There’s been a tremendous amount of media attention on the subject of food allergy. While the exact prevalence is unknown, recent estimates suggest that 6% of young children and up to 4% of adults in North America are affected and that these numbers are continuing to rise.^1-3 In the U.S., approximately 100-200 deaths per year⁴ are attributed to food allergy. Accurate diagnosis is critical, as accidental exposure to even minute quantities of the food may result in anaphylaxis.

The priority allergens defined by Health Canada are:

• peanuts
• tree nuts
• eggs
• milk
• soy
• wheat
• sulphites
• fish
• shellfish
• sesame
• mustard

Allergy to peanut, tree nuts, fish and shellfish is lifelong in most, although up to 20% of individuals may outgrow peanut allergy.⁵ The majority of children will outgrow milk and egg allergy by school age.⁶

Peanut and tree nuts are responsible for most serious allergic reactions and food allergy related fatalities,⁷ but any food may be potentially implicated.

Signs and symptoms

Food allergy presents with well-defined signs and symptoms, based on immunological mechanisms. The immune response can be IgE-mediated (immediate reaction), as in the case of anaphylaxis and oral allergy syndrome (OAS), or non-IgE-mediated (delayed onset), as in the case of milk-induced enterocolitis and celiac disease. Food allergy is NOT a cause of conditions such as:

• migraines
• behavioural or developmental disorders
• arthritis
• seizures

Allergic individuals will experience reactions of varying severity after ingesting the culprit food. The most severe allergic response is anaphylaxis, which may involve any organ system (Table 1).

It was only in 2006, that a definition of anaphylaxis was reached by consensus. Anaphylaxis is felt to be highly likely when the following 3 criteria are fulfilled following exposure to an allergen:

• Acute onset of an illness (minutes to hours) with involvement of the skin and/or mucosal tissue with at least one of respiratory compromise or reduced blood pressure.
• Symptoms involving two or more organ systems (skin/mucosal, respiratory, cardiovascular, GI) that occur rapidly after exposure to a likely allergen for that patient.
• Reduced BP following exposure to a known allergen for that patient.^8,9

The mildest IgE mediated reaction is an “oral allergy syndrome” — a tingling and itching of the mouth and pharynx. This is typically triggered after consumption of fresh fruits and vegetables in pollen-allergic individuals. It’s caused by cross reactivity of IgE antibodies against certain pollens with proteins in some fresh fruits and vegetables (Table 2). These proteins are heat labile, enabling allergic individuals to eat these foods when cooked. Allergy skin tests are usually negative. In a small proportion of OAS, there may be progression to systemic symptoms.

Diagnosis

For suspected food allergy, referral to an allergist is essential. Patients should avoid the food in question until assessment. An epinephrine auto-injector should be prescribed even if the diagnosis is uncertain, especially if peanut, tree nuts, fish or shellfish are involved.

Diagnosis of a food allergy includes a detailed history and physical examination, and diagnostic tests such as skin prick tests (SPT) and/or an ImmunoCAP^®, a blood test which measures specific IgE to the food. History should elicit symptoms, timing, reproducibility, and associations such as alcohol or exercise. Physical examination is used to look for supporting evidence of atopy, and to exclude non-allergic causes of symptoms. The SPT is a quick, safe and sensitive method to diagnose food allergy. A positive SPT appears as a wheal and flare reaction when the responsible food is applied and pricked into the skin. A positive SPT has a sensitivity of about 90%, however, the specificity is only around 50%. Therefore, a positive SPT alone is not enough to diagnose a food allergy — the patient must have a supportive history.

If SPT can’t be done, the ImmunoCAP^®, although somewhat less sensitive than SPT, can be used for diagnosis. The level of specific IgE may also be used to determine the patient’s risk of reaction if challenged with the food.

The negative predictive value of a negative SPT is > 95%. If there’s still clinical suspicion, a food challenge may be appropriate, and should be considered in consultation with the allergist. Assessment by a specialist in gastrointestinal diseases may also be appropriate.

Treatment

There’s currently no proven treatment for food allergy, beyond avoidance of the responsible foods. In the case of accidental exposure, the treatment of choice is epinephrine. There are currently two epinephrine auto-injectors available in North America: EpiPen^® and Twinject^®. Both products come in two dosages (0.15 mg and 0.30 mg), which are prescribed based on weight. According to product instructions, the 0.30 mg dosage of both the EpiPen® and Twinject^® auto-injectors should be used for those weighing 30 kg or more; and the 0.15 mg dosage for children weighing between 15 kg to 30 kg. Antihistamines are only a second-line treatment.

Patients and caregivers must be educated on food avoidance, recognition of allergic symptoms, and early management of anaphylactic reactions. They must read food labels carefully, watching for hidden ingredients such as “natural flavour,” and “may contain” warnings. All food-allergic patients should have medical identification (such as a MedicAlert bracelet/necklace), and carry an epinephrine auto-injector.

Signs and symptoms typically develop within two hours of food exposure. Reactions can be unpredictable: they vary from person to person, and also within the same person from attack to attack, so early symptoms shouldn’t be ignored. This is especially true if there’s a history of a previous anaphylactic reaction.

Recent research has shown that peanut-allergic kids can be desensitized by feeding them increasing amounts of peanut under close supervision.¹⁰ Results of these studies are promising, but they’ve involved only small numbers of subjects, and further confirmatory studies are needed. Similar work has been done on egg and milk allergy.

Prevention of food allergy is also being extensively studied. Older recommendations suggesting avoidance of highly allergenic foods in the diets of all infants and pregnant/breastfeeding mothers haven’t been shown to decrease the prevalence of food allergy or atopic disease. The updated 2008 American Academy of Pediatrics guidelines now state that “no current convincing evidence exists to recommend specific avoidance of certain foods beyond 4-6 months of age for the prevention of allergy.”¹¹

Food allergy is an important clinical problem of increasing prevalence. Assessment by an allergist is essential for appropriate diagnosis and treatment.