The incidence of childhood allergy to peanuts is increasing and, although in some cases reactions are relatively mild, with diarrhoea or vomiting, urticaria, sneezing, rhinitis or watering eyes, fatalities due to anaphylaxis do occur.
Some 4 to 6 per cent of children are affected by food allergy. However, in some countries peanut allergy and tree nut allergy are relatively rare. Peanut allergy is more common in children from atopic families, particularly those with eczema at an early age.
Allergy specialists traditionally refer to 'the allergic march'. Babies with eczema in the first few months are more likely to develop food allergies before the age of three.
Hay fever and later asthma are more common in these patients. Up to 20 per cent of babies diagnosed with eczema or egg allergy will develop peanut allergy by the age of five. Only 20 per cent of those with peanut allergy later grow out of their allergy.
It is now known that, in some babies, sensitisation to food allergens (estimated by skin prick testing at six months) precedes and predicts eczema, whereas in others the reverse is true.
There is also concern that inflamed skin, for example of babies with eczema, is particularly vulnerable to sensitisation to topical allergens. So babies exposed to creams and lotions with any nut-related content might become sensitised.
Whereas topical exposure to allergens may sensitise allergy-prone patients, oral exposure may actually reduce the risk of developing allergy.
Current DoH guidelines recommend that those with a strong family history of allergy should avoid peanut-containing foods up to the age of three.
In recent years, pregnant and breastfeeding women from atopic families and those who have a child already affected, have also been encouraged to avoid peanuts. However, evidence that avoidance reduces the incidence of peanut allergy is lacking.
For those who have had anaphylactic reactions to food allergens, dietary avoidance is essential. This has become a major issue in classrooms across the country, as teachers have to ensure that no foods with any trace of peanuts are found on school premises. School nurses have to be well versed in the use of Epipens and action plans produced for those children at risk.
Food industry concerns
Contamination of manufactured foods with allergens is a major concern for the food industry. French studies have attempted to define the threshold doses of food allergens that precipitate an allergic reaction. They suggested that detection tests should ensure a sensitivity of 24 peanut parts per million (ppm) and if peanut oil allergy is considered, the limit of sensitivity should be five ppm.
For those with milder symptoms, restrictive diets arbitrarily introduced by parents who suspect an allergy can be positively hazardous.
In a study of six-year-olds, 11.8 per cent were reported by parents to have a food hypersensitivity. Actual prevalence after clinical assessment and testing was only 1.6 per cent.
Cases of rickets have been described due to dietary restriction imposed without professional advice. It is important that those with suspected allergies are referred for specific skin prick testing or have IgE levels measured.
Most children with peanut allergy present at 14-24 months. Diagnosis can be confirmed by skin prick testing or IgE measurement. Skin prick testing should be performed under standard conditions on non-inflamed skin, in an environment equipped with full resuscitation facilities.
A healthcare practitioner experienced in the procedure should read the result. IgE levels can be requested by GPs (as a RAST test), with the suspected allergens specified on the immunology request form.
The LEAP (Learning Early About Peanut allergy) study at St Thomas' Hospital hopes to determine the best way of preventing peanut allergy in high-risk children.
Children aged between four and 10 months, with severe eczema or egg allergy, will be randomised either to avoid peanuts or to consume a peanut-based snack three times a week until the age of three years.
Skin prick testing and IgE measurement will be used to monitor the children.
Previous or current consumption of peanuts, a previous allergy to peanuts or a family member with peanut allergy would exclude children from the study.
In considering the results of this study it will be important to consider that many breastfeeding mothers use creams to ease symptoms of nipple discomfort and inflammation.
Such creams may contain nut traces and so babies may ingest nut allergens from the mother's skin while feeding. Parenting websites often recommend almond or other nut oils for baby massage, meaning many babies have had exposure to nut allergens topically from an early age.
An Australian study has found that prognosis can be predicted by the rise or fall in IgE to peanut by the age of three years. A rise or fall in the diameter of serial skin prick tests between the age of one and four years could also predict prognosis.
Allergies to tree nuts and sesame were more common in those with a poor peanut allergy prognosis.
Specific immunotherapy has been widely used to desensitise individuals with allergic rhinoconjunctivitis to grass pollens.
A San Francisco study showed that immunotherapy using peanut allergen with heat-killed listeria reduced the reaction to oral peanut in allergic dogs and reduced the size of the skin prick response.
Injections of peanut protein and a bacterial adjuvant have also been shown to protect peanut-allergic mice from anaphylaxis.
A Chinese herbal medicine formula is being studied after it was found to block anaphylaxis for up to five weeks. A small study has reported some success with oral immunotherapy for peanut allergy, with tolerance gradually increasing over 21 or more months.
Finally, a promising clinical trial of monthly injections of humanised recombinant anti-IgE antibodies increased the threshold for allergic responses to peanut in subjects, but only as long as they were continued.
Dr Miller is a GP in West London.
LEAP study website: www.leapstudy.co.uk
Halken S. Pediatr Allergy Immunol 2004; 15:4-5, 9-32.
Li XM. Curr Opin Allergy Clin Immunol 2005; 5: 287-92