Peanut Allergy

Peanut allergy is an IgE-mediated reaction to peanut proteins and is a leading cause of fatal food anaphylaxis. It usually persists lifelong.

Pathophysiology. Peanut is a legume. Storage proteins Ara h 1, Ara h 2, Ara h 3 and Ara h 6 cause systemic reactions, while Ara h 8 (birch pollen cross-reactive) causes mild oral symptoms. Sensitisation can follow skin exposure in infants with eczema.

It affects roughly 1 to 2% of children in high income countries. About 20% of affected children outgrow it, and persistence is more likely with high Ara h 2 specific IgE, early onset and severe reactions.

Reactions occur from very small amounts, and asthma raises the risk of severe or fatal reactions.

• Urticaria, angio oedema, flushing within minutes
• Oral itching and throat tightness
• Vomiting, abdominal pain, diarrhoea
• Wheeze, cough, rhinitis, stridor
• Hypotension, collapse and anaphylaxis
• Reactions from traces, kissing, or shared utensils
• Mild oral symptoms only in birch pollen related (Ara h 8) sensitisation

• History: food, dose, timing, route and severity.
• Skin prick test: a wheal of 8 mm or more makes a reaction likely, and under 3 mm makes it unlikely.
• Specific IgE to peanut and to components (Ara h 2, Ara h 1, Ara h 3, Ara h 6, Ara h 8): Ara h 2 over 0.35 kUA/L raises the likelihood of true allergy and risk of systemic reactions, and higher values predict reactions to smaller doses. Isolated Ara h 8 sensitisation suggests mild oral symptoms.
• Supervised oral food challenge is the reference test when results are uncertain.
• Basophil activation test where available.
• Asthma and atopy assessment.

Differential diagnosis.

• Tree nut and sesame allergy
• Pollen food allergy syndrome (Ara h 8)
• Lupin and soy cross-sensitisation
• Cow's milk and egg allergy
• Mastocytosis and idiopathic anaphylaxis
• Intolerance and anxiety related symptoms

• Strict avoidance of peanut and products labelled as containing it. Read labels and ask about cross-contact.
• Adrenaline auto injector carried at all times: 150 microgram for 7.5 to 25 kg, 300 microgram for 25 to 50 kg, 500 microgram for over 50 kg, with a written action plan and training of carers and school.
• Anaphylaxis: adrenaline 0.01 mg/kg intramuscularly (maximum 0.5 mg) into the anterolateral thigh, repeated after 5 minutes if no improvement, plus oxygen and intravenous fluids, and observation for at least 6 hours.
• Mild reactions: cetirizine (children 1 to 5 years 2.5 mg twice daily; 6 to 11 years 5 mg twice daily; over 12 years and adults 10 mg once daily). Do not delay adrenaline in a patient with asthma or a rapidly progressing reaction.
• Asthma control: optimise, because poorly controlled asthma raises the risk of fatal reaction.
• Oral immunotherapy (specialist only, ages 4 to 17 years): a build up from 3 mg to 300 mg of peanut protein daily over about 6 months, then 300 mg daily maintenance, which raises the reaction threshold but does not allow free eating of peanut and carries a risk of reactions and eosinophilic oesophagitis.
• Omalizumab (dosed by weight and baseline total IgE) as an adjunct in selected severe cases.
• Early introduction: infants with severe eczema or egg allergy have allergy assessment first and then peanut introduced from 4 to 6 months (about 2 g peanut protein three times weekly, for example smooth peanut butter or peanut puff). Other infants take peanut from about 6 months.

Special groups.

• Adolescents and young adults: the highest risk of fatal reactions, so review adherence, auto injector carriage and alcohol, exercise and illness as cofactors.
• Mild disease limited to Ara h 8: reassurance and oral antihistamine for oral symptoms.

Monitoring. Review every 6 to 12 months. Repeat skin prick or Ara h 2 specific IgE every 1 to 2 years in children to judge resolution. Check auto injector dose, expiry and training.

Referral and escalation. Refer to allergy for any peanut reaction. Emergency for anaphylaxis, or wheeze and throat swelling after exposure.