A Comprehensive Guide on Shellfish Allergies, as Advised by a Doctor

Consuming seafood has become a popular food choice globally. However, there have been increasingly reports of adverse health reactions following consumption of seafood. A body’s adverse reaction towards shellfish can be allergic-related vs toxic-related.

Allergies to seafood or shellfish is one of the common food allergens that occur in both children and adult. Interestingly, one with shellfish allergies tend not to outgrow from childhood. Shellfish allergic symptoms tend to be more severe when compared to other food allergies. Shellfish allergy affects approximately 0.5-2.5% of populations, though the actual cases of shellfish allergies are likely under-reported or un-diagnosed.

While one can have true allergic reaction following consumption of shellfish, the body can also react badly as a result of toxins and infections from eating shellfish. Shellfish can also be contaminated by bacteria such as salmonella spp, vibrio cholera, listeria spp. These noxious bacteria when consumed into human body, have capabilities to release toxins to the bloodstream and trigger a cascade of inflammatory and infective response.

Understanding one’s clinical presentation and underlying the cause of adverse reaction to shellfish can be important in determining subsequent management and prevention of similar detrimental health symptoms.

What is defined as shellfish?

Although 'seafood' and 'shellfish' are both used loosely, interchangeably and ubiquitously, they may have different meaning. 'Seafood' is an umbrella term to include any 'edible particles found in the sea', while 'shellfish' includes those with 'shells' as part of their skeletal system. 

The shellfish family can be sub-categorised into crustaceans and mollusks. 

1. Crustaceans include: 

• crab
• prawn
• shrimp 
• lobster
• crayfish 

2. Mollusks can further be subdivided into

• Gastropods that includes abalone, limpet, snail
• Bivalves that include clam, mussel, scallop, oyster, cockle
• Cephalopods that include squid, octopus, cuttlefish

What are the symptoms of shellfish allergies?

Allergies occur when the body's immune system is exposed to a substance (known as allergen) that is generally harmless. The immune system overreacts and produces excessive inflammatory response leading to clinical manifestations of allergic symptoms.

The onset of shellfish allergic symptoms tends to be within seconds to minutes with involvement of the skin, airway and gastrointestinal system. There can be an array of clinical presentation:

• mild symptoms, such as:
  • skin/mouth/throat itching and burning 

• more severe symptoms such as:
  • retching
  • vomiting
  • abdominal pain
  • diarrhoea 

In very severe cases, one can succumb to shellfish allergy due to development of life-threatening anaphylactic reaction. Anaphylaxis presents with generalised involvement of allergic symptoms with feeling of 'impending loom', fainting spells, tightening of airway, facial swelling, difficulty of breathing, wheezing and widespread body rashes. Immediate medical attention can reverse such situation, and delay of treatment can result in death.

One can also develop allergic symptoms following the inhalation or direct touching of the shellfish protein allergens. This can occur in fish handlers, or one who works in kitchen preparing shellfish meal, leading to sensitisation of the airway and the skin. In such cases, one can present with wheezing, asthma symptoms or itchy hives rashes upon close contact with shellfish. Hence, shellfish allergy can develop even without consuming shellfish. 

Why does a person develop shellfish allergy?

Tropomyosin is the main allergen found in shellfish that plays a vital role in shellfish allergies. Tropomyosin is a protein found in the muscle cells. It can be found in shellfish in both crustaceans and mollusks. 

One can have cross allergy reactivity between the shellfish family. For example, one may be allergic to crustaceans such as crab and also present allergic symptoms when consuming mollusks such as clam. Tropomyosin is stable to heat- hence one can still develop allergic symptoms after consuming shellfish that is boiled or cooked with heat. 

As tropomyosin allergen can also be found in dust mites, cockroach particles,  parasites and insects, there can be cross allergy reactivity between these substances. A person with shellfish allergy can have concurrent allergy towards dust mites, insects, parasites and etc, vice versa. 

Thankfully, those who has shellfish allergies usually do not have fish allergies as there is no cross reactive protein allergens between the two groups although both are aquatic edibles. The belief that shellfish allergy is due to iodine is untrue, hence one with shellfish allergies does not have increased risk of radiocontrast iodine allergy.

Other allergens that are found in shellfish include arginine kinase, myosin light chain kinase, and sarcoplastic calcium binding protein which can cause allergy reaction in the children group.

Shellfish allergy vs. Shellfish poisoning

While we tend to assume shellfish adverse reactions are due to an allergic response, one may have similar unpleasant symptoms due to shellfish poisoning. 

Shellfish poisoning or shock syndrome can present with symptoms mimicking an allergic reaction depending on the amount of shellfish and toxin concentration ingested. Symptoms, within minutes of consumption of poisonous shellfish, include: 

• dizziness
• nausea
• vomiting
• abdominal cramps
• diarrhoea
• tingling or numbness of the lip, mouth or throat 

In more severe cases, neurological symptoms such as numbness and weakness of face, neck, arms and legs, or even paralysis up to 72 hours may occur. Shellfish poisoning can also be associated with headache, confusion and short-term memory loss.  

Occasionally, adverse symptoms associated with shellfish may not be the poison directly from shellfish. Shellfish can be contaminated with bacteria such as vibro spp, clostridium botulinum, staphylococcal spp, salmonella spp or even Norwalk virus that can present itself with acute gastrointestinal symptoms such as nausea, vomiting, diarrhea, or even fatality.

How is a shellfish allergy diagnosed?

• Medical history — A good accurate history may be able to pick up shellfish allergy. However as there are overlapping symptoms of both shellfish allergy and toxicity, it may be much more difficult in reality to delineate shellfish allergy from history alone.
  
• Allergy tests — Allergy tests such as skin prick tests, blood RAST test can be safely offered to facilitate the diagnosis of shellfish allergy.
  
• Oral challenge tests — Oral challenge test may be offered provided there is no risk of life threatening anaphylaxis reaction.

If you have concern of shellfish allergies, you are encouraged to discuss your symptoms and concerns with your health care providers for further guidance on diagnosis and management of your symptoms.

How can we treat shellfish allergies?

• The key in managing shellfish allergy is by recognising and avoiding consuming or having close contact with the offending shellfish. 
  
• Symptomatic medications such as antihistamine, corticosteroid may be useful. 
  
• Adrenaline injections can be used to reverse life-threatening anaphylaxis reactions.
  
• Although there are researches on immunotherapy for food allergies, hitherto, there remain no such treatment option for shellfish allergy.

Key learning points about shellfish allergies

• Adverse health symptoms following consumption of shellfish can be due to an allergy or a toxic reaction.
  
• Shellfish allergy and shellfish poisoning have similar clinical symptoms.
  
• Recognising shellfish allergies can avoid future exposure to the culprit shellfish hence avoiding allergy.
  
• Shellfish allergy testing can be used to confirm the diagnosis of shellfish allergy.

• Main stream management of shellfish allergy is via avoidance.
  
• Those who are immunocompromised should only consume shellfish that is cooked to reduce risk of shellfish poisoning or infection from contamination.

References:

1. Woo CK, Bahna SL. Not all shellfish "allergy" is allergy! Clin Transl Allergy. 2011 Jun 10;1(1):3.
2. Lopata, A.L., Kleine-Tebbe, J. & Kamath, S.D. Allergens and molecular diagnostics of shellfish allergy. Allergo J Int. 2016; 25, 210–218. 
3. Alonso LL, Armstrong L, Warrington SJ. Shellfish Allergy. [Updated 2022 Sep 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK448089/