Hen’s egg plays an important role in the nutrition all over the world and is used as an ingredient in a vast variety of foods. Egg allergy is now one of the most frequent food allergies in childhood. Egg allergy is a so-called IgE-mediated food allergy. IgE (Immunoglobulin E) is the allergy antibody.
Eggs from birds contain several proteins capable of inducing an immune response in man – most of these are present in the egg white. Allergenic proteins in the yolk only contribute to a minor degree and isolated allergy to egg yolk proteins are rare (see cross reactions).
Most of the allergenic proteins present in the egg white are both heat- and digestion resistant and may also be found in the house dust in normal households.
Although not thoroughly investigated, sensitization is believed to take place by ingestion of egg proteins in the diet – even the minute quantities present in human milk can sensitize the infant in some cases, where a reaction is elicited the very first time the infant eats a meal containing egg.
Ingestion of egg will in sensitized individuals elicit an immediate allergic reaction starting with itching of the mouth and pharynx, followed by nettle rash (urticaria), vomiting, rhinoconjunctivitis, asthma and in rare cases anaphylaxis. Exacerbation of atopic eczema may be seen also, but isolated reactions taking place hours to days after intake, without a previous immediate reaction, are rare.
Related foods (cross-reactions)
Eggs from other birds contain allergenic proteins similar to those in egg and will also elicit reactions in egg allergic patients, whereas ingestion of meat from poultry rarely causes reactions. Patients sensitized by inhalation of allergenic proteins from birds (e.g. canary bird) may experience symptoms when ingesting egg yolk, due to cross.
Who, when, how long and how often?
In recent epidemiological studies, egg allergy is one of the most common food allergies in childhood, and is found in approx 2 per cent of the young children. More than half of the children will outgrow their clinical reactivity to egg before adulthood, leaving only less than 1 per cent of the adult population allergic to egg. Early sensitization to egg in infancy is, however, also a predictor for later development of asthma and allergy to inhalants.
How much is too much?
Most patients react to doses of egg in the mg to g range – in controlled challenges, 5 per cent of the patients will react to doses below 5 mg, whereas 50 per cent will experience symptoms when ingesting an amount above 100 mg. It is important to know the threshold dose for individual patients, enabling them to implement a tailor-made avoidance strategy.
The diagnosis of egg allergy follow the international guidelines initiated with a careful case history with detailed information of a close correlation between intake of food containing egg and development of allergic symptoms and signs. The case history is followed by skin prick testing and measurement of specific IgE against egg white and yolk and should always end up with a controlled challenge in order to verify or rule out the diagnosis of egg allergy, and most important to the single patient, also to establish the threshold value, which will enable a specific and tailor-made avoidance strategy. Atopy patch testing mimicking late phase eczema reactions has gained considerable interest in the last years, but should be used with caution outside scientific protocols.
Data demonstrating that a certain serum level of specific IgE can predict clinical egg allergy has been published from various centres (USA, Spain, Germany and Denmark), but the actual level predicting clinical allergy has varied from 0,35 in Spain to 17,5 in Germany and can therefore only be used with caution. Furthermore, the value of specific IgE is not correlated to the single patient’s thresholds value, and therefore can not substitute for controlled food challenge.
Since approximately half of the patients will outgrow their egg allergy, repeated evaluations with appropriate time intervals are important. It is helpful to families to have evidence that their child’s egg allergy has been outgrown, because avoiding egg in trace amounts is very difficult to do.
Where do I find hen’s egg?
Egg protein is found in everyday diets all over the world in many types of foods ranging from cooked whole egg and pasta to cakes and cookies. Egg has also been used in raw forms, but nowadays pasteurized raw eggs are used instead e.g. in mayonnaise due to a risk of bacterial contamination (Salmonella).
Egg protein is also used in cosmetic products, in particular in shampoos but also lotions and crèmes. Egg yolk lecithin is used in pharmaceutical products.
The only established treatment of egg allergy is dietary avoidance. Since egg is so widely used as an ingredient complete avoidance is difficult. Not every statement on a food label sets the alarm clock off, like albumin, ovalbumin, globulin, ovomucin, ovomucoid, vitellin, ovovitellin, yolk and silici albuminate. According to the new EU labelling directive (2003/89/EC) and the list of the Codex Alimentarius Commission, foods containing egg-derived ingredients must always be labelled. Dietary advice should be given based on the clinical threshold of the actual patient. Only the most sensitive patients should avoid foods with a very low amount of possible contamination by egg and only these patients should be careful with vaccines developed in egg yolk.