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

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