Shellfish and Fish Allergies Explored

in Fish & Shellfish
Published: September 10, 2010

From the Allergic Living Archives.

A quick test: what’s the most widespread food allergy in North America today?

If you answered ‘peanut’, that’s incorrect. But you could hardly be blamed given that legume’s notoriety. The right answer is seafood – from fish to crustaceans and mollusks.

At a time when every “must-try” new restaurant is an over-priced sushi bar, an astounding 2.3 per cent of Americans are now allergic to one form or another of seafood. (There is no such specific Canadian data, but general rates of allergic prevalence are considered comparable.)

If that incidence comes as a surprise, however, you’re in good company. Even the researchers who surveyed 5,500 households for the Food Allergy & Anaphylaxis Network (now FARE) in Virginia and the Mount Sinai School of Medicine in New York to arrive at this finding were taken aback.

“We said, ‘can this be real?”‘ recalls Anne Munoz-Furlong, founder and former CEO of FAAN and co-author of the resulting seafood prevalence study published in 2004. “We learned that shellfish and fish is about double the incidence of peanut and tree nut combined.”

In fact, the researchers reviewed the data to see if something was, well, fishy. But it wasn’t. Those who were accepted as allergic in the detailed phone survey “were very clear in their answers and the symptoms were very clear,” says Munoz-Furlong. Supporting those findings was a 15-nation study that showed similarly high levels of seafood allergies.

Shrimp was the most common cause of reactions in the FAAN-Mount Sinai survey. In fact, shellfish as a group – crustaceans and mollusks – proved by far the larger culprit, with 2 per cent of survey participants reporting shellfish allergy while 0.4 per cent reported fish.

Another eye-opening finding was that this is largely an adult phenomenon. The survey found 2.8 per cent of adults were seafood allergic compared to 0.6 per cent of children. And that can be one of the most difficult aspects of this condition – since it strikes grownups, who’ve been eating for years as they pleased, they often don’t know what hit them.

What happened to Chris Oleson is not uncommon. The 41-year-old is a self‚Äëdescribed “beach guy” who lives in San Francisco and grew up surfing on the California coast.

One day in 2002, he was with his wife Julia at an upscale sushi restaurant. He’d eaten about five or six very tasty” items from the trays on offer, when a waiter approached. “Sir, your ears are turning red. You may want to get that checked out,”‘ Oleson recalls him saying.

Oleson was suddenly rather feverish and unwell, but he wasn’t that concerned. He and his wife just thought he was coming down with a bug and had better get home. Oleson has asthma, and on the drive home an attack started coming on. He was also feeling woozy when Julia said, “you know, you’ve got hives.”

By the time they got through the door at a nearby hospital, Oleson was staggering. He lost consciousness on a stretcher. “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?”

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