Family doctor



PEANUT ALLERGY - a patient's guide


This article provides detailed information about the current understanding ,investigation and treatment of this allergy.

peanut allergy

What is it?

Peanut allergy occurs if a person develops allergic antibodies (IgE) specific to peanuts. If the person is then exposed to peanut, the peanut reacts with the person's own IgE antibodies to cause a reaction.

 Who gets peanut allergy?

Peanut allergy is most often first diagnosed during early childhood. Children are at increased risk of developing peanut allergy if there is a family history of other allergy problems (food allergy, eczema, rhinitis and asthma). Why some of the at risk children become allergic to peanuts and others do not is unknown.

Occasionally peanut allergy can develop later in life in people who were previously tolerant of peanuts, but this is less common.

 How common is peanut allergy?

We do not have very good figures in New Zealand, but we estimate that 0.5-1% of the population is affected. This means there may be 17-35,000 people with peanut allergy in New Zealand.

 Is it becoming more common?

Again, we do not have good scientific data to prove this. Most Doctors who work in the area of allergies agree that food allergy, and peanut allergy in particular, seems to be an increasingly common problem.

 Do you have to eat peanut to get a reaction?

No, although most reactions are due to eating foods containing peanut. In one study 14% of the children having their first reaction to peanut reacted to skin contact only (eg. a smear of peanut butter on their skin).

Infrequently people can react to peanut in the air &endash; this has been reported for example on aeroplanes when a lot of people open a bag of peanuts at the same time in a confined space. This type of reaction is relatively uncommon &endash; only 14 of 3704 people on a peanut/Tree nut allergy registry in the United States reported such reactions.

 Can you react the first time you are exposed to peanut?

Yes. This doesn't seem to make sense, as you do need to be exposed to an allergen before you can develop an allergic antibody. However the exposure may be through breast milk, or through "trace" exposure in a food not thought to contain peanut. Overall about 75% of peanut allergic children react the first time they knowingly exposed to peanut.

There have been recent media reports of a possible link between peanut allergy and eczema treatment, as some skin care products contain peanut oil. However most people who are allergic to peanut do not react to highly purified peanut oil, as it contains virtually none of the protein allergen. Regardless peanut allergic patients should avoid peanut oil, as there will be some of the allergenic protein in the less refined/cold pressed peanut oils.

 What is a peanut allergic reaction like?

You can react to peanut in a variety of ways.

  • Most people will have a skin reaction, which can be hives (or urticaria), itching, and swelling, or in some cases a flare of eczema.
  • Reactions frequently cause respiratory symptoms, which can be non-life-threatening such as a runny nose, sneezing, or an itchy mouth.
  • Potentially life-threatening respiratory symptoms include swelling affecting the airway, wheeze, cough, and shortness of breath.
  • Some people also get vomiting, tummy cramps and diarrhoea as part of their reaction.
  • Reactions can also affect the cardiovascular system with a drop in blood pressure, and collapse.
  • Anaphylactic reactions are life threatening allergic reactions, usually with breathing or cardiovascular symptoms. 

How do you treat a peanut allergic reaction?

The treatment needed will depend on the type of symptoms. If you think you or someone you are with might be having an allergic reaction to peanut it is important to seek urgent medical attention. The treatment required may include:

  • Adrenaline &endash; this is the key to treating severe allergic reactions. It is given by injection, and will usually quickly counter the signs and symptoms of a severe reaction.
  • Antihistamines &endash; these can be given by injection or by mouth. They will improve symptoms, reducing itch and helping make hives/urticaria resolve. By themselves they will not treat a more severe reaction.
  • Steroids &endash; a dose of oral steroids may be given after a significant allergic reaction, aiming to prevent a late recurrence of symptoms. 

What next, after a peanut allergic reaction?

It is important to seek ongoing medical care after a peanut allergic reaction. The aims of this are to:

  • Confirm the allergy on testing.
  • Discuss appropriate avoidance strategies.
  • Develop an "Allergy Action Plan" to help manage any further reactions on accidental re-exposure.
  • Decide on follow-up, particularly for children who have a chance that they might grow out of this allergy. 

How do you test for peanut allergy?

Peanut allergy can be tested in 3 ways.

1-Allergy skin test &endash; this is a simple and fast way to test for peanut allergy. It involves placing a drop of a weak solution of peanut (plus you need positive and negative control solutions to interpret the tests properly). The drops are pricked and the results are measured 15 minutes later. A positive result looks like a little mosquito bite. If there is a good history of a peanut allergic reaction the skin test is a simple way to confirm the allergy. If there is no history of a reaction to peanut a positive test needs careful interpretation. The size of the skin test reaction does not indicate what reaction you may have.

