Shellfish Allergy

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Continuing Education Activity

Shellfish allergies can occur due to immunologic or non-immunological processes, and individuals may react to crustaceans, such as crab or shrimp, or mollusks, such as clams or scallops, or both. About 14% of individuals are allergic to both crustaceans and mollusks. Therapy is directed by the patient's presentation and underlying etiology. This activity reviews the presentation, evaluation, and management of shellfish allergies and emphasizes the role of the interprofessional team approach in the care of affected patients.


  • Describe the epidemiology of shellfish allergies.
  • Summarize the exam findings typically seen in patients with shellfish allergies.
  • Review the management strategies for shellfish allergies.
  • Explain modalities to improve care coordination among interprofessional team members in order to improve outcomes for patients with shellfish allergies.


Shellfish reactions can occur as a result of immune system-mediated effects and also through non-immunological processes. It is important to determine the underlying cause for the patient’s presentation to best direct therapy and understand management implications. Tropomyosin is the most common shellfish allergen found.[1] There is a wide array of consumables that falls under the category of shellfish, including crustaceans (such as crab or shrimp) or mollusks (such as clams or scallops), and this adds to the difficulty in dealing with “shellfish” allergies. Not all individuals react to both. One group notes only 14% cross-reactivity between crustacean and mollusk allergies.[2][3][4]


Reactions or symptoms due to shellfish consumption may be attributable to a variety of factors. Some reactions are not a true allergy and are due to infectious agents such as parasites, bacteria, viruses, and parasites.[1] Bacteria implicated with shellfish include Vibrio, Listeria, and Salmonella. Toxin-mediated reactions may also occur and trigger symptoms such as ciguatera or saxitoxin. True allergic reactions may occur to substances in the shellfish (such as tropomyosins), or from a component ingested with the shellfish such as spices or chemical additives.[5][6]


Fish and shellfish combined are suspected to be responsible for a significant portion of allergic reactions. The actual incidence of shellfish allergies is difficult to identify due to the numerous possible etiologies and under-reporting. Even in cases where a true allergic reaction is identified, it is difficult to determine if there could have been a chemical agent or another additive that could have been the triggering agent. Overall, food allergens are responsible for approximately 30,000 anaphylactic events. Foods as a whole are responsible for around one-third of anaphylaxis cases.[7][8]


There are multiple allergens responsible for crustacean allergies. Domoic acid is a strong neurotoxin produced by many diatoms of the genus Pseudo-nitzschia. It acts as a glutamate agonist. The consumption of contaminated shellfish can lead to human exposure.[9] Tropomyosins being one of the better-characterized agents. Tropomyosins can be heat-stable and act through an immunoglobulin E (IgE)-dependent mechanism with antibody binding. Other agents responsible include a substance thought to be similar to arginine kinase, a myosin light chain, and a protein that binds to sarcoplasmic calcium. The agents responsible for mollusk allergies are not well identified.


While shellfish toxicity is secondary to a non-allergic pathway, it is crucial to consider it in cases that are challenging to differentiate. As mentioned above, there are multiple potential toxins to be aware of in shellfish toxicity. Saxitoxins are responsible for causing paralytic shellfish poisoning and are secondary to dinoflagellates.[10] The toxin is a voltage-gated sodium channel antagonist, which can result in neurologic symptoms, including paralysis and death from respiratory suppression. In a small case series, the effects began to resolve after four hours, with the patient returning to normal after 12 hours.

Neurotoxic shellfish poisoning can also occur and may resemble paralytic shellfish poisoning, but is much milder.[11] Symptom onset is within three hours and includes gastrointestinal symptoms in addition to neurologic symptoms. Symptoms include reversal of hot and cold temperature sensation, paresthesias, aches, nausea, vomiting, and diarrhea. It is also believed secondary to a dinoflagellate, and specifically due to brevetoxins.[12]

Another disease state secondary to shellfish is amnesic shellfish poisoning, which occurs due to domoic acid. These patients demonstrate gastrointestinal symptoms within 24 hours of ingestion and may develop neurologic symptoms such as confusion, disorientation, or memory loss within 48 hours.[13]

Azaspiracid shellfish poisoning is due to marine toxins that can accumulate in shellfish and trigger severe gastrointestinal symptoms. While the toxin may be secondary to dinoflagellates, its origin is not definitively known. Symptoms persist for two to three days, and the substance can induce widespread organ damage.[14]

The mildest toxin-associated illness found with shellfish ingestion is diarrhetic shellfish poisoning, and it is also associated with biotoxins from dinoflagellates. Symptoms are primarily gastrointestinal, but may also include fevers, chills, or headaches. Timing is generally between 30 minutes to six hours after ingestion.

History and Physical

Asking the patient about recent ingestion or their handling of crustaceans or mollusks will help narrow etiologic agents. Most allergic reactions begin within minutes to a few hours after the ingestion of the food. The severity of symptoms can vary widely from one individual to another. Mild allergies will manifest with pruritus and urticaria, while severe cases can have true anaphylaxis with respiratory compromises such as angioedema or wheezing and hypotension. Other signs and symptoms associated with shellfish allergies are atopic dermatitis (eczema), coughing or sneezing, coryza, circumoral paresthesias, nausea, diarrhea, vomiting, dizziness, and fainting.


