Dr. Scott Sicherer: You are not alone in developing shrimp allergy as an adult. In our U.S. prevalence studies and studies in Canada that tracked peanut, tree nut, fish, shellfish and sesame allergies, shellfish was the most common self-reported allergy.

We know food allergies are affected by the immune system, heredity, environmental factors, and the characteristics of the foods themselves. We know that many food allergies are outgrown, so there are clearly differences between children and adults. What we are missing are the exact details underlying each factor and how the factors interrelate. We only have theories to address your question.

The characteristics of food proteins likely play a role. Proteins responsible for persistent and severe allergies are more resistant to digestion and more likely to be recognized by the immune system. It may be that the child’s immature gut or immune system is more prone to attack the proteins, but shrimp may be an example of a protein that is particularly capable of triggering an immune attack even for adults.

A route of exposure other than through the mouth may be a contributing problem for adult-onset allergy. Most food allergies that begin beyond childhood are mild reactions to raw fruits and vegetables. This type of allergy is called oral allergy syndrome and is related to proteins in pollens. For example, apple shares similar proteins with birch pollen. Thus, becoming allergic to similar proteins in the air starts the trouble. A theory has also been proposed that environmental exposure to peanut, without actually eating peanut, may increase the risk of peanut allergy. Interestingly, shellfish proteins are similar to ones found in dust mite and cockroach, although most people with those allergies tolerate shellfish.

Next: Digestion Changes

Tropomyosin, the allergenic protein in shrimp, is also found in dust mites, cockroaches and other insects. The goal of the study was to determine whether there was a strong correlation between shrimp, cockroach and dust mite IgE levels.

Researchers at New York’s Mount Sinai School of Medicine examined blood samples of asthmatic children from seven inner-city areas across the United States. The children who had a high exposure to cockroaches at home had significantly higher levels of shrimp IgE than children who had low exposure to cockroaches.

There was not a high correlation between dust mite exposure and shrimp IgE levels.

Maybe you feel like you’re going to be sick to your stomach, or feel your throat tightening and your lungs beginning to wheeze. You may feel flushed and hot as your face turns beet red. Maybe you break into hives from head to toe, or feel like you’re going to pass out as your heart takes off like a bullet train.

Chances are that you are having an allergic reaction to seafood – the most common culprits in the allergy world.

What’s The Problem?

Seafood can be divided into two groups: shellfish and fish. Shrimp is behind most of the reactions, and shellfish as a group – which can include oysters, mussels, scallops, squid, crab, snails and many more – tends to be more problematic than finned fish. (In one survey by FAAN and Mount Sinai researchers, two per cent of respondents reported a shellfish allergy, whereas just 0.4 per cent reported an allergy to fish.)

Symptoms of a reaction to seafood can include flushing and swelling in the face, mouth and throat; digestive tract symptoms such as nausea and diarrhea; itching and hives; difficulty breathing; lightheadedness or faintness; a sudden drop in blood pressure and a rapid heartbeat.

In severe cases, a person may experience anaphylactic shock, a reaction that includes one or more of the body’s systems and can result in cardiac arrest and death.

Widespread Incidence

A whopping 2.3 per cent of Americans are allergic to some form of seafood, whether it’s fish, crustaceans or mollusks. In Canada, 1.69 per cent of people are allergic to shellfish and 0.48 per cent are allergic to fish. What’s more, a 15-nation study showed the incidence is similarly high right around the world.
See: Seafood Allergy Statistics

Because of its high incidence, Health Canada has named both shellfish and fish “priority food allergens”, which means tougher labeling rules for Canadian manufacturers. Other regions, including the United States and Europe, have also included seafood on their lists of top allergens.
See: Label Aware

It’s a Grown-Up Thing

What’s different about seafood allergies is that they are largely an adult phenomenon. In Canada, 1.69 per cent of adults have shellfish allergy, compared to just 0.5 per cent of children. According to the FAAN-Mount Sinai survey, 2.8 per cent of adults in the United States reported a seafood allergy, as compared with just 0.6 percent of children.

This can be especially problematic, since most adults are accustomed to eating seafood, so when they react, it doesn’t seem to make sense to them. Often they dismiss earlier, less severe reactions as food poisoning or simply “something they ate”; but then they get a tough lesson in science as the allergy grows more severe and the reactions more extreme.