2-Peanut RAST test &endash; the RAST test is done on blood, and measures the amount of IgE against peanut. As with the allergy skin test, if there is a good history of a peanut allergic reaction the RAST test is a simple way to confirm the allergy. The RAST result is usually reported in two ways &endash; either as 1+ to 4+ or as an exact number.

The exact number gives a more precise indication of how likely it is a reaction will occur on exposure to peanut. For example a result of 15kUA/l means it is 95% likely there will be a reaction. It does not tell you what the reaction will be. Three people with levels of 15kUA/l could have very different reactions such as eczema in one, hives in another, and anaphylaxis in the third.

The RAST can also be used to assess the chance that peanut allergy has resolved (see below).

3-Peanut challenge &endash; very occasionally there is a good history suggesting peanut allergy but negative results on skin and RAST tests. In this situation a peanut challenge may be considered. This should only be undertaken with careful hospital supervision in case a reaction occurs. The challenge will usually start with a tiny amount of peanut (eg. 1/8th of a gram) to try and avoid causing a significant reaction.

Peanut challenge may also be suggested for older children who have possibly outgrown their peanut allergy (see below).

 Can the test tell me if I'm "anaphylactic"?

No. The test tells that you have an allergic antibody to peanut present. It does not tell what sort of reaction you may have. As noted above three people with the same result could have very different reactions.

 Can you test positive but not be peanut allergic?

Yes. In one UK study looking at 4 year old children 1% had a positive skin test, but only 0.5% had a history of reacting to peanut. Interpreting a positive test if there is no history of a peanut reaction needs input from your Doctor.

 How do I avoid peanut?

Avoiding peanut is very difficult. All foods containing definite or possible peanut should be avoided. Peanut can be an unintentional ingredient in a lot of commercially produced foods, and many labels include a warning about this. All these foods should be avoided, as probably about 20% of them do contain peanut.

Most people with peanut allergy will be advised to avoid tree nuts as well (eg. walnuts, hazelnuts, brazil nuts etc) even though they are from different botanic families and don't cross react. There is a risk that other nuts may be contaminated with peanut. Also, people with peanut allergy seem more likely to develop allergy to other nuts.

Reviewing peanut avoidance with a dietician can be very useful. A food database can be used to generate lists of commercial foods that are definitely peanut free, making supermarket shopping easier.

 Do I need to avoid other legumes?

No. Although peanut is a legume most peanut allergic people are not allergic to other legumes (eg. peas, soy). If you think there may be allergy to other legumes this should be discussed with your Doctor and may need further investigation.

 What is an "allergy action plan"?

As part of the management of peanut allergy you need a written plan of how to manage further reactions on accidental exposure. The plan should include precise details of what to do for particular symptoms. A copy of the plan should be at home, but the school/day care also need a copy. The plan can also be used to explain to other family members/baby sitters etc about an allergic reaction and what to do if it occurs.

For most people with peanut allergy the plan will also include adrenaline for you to use in an emergency. The options for this include an EpiPen, an AnaKit, or adrenaline ampoules plus a needle and syringe. It is important these options are reviewed with your Doctor. You will need careful education about the appropriate use of the device you choose.

Other family members or your child's school/pre school will also need education about the allergy action plan and adrenaline device. You should discuss with your Doctor the best way to do this. In some areas they may be able to arrange for a Public Health Nurse to help with this education.

 Will the allergy go away?

Up until about 1996 most people were told that their peanut allergy was likely to be life long. In 1996 a research study in the UK showed that a third of children who had been peanut allergic in early childhood were no longer allergic at age 5 years. This study has been confirmed a number of times since.

The problem is that all the studies show that most of the children who do grow out of peanut allergy will continue to have positive skin tests and RAST tests to peanut. This means the tests alone cannot determine which children have grown out of the allergy. However if the RAST result is very high (eg. above 15kUA/l) it is extremely unlikely the allergy has gone away.

Given the difficulty of managing peanut allergy, repeat evaluation is warranted. It is probably best to delay this until age 4 years in order to give the best chance that a child has outgrown the allergy. At that time it's worth repeating the allergy tests if there have been no recent reactions on accidental exposure.

  • If the skin test/RAST test is negative then a cautious peanut challenge should be considered.
  • If the skin test is positive and the RAST test is low positive then a cautious peanut challenge should be considered.
  • If the RAST test is highly positive then it's very likely the allergy is persistent, and most likely that it will be life long. Ongoing avoidance and an up to date action plan are needed. 

Can you prevent peanut allergy?

The answer to this is not yet clear. Particularly if there is a family history of allergy however it may be worth trying to prevent peanut allergy. The New Zealand Ministry of Health suggests that if there is a family history of allergy the steps that could be taken to reduce the risk of peanut allergy include:

  • The mother avoiding peanut during pregnancy and breast-feeding.
  • The infant being breast fed (ideally to 6 months)
  • Delaying the introduction of peanut products into the child's diet until at least 3 years of age. 

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