Tree Nut Allergy

Tree nut allergy is an IgE-mediated reaction to nuts such as walnut, cashew, pistachio, hazelnut, almond, pecan, Brazil nut and macadamia. It tends to persist, and reactions are often severe.

Pathophysiology. Storage proteins (for example Jug r 1 and Jug r 4 in walnut, Ana o 3 in cashew) cause systemic reactions. Heat and digestion stable proteins explain reactions to cooked or processed nuts, while hazelnut, almond and other nuts can also cause mild oral symptoms through birch pollen cross-reactivity (Cor a 1).

It affects roughly 1% of the population. Many patients are allergic to more than one nut, because of cross-reactivity between cashew and pistachio, and between walnut and pecan, but others tolerate some nuts.

Coconut and nutmeg are not botanically tree nuts, and coconut allergy is uncommon.

• Urticaria, angio oedema, flushing
• Oral itching, throat tightness, vomiting, abdominal pain
• Wheeze, cough, stridor
• Anaphylaxis, which is common with walnut, cashew and hazelnut
• Mild oral symptoms in birch pollen related hazelnut or almond sensitisation
• Coexisting peanut or sesame allergy

• History of each nut: which nuts have been eaten without reaction and which have caused reactions.
• Skin prick test and specific IgE to individual nuts and components (walnut Jug r 1, Jug r 4; cashew Ana o 3; hazelnut Cor a 9, Cor a 14; Cor a 1 for birch related disease): skin wheals of 8 mm or more, or high storage protein IgE, raise the chance of true allergy. Sensitisation to Cor a 1 alone suggests mild oral symptoms.
• Supervised oral food challenge to clarify tolerance of nuts that have not been eaten, especially when sensitisation is present without a history.
• Asthma and atopy assessment.

Differential diagnosis.

• Peanut allergy
• Sesame and seed allergy
• Pollen food allergy syndrome
• Contamination or cross-contact reactions
• Idiopathic anaphylaxis
• Food intolerance

• Avoidance of the nuts shown to cause reactions, and of products with cross-contact risk. If a person has reacted to several nuts or has high risk results, avoid all tree nuts until each is cleared by assessment.
• Introduce nuts that are tolerated after a negative supervised challenge, since eating tolerated nuts regularly keeps tolerance.
• Adrenaline auto injector: 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.
• 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).
• Asthma control is optimised, since asthma raises the risk of severe reactions.
• Oral immunotherapy for tree nuts (cashew, hazelnut, walnut) is experimental and performed only in specialist centres.

Special groups.

• Birch related oral symptoms: reassurance, and oral antihistamines; roasted nuts are often tolerated.
• Cashew and pistachio: cross-react in most cases, so test both.

Monitoring. Review every 6 to 12 months. Repeat testing every 1 to 2 years in children. Check auto injector dose and expiry.

Referral and escalation. Refer to allergy for any tree nut reaction or multiple nut sensitisation. Emergency for anaphylaxis.