Oral allergy syndrome causes symptoms around the mouth that are triggered by certain foods. It is associated with other conditions caused by plant allergies, such as hay fever and allergy to birch pollen.
What is oral allergy syndrome?
Oral allergy syndrome (OAS) is an allergic condition which causes itching and swelling of the lips and mouth after eating raw fruit, vegetables or certain nuts. All raw foods that cause symptoms should be avoided.
Usually no symptoms occur after eating the same fruit, vegetable or nut when it has been cooked (eg, jam) or processed (eg, fruit juice). However, lightly cooked foods, such as stir-fried vegetables or steamed or poached fruits or vegetables, may also cause a reaction.
If you do also have symptoms with cooked fruits or fruit juices as well as when raw, then you have a more severe type of allergy to fruits and vegetables.
What causes oral allergy syndrome?
The symptoms are caused by a reaction to plant proteins (allergens), which are similar to those in pollen. There are several allergens that can be responsible for the condition.
The most common allergen in the UK is birch pollen. It has been estimated that as many as 3 in every 4 people with allergy to birch experience a sensation in the lips or tongue after eating raw apples, which is a mild form of OAS.
Many people with OAS have had hay fever symptoms (such as itchy eyes and nose, and sneezing) between February and July for a number of years before developing OAS.
People who have hay fever from February to May are usually allergic only to tree pollen and are more likely to react only to fruits and some nuts such as hazelnuts.
People who have hay fever from March to the end of July are probably allergic to both grass and tree pollen and are not only more likely to develop OAS, but will be potentially allergic as well to a much wider range of foods, including fruits, vegetables, tree nuts and peanuts.
Allergy to grass and mugwort pollens can also cause OAS and a similar condition occurs in some patients with rubber latex allergy.
There is, at present, no way to predict who will develop a problem with any particular food.
Which foods are involved?
Most people with OAS are sensitive to one or two foods, although some will find they may develop problems with a number of the different foods.
Foods that are more common causes of oral allergy syndrome include: apple, carrot, walnut, cherries, peach, potato, peanuts, plum, pear, fennel, wheat, honey, nectarine, spinach, hazelnuts, almonds, strawberries, brazil nuts, celery, apricots, melon, watermelon, cucumber, tomato, camomile tea, and spices (eg, cumin, coriander, parsley).
- Birch pollen cross-reacts with almond, apple, apricot, raw carrot, raw celery, cherry, coriander, fennel, hazelnut, kiwi, nectarine, parsley, parsnip, peach, pear, peppers, plum, raw potato, prune, tomato, and walnut.
- Rye-grass pollen cross-reacts with melon, peanut, tomato, and watermelon.
- Rubber latex cross-reacts with almond, apple, apricot, avocado, banana, chestnut, cherry, dill, fig, ginger, kiwi, mango, melon, oregano, papaya, passion fruit, peach, pear, plum, raw potato, sage, and raw tomato.
How common is oral allergy syndrome?
Many people do not realise that they have a mild form of food allergy and the condition is therefore not always diagnosed.
Children and teenagers occasionally develop OAS but it is more common in adults.
What are the symptoms of oral allergy syndrome?
The main symptoms are redness, swelling and itching in the mouth immediately after contact with certain foods. There may be a blotchy, pimply or even blistering rash on the lips, tongue, the inside of the mouth, and the soft palate.
Symptoms in the gullet (oesophagus) or stomach occur only very occasionally. This causes discomfort, heartburn, nausea or even vomiting.
General allergy symptoms, such as urticaria, rhinitis and asthma, are relatively unusual but when they do occur, it is normally after a period of minutes or hours. These symptoms are more likely if you ignore the initial mouth symptoms and eat all of the food causing the reaction.
Anaphylaxis is a rare complication of OAS. See the separate leaflet called Anaphylaxis for more information.
What are the tests for oral allergy syndrome
If it is essential to accurately identify exactly which fresh fruits or raw vegetables are causing OAS.
The symptoms and association with foods are much more important than tests in the diagnosis of OAS.
The standard skin prick and specific IgE (RAST) blood tests that are used for food allergy are unreliable. Food allergens causing OAS are usually damaged or destroyed by heat and processing, and so are likely to be destroyed during the process of making the extracts used in these tests.
Allergy skin prick, testing using the juices from the fresh fruits and raw vegetables themselves, can be tried. This is a more sensitive test and can be effective. A skin prick test for birch pollen is the most useful of the standard allergy tests.
Standard allergy tests covering peanuts and all the tree nuts are also often helpful. Almond, hazelnut and walnut are the nuts often involved in OAS.
If you are worried about eating other foods then you can safely test them yourself:
- Rub the food on the outside and then the inside of your lip.
- If you feel no tingling or itchiness or swelling then take a bite, chew well and spit it out.
- Then wait for 15 minutes to see whether you develop any symptoms that suggest OAS.
NB: You must only perform this test if your usual symptoms are mild and only involve your mouth. If your symptoms usually include swelling, wheezing or tummy (abdominal) pain then you must not test any food except under medical supervision.
What are the treatments for oral allergy syndrome?
All raw foods which cause symptoms should be avoided. If lightly cooked foods also cause a reaction, they should also be avoided. If your symptoms are not just in your mouth, you may also need to be careful with cooked foods. If you have symptoms of OAS caused by nuts then you should avoid these in both their raw and their roasted state.
There are currently no desensitisation treatments for OAS. However, several studies of specific immunotherapy using birch pollen have shown at least a temporary improvement of symptoms among people with OAS who are allergic to birch pollen.
What is the outcome (prognosis)?
OAS is usually a lifelong condition and allergies to other fruit and vegetables may also develop. However, it is relatively harmless if you can avoid any fruit, vegetable or nut that causes symptoms.
Further reading and references
; Oral allergy syndrome. Allergol Int. 2009 Dec58(4):485-91. doi: 10.2332/allergolint.09-RAI-0136. Epub 2009 Oct 25.
; Oral allergy syndrome. CMAJ. 2010 Aug 10182(11):1210-1. doi: 10.1503/cmaj.090314. Epub 2010 Jun 21.
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