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 of which to be aware in shellfish. Saxitoxins are responsible for causing paralytic shellfish poisoning and are secondary to dinoflagellates. 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. 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.

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.

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.

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.

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.

There is no specific treatment for shellfish allergies. 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.[9][10][11]

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.

• Acute urticaria

• Bacterial gastroenteritis

• Bulimia nervosa

• Celiac disease

• Chronic gastritis

• Constipation

• Diverticulitis

• Dumping syndrome

• Esophageal spasm

• Esophageal stricture

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.