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The Seafood Section

Shellfish and Fish Allergies Explored

“The last thing I remember was they took my blood pressure and said, ‘this guy’s lucky to be alive’ It was very, very low.”

Oleson saw an allergist for allergy skin and blood tests. He turns out to be part of a minority allergic to both shellfish and finned fish (though his shellfish sensitivity measures much higher).

What he realizes today is that this was actually his second seafood reaction. The first came eight months earlier, at a gathering at his in-laws home in Washington state. The so-called “bad flu” he suffered had overcome him right after a dinner of fresh cooked lobster. While he’d eaten crab all his life, that was his first experience eating lobster.

Those who suddenly find themselves allergic wonder: what causes a seafood allergy to switch on in adulthood?

“No one knows for sure,” says Dr. Scott Sicherer, an allergist and associate professor of medicine at Mount Sinai. Sicherer is a co-author of the U.S. prevalence survey and also has written a two-part review of seafood studies from around the world,  published in 2006 in UpToDate, an evidence-based clinical information resource for physicians.

While there are no firm answers as to “why,” he identifies a couple of theories. Scientists are intrigued that the main allergen in shellfish, the muscle protein tropomyosin, is very similar to the tropomyosin found in dust mites and cockroaches, highly allergenic pests.

“One theory would be air exposure to these [insect] proteins could drive a shell­fish allergy,” says Sicherer, which might explain studies in which orthodox Jews, who had never eaten a crustacean, tested positive to shellfish.

But like most allergy theories, this one has holes: how to resolve, as Sicherer notes, that “the vast majority of people testing positive to dust mites are fine eating shellfish?”

Another hypothesis is that adults who become allergic eat seafood on occasion rather than regularly, and that may affect their tolerance.

“For example,” Sicherer says, “we have seen some people re-develop a peanut allergy when they did not consume [peanut] for about a year after tolerating it in a doctor-supervised feeding test. Maybe not having these foods in the diet routinely allows the immune system to misdirect its responses – sort of forgetting that the food is `innocent’.”

The key allergic proteins in fish and shellfish are unrelated – so people are usu­ally only allergic to one or the other seafood group. But if you’re highly allergic to shrimp and want to try eating a fish not in your diet, Sicherer recommends speak­ing to an allergist first and probably testing “to avoid surprises”.

He sees another confounding aspect of these allergies: there can be a great variability of triggers from person to person. You might, for instance, be allergic to caviar (fish eggs) of a fish, but not the fish’s meat.

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