Unfortunately, once an adult develops a shellfish allergy, he or she likely has that allergy for life.

Next page: Seafood Allergy Myths

There would still be protein traces and no one is suggesting that the one in 50 Americans who are allergic to shellfish (the family that includes shrimp, lobster, clams and crab) begins tucking into a well-boiled chowder.

But the study, published in the Journal of Food Science in January, shows promising results on the possibility of lowering the allergenic properties of shrimp. Tropomyosin (TM) is the main allergen in seafood, including shrimp.

Food scientists have found that boiling shrimp for 10 minutes seems to reduce the  allergenic potency. Researchers compared raw and boiled shrimp that had been ground and freeze-dried and found that the boiled shrimp had fewer allergenic properties than its raw counterpart.

While a far cry from a cure, lead researcher Guang Ming Liu believes the findings are a step in the right direction: “Understanding the allergenic properties of shrimp as affected by the cooking process is critical for shrimp allergic individuals.”

Fish: includes salmon, tuna, cod.
Crustaceans: includes shrimp, lobster, crab.
Mollusks: snails, bivalves (mussels, scallops, oysters), squid.

Cross-Reactions

Within a seafood group …

Dr. Scott Sicherer, author of Understanding and Managing Your Child’s Food Allergies and associate professor of medicine at the Mount Sinai School of Medicine’s Jaffe Food Allergy Institute, concludes from his 2006 review of seafood allergy studies:

- That 30 to 50 per cent of those with an allergy to one of the seafood groups will react to more than one type of fish or shellfish.

However, you can also be allergic to just one type of fish or shellfish. It’s even possible to be allergic to just one type of shrimp.

- That it is common for people to be allergic to more than one shellfish. Sicherer finds up to 80 per cent who are allergic to one crustacean may be sensitized to others, and “40 per cent may react upon ingestion.”

In one study reviewed, blood samples from nine patients with shrimp anaphylaxis reacted to the proteins of 13 crustaceans and mollusks. As well, he finds between 10 and 15 per cent allergic not only to other crustaceans, but to mollusks as well.

- That the risk of reacting to more than one fish is high. Sicherer suggests to speaking to your allergist: “Your allergist may take into consideration the severity of your allergy, test results, and dietary preferences to determine what it is you can and can not eat.”

- That about 50 per cent with a mollusk allergy react to more than one mollusk.

Between shellfish and fish …

According to Sicherer, only about 10 per cent react to both, which have quite different key allergenic proteins (tropomyosin in shellfish; parvalbumin in fish).

Remember, however, that there may be cross-contact between the two groups at grocery seafood counters or in restaurants.

Sources: UpToDate 2006; interview Dr. Scott Sicherer

Fish allergy is less common, affecting 0.18 per cent of children and 0.56 per cent of adults.

In general, food allergy is on the rise in North America and other developed countries In Canada, an estimated 7.5 per cent of people have food allergies, representing more than 2.5 million people. In the United States it’s estimated that 12 million Americans (or just under 4 per cent of the population) have food allergies.

A major study in the United States recently found that cases of peanut allergy in children more than tripled in a decade, and that more than 3 million Americans now have a peanut or nut allergy.

Here are the rates of some of the major allergens in Canada and the U.S.

CANADA

Source: Surveying Canadians to Assess the Prevalence of Common Food Allergies and Attitudes towards Food LAbelling and Risk (SCAAALAR) study. Published in the Journal of Allergy and Clinical Immunology, June 2010.

UNITED STATES

Peanut, Tree Nut or both:
Children -  2.1 %
Adults – 1.3 %

Source: US prevalence of self-reported peanut, tree nut, and sesame allergy: 11-year follow-up. Published in the Journal of Allergy and Clinical Immunology, June 2010.

Milk and egg allergy are both common in children. In the United States, 2.5 per cent of children under the age of 3 have a milk allergy. Egg allergy affects 1.5 to 3.2 per cent of children.

The good news is many children outgrow their allergies to milk and egg. Research shows 80 per cent of milk allergy is outgrown by age 16, while 68 % of egg allergy is outgrown by that age.

Source: AAAAI