Dr. Susan Waserman: Most food allergies start in childhood, but you’re right, they can develop at any time in a person’s life.

Factors that favour the development of food allergy in adults include sensitization to pollens and house dust mites, as well as occupational sensitization (which occurs when people who regularly handle and inhale food become sensitized to it due to repeated exposures).

As well, adults with birch pollen allergy may develop “oral allergy syndrome,” which leads those affected to experience itchy mouth when they eat certain raw fruits and vegetables. This is the result of cross-reactivity between the food and the pollen.

Interestingly, those sensitized to house dust mites may end up with shrimp allergy because both contain the protein tropomyosin.

Occupational sensitization is also associated with crab and pork allergies.

Dr. Susan Waserman is an allergist and Professor of Medicine in the Division of Allergy and Clinical Immunology at McMaster University in Hamilton, Ont. She is also a past president of the Canadian Society of Allergy and Clinical Immunology.

We welcome your question to Allergic Living’s Ask the Allergist. Thank you for understanding that the specialists aren’t able to answer every question received.

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

It’s crucial that people with a severe allergy to seafood carry epinephrine (brands include EpiPen, Auvi-Q, Allerject) with them at all times, as well as other medications, such as antihistamines and inhalers, that may be recommended by their allergists.

It’s also important to remember that epinephrine is considered an emergency measure – not a treatment – so people should avoid taking unnecessary risks. But with a few simple precautions, people with allergies to seafood should be able to lead full, normal lives.

What’s Safe?: It’s possible to be allergic to just one or two forms of fish or shellfish – for example, some people can eat lobster but not scallops, while others can eat cod but not salmon. But because there is a high level of cross-reactivity within the food groups, many need to avoid either fish or shellfish in all their forms.

It is important to note, however, that the key allergens in fish and shellfish are completely unrelated, so even if you are allergic to shellfish, finned fish might be just fine. (There are people who are allergic to both fish and shellfish, but this is rare.) If you’re unsure of what’s OK and what’s not, make sure to talk it over with your allergist.

Acceptance: Most people with seafood allergies develop them later in life, which can be tricky, because they may be accustomed to eating without restrictions. “But I’ve never had a problem with seafood,” is a common refrain, so sometimes people take unnecessary risks and try to eat the food that has caused them to react. But once you have a seafood allergy, it’s very important to avoid the allergen altogether, as the allergy can worsen with more exposure.

Know What You’re Eating: Seafood comes in many different forms.
Shellfish can include mollusks such as clams, mussels, and oysters, as well as crustaceans such as shrimp, lobster and crabs. Other forms include squid (the main ingredient in calamari), octopus, prawns, periwinkle, limpets, abalone, cockles, quahogs, snails (or “escargot”), langoustines and sea urchins. The most allergenic type of shellfish is shrimp.

There are many different types of fish, including anchovies, bass, bluefish, catfish, char, chub, cod, eel, flounder, grouper, haddock, hake, halibut, herring, mackerel, mahi-mahi, marlin, monkfish, perch, pickerel, pike, pollock, rockfish, salmon, sardines, shark, smelt, snapper, sole, sturgeon, swordfish, trout, tuna, turbot, whitefish and more.

It is very important that you understand the different names of the food you are allergic to, and carefully read all food labels so you can avoid them.
See: Label Aware

One thing to be cautious of with seafood allergies: imported foods. Not all countries have the stringent labeling requirements of the United States, Canada and the European Union. Don’t take chances if you suspect fish or shellfish could be ingredients of an import.

Next Page: Hidden Sources

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