Shellfish are often blamed for causing an “upset stomach,” and the assumption may be that it is caused by a “shellfish allergy.” This may indeed be the case because shellfish are one of the commonest causes of allergy and in some cases reactions can be severe. Any significant reaction to shellfish should be investigated by appropriate allergy testing and on no account should someone who suspects that a reaction was due to allergy, try a home “challenge” in case the most severe type of allergic reaction known as “anaphylaxis” is induced. Such challenges, if necessary at all, should only be done in a hospital setting by someone trained in diagnosing and managing severe allergic reactions.
However, not all shellfish reactions are allergic in nature. This paper describes other causes of reactions that may be confused with allergy.
COUNTRY : USA
The popularity of shellfish has been increasing worldwide, with a consequent increase in adverse reactions that can be allergic or toxic. The approximate prevalence of shellfish allergy is estimated at 0.5-2.5% of the general population, depending on degree of consumption by age and geographic regions. The manifestations of shellfish allergy vary widely, but it tends to be more severe than most other food allergens.
Tropomyosin is the major allergen and is responsible for cross-reactivity between members of the shellfish family, particularly among the crustacea. Newly described allergens and subtle differences in the structures of tropomyosin between different species of shellfish could account for the discrepancy between in vitro cross-antigenicity and clinical cross-allergenicity. The diagnosis requires a thorough medical history supported by skin testing or measurement of specific IgE level, and confirmed by appropriate oral challenge testing unless the reaction was life-threatening.
Management of shellfish allergy is basically strict elimination, which in highly allergic subjects may include avoidance of touching or smelling and the availability of self-administered epinephrine. Specific immunotherapy is not currently available and requires the development of safe and effective protocols.
But symptoms after consuming shellfish may be due to something else:
Shellfish poisoning frequently masquerade as an allergic reaction. Ingestion of contaminated shellfish results in a wide variety of symptoms depending on the concentration of toxins and amount consumed. Five types of shellfish poisoning have been identified . Scombroid poisoning  has been linked to fish by the action of bacteria on muscle histidine and production of histamine. To the best of our knowledge, we did not encounter any reports in the English literature on scombroid poisoning from shellfish consumption.
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