussels (14). 

Worldwide distribution

Mussel has a worldwide distribution and consumed in different parts of the world. Mytilus spp. such as M. edulis, M. galloprovincialis, M. trossulus, M. californianus, and M. coruscus are found in Northern Hemisphere, while M. galloprovincialis, and M. platensis are found in the southern hemisphere. However, the native distribution of M. edulis is in the North Atlantic region, M. galloprovincialis in the Mediterranean, M. trossulus in the northern Pacific and Baltic Sea, M. californianus in the North Eastern Pacific Ocean, M. coruscus in the subtropical western Pacific Ocean, and M. platensis in South America (4). In Europe, blue and Mediterranean mussels are the largest consumed mussels (3).

Main

The primary route of exposure to mussel allergen is ingestion (oral route) of raw, or cooked, or processed mussel (15).

Secondary

Bivalve allergy is reported through inhalation of aeroallergen at the workplace, which is generated during cutting, cleaning, cooking or boiling, drying, and scrubbing. Also, they are reported to induce allergy through skin contact during handling or cutting (8). Occupational exposure is reported among restaurant workers, or workers handling bivalves in shellfish processing plants (8).

Oral allergy symptoms and Anaphylaxis

Bivalve allergy symptoms have similarities with 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 (8). Exposure of mussel in sensitized individuals has reported causing severe allergic reactions, urticaria, angioedema and anaphylaxis. Also, restaurant workers or food handlers are reported to have contact dermatitis when exposed to mussels (16).

A case study by Nettis et al. (2001) evaluated 11 adult patients (with family history and/or personal history of atopy), who reported allergic reactions on the consumption of raw and/or cooked mussels. The primary symptoms in all 11 patients were urticaria and angioedema. In addition, GI symptoms such as nausea, vomiting and/or diarrhea were observed in four patients (15).

A retrospective study by Thong et al. (2018) evaluating the pattern of food allergy in 120 adults in Singapore found that 4 out of 10 (40%) patients who had mussel positive SPT showed 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 reactions to shellfish included skin (77.7%), respiratory (50.5%), and GI symptoms. Vascular symptoms were less common and reported in 10-33% of shellfish allergy patients. Skin and respiratory symptoms were reported by mussel allergic patients. Also, it was found that mussel allergic patients had more than one organ system involved (13).

In a prospective study with 45 mollusk-allergic patients (allergic to gastropods, bivalve, and cephalopods), 69% of patients (31/45) experienced systemic reactions; among them, 22% showed oral allergy syndrome (OAS) and 15% contact urticaria. Moreover, in 80% of bivalve-allergic patients, systemic reactions were detected (14). 

Asthma

Shellfish are known to induce occupational asthma among workers, working in seafood processing plants (18). A cross-sectional study by Glass et al. (1998) evaluated the prevalence of respiratory symptoms (work and nonwork-related) in 224 mussel openers from mussel-processing sites at New Zealand. The prevalence of reported respiratory symptoms were any wheeze (35%), work-related wheeze (23%), any chest tightness (30.5%), and work-related chest tightness (20.2%). About 32.3% (72/224) mussel openers gave a positive response to at least 1 of the 4 questions associated with work-related symptoms (which are defined as symptoms improving on days of rest or worsened during work) (19). In a case study by Nettis et al. (2001) on 11 patients (allergic to cooked or raw mussels), respiratory symptoms, like rhinitis and asthma were reported by three and two patients respectively (15). Also, boiling shellfish in an open container may be a potential source of respiratory and contact allergen, which may induce respiratory allergic symptoms and dermatitis in cooks (20).

Prevention strategies

Avoidance

To prevent allergy to bivalves, the primary approach is to avoid ingestion of all the bivalve species (oysters, clams, mussels, etc.) by sensitized individuals. Also, bivalve allergic patient is recommended to avoid both molluscan and crustacean species (such as shrimp, lobster, and crab) (8). 

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% of the identical amino acid sequence. Tropomyosin is a heat-stable protein with a molecular weight ranging from 33 to 38 kDa (6). In mollusks, the allergenic molecules are not well defined (7).

Similar to other shellfish, tropomyosin is the major allergenic molecule identified in mussels; for e.g. Myt e 1 in blue mussel (Mytilus edulis), Myt g 1 in Mediterranean mussel (Mytilus galloprovincialis), and Per v 1 in Asian green mussel (Perna viridis) (5).

In a study with mollusk allergic patients, the allergenic proteins were identified by Mass Spectrometry. The mussel allergic protein identified as a putative C1q domain-containing protein of molecular weight 42 kDa (14).

Cross-reactivity

Patients allergic to one species of bivalve mollusks (clams, mussels, oysters, and scallops) may cross-react to other species of bivalves (18). Cross-reactivity was reported among mussels, snails, oysters, and squid (10). Individuals allergic to mollusks (oysters, squid, mussels, and scallops) may also be allergic to crustaceans (shrimp, lobster, crawfish, or crab) due to similar allergenic protein content and people are expected to react similarly to both crustaceans and mollusks. 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 parasites (6). Tropomyosin from oyster, mussel, clam, and scallop has 70-100% amino acid homology (10). Mussel tropomyosin is found to have high similarity with the tropomyosin of gastropods (such as abalones, turban shell, and whelk) showing 71.5–75.4% identity to abalones, 74.3–74.6% identity to turban shell, and 69.0–70.1 similar to whelk (21).

House dust mite allergen tropomyosin found to cross-react with blue mussel (Myt e 1), Mediterranean mussel (Myt g 1), and Asian green mussel (Per v 1) tropomyosin (5). 

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: December 2020