The evaluation and diagnosis of shellfish allergy in an acute setting are on clinical grounds as no rapid tests exist to diagnose this entity accurately. Assessment of vital signs, with a particular focus on respiratory status and blood pressure, is crucial in distinguishing anaphylaxis from a more benign course. A thorough examination of the oropharynx and auscultation of lung sounds to detect edema or wheezing is essential. Many patients will have gastrointestinal symptoms, so a comprehensive exam of the abdomen is also advised. Close examination of the skin for exanthems or edema should also occur.

Mass spectrometry, liquid chromatography, and enzyme-linked immunosorbent assay (ELISA) methods have been devised for domoic acid.[15] Saxitoxin can be assessed by a mouse bioassay, ELISA, and high-performance liquid chromatography. Lastly, brevetoxin can be checked by an antibody radioimmunoassay (RIA), mouse bioassay, and ELISA.

Treatment / Management

There is no specific treatment for shellfish allergies but preventing contaminated fish from entering the food supply can prove to be very useful.[16] Intravenous fluids are often given to patients who have been vomiting. Standard allergic reaction therapy includes antihistamines (H1 and H2) and steroids. In cases of anaphylaxis, intramuscular (IM) or intravascular (IV) epinephrine should be given immediately, followed by antihistamines, steroids, and IV fluids. In rare cases, refractory to standard treatment intubation may be necessary to protect the airway. The duration of symptoms and response to treatment is highly variable, and there is no one agreed-upon period of observation. In general, benign presentations that respond to standard treatment and improve while in the emergency department can be safely discharged home with oral antihistamines and steroids with clear instructions to return for worsening symptoms. True cases of anaphylaxis require admission for further monitoring and close observation.[17][18][19][19]

Following treatment, patients should receive instructions to avoid the offending agent or food. For example, patients who have an allergic reaction triggered by crab should avoid other crustaceans. Despite avoidance, consideration of a prescription for an epinephrine auto-injector may be wise due to the potential for anaphylactic reactions and cross-reactivity.

Differential Diagnosis

Following are some important differential diagnoses that should be considered while making the diagnosis of shellfish allergy:

  • Acute urticaria
  • Bacterial gastroenteritis
  • Bulimia nervosa
  • Celiac disease
  • Chronic gastritis
  • Constipation
  • Diverticulitis
  • Dumping syndrome
  • Esophageal spasm
  • Esophageal stricture


In 2017, the National Poison Data System (NPDS) of the Annual Report of the American Association of Poison Control Centers observed patients with a single exposure to paralytic shellfish poisoning. They reported 3 major outcomes, 20 moderate outcomes, 17 minor outcomes, and no deaths among 109 patients.[20] Although anyone eating shellfish harboring HAB toxins could become ill, patients with some chronic illnesses, such as liver disease, may potentially have a more severe form of toxicity.

Mortality rates from paralytic shellfish poisoning, the most severe of the shellfish poisoning syndromes, ranges between 1% and 12%. In severe cases, muscle paralysis along with respiratory failure can cause death in 2–25 hours. However, the risk of death is decreased if healthcare providers have prompt access to advanced life support.  


The following five distinct shellfish-poisoning syndromes can result from the toxicity:

  • Paralytic shellfish poisoning (PSP) - Symptoms usually commence within two hours of exposure, but can begin anywhere between 15 minutes and 10 hours after eating contaminated shellfish.[10] At the onset, paresthesias of the perioral area, lips, tongue, and gums are noted. After onset, there is rapid progression to the distal extremities. Other symptoms may include headache, paralysis, ataxia, muscle weakness, and cranial nerve dysfunction.
  • Neurologic shellfish poisoning (NSP) - The illness associated with NSP is less severe than that with PSP. Patients usually present with gastroenteritis, burning sensation in the rectum, paresthesias of the face, limbs, and trunk. There may also be myalgias, ataxia, vertigo, and inability to distinguish hot/cold sensation.[11][12]
  • Diarrheal shellfish poisoning (DSP) - It is associated with stomach and intestinal symptoms that begin between 30 minutes and a few hours after exposure and include severe diarrhea, abdominal cramps, nausea, vomiting, and chills.
  • Amnesic shellfish poisoning (ASP) - Mostly, diarrhea, vomiting, and abdominal pain appear within 24 hours of consuming toxic shellfish. These symptoms are followed by headache, loss of memory, and cognitive decline.[13]
  • Azaspiracid shellfish poisoning (AZP) - Consuming contaminated shellfish may result in symptoms such as nausea, vomiting, diarrhea, and stomach cramps.[14]

Deterrence and Patient Education

Patients should be made aware of the food items that can result in toxicity. They should also be given information on the early signs and symptoms of shellfish allergy so that they can seek medical attention earlier. Interprofessional teams should make patients aware that medical treatment should be sought immediately if they are unable to tolerate oral liquids if fever is noted, if blood is observed in the stool, or if other worrying symptoms develop.

Enhancing Healthcare Team Outcomes

Shellfish allergies do not increase the risk of radiocontrast material allergic reactions. The myth that shellfish allergy is secondary to iodine is still prevalent despite the dispelling of this false information in the literature. With that noted, any food allergy or a history of atopy is associated with an increased risk of reaction to contrast. Healthcare workers, including nurse practitioners, should always strive to get a thorough history of food and medication allergies and record it in the chart.



9/26/2022 5:56:46 PM



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