Hymenoptera Venom Allergy

Hymenoptera venom allergy is an IgE-mediated reaction to stings from honeybees, vespids (wasps, yellow jackets, hornets) and, in some regions, ants and bumblebees. Systemic reactions can be fatal, and venom immunotherapy prevents them.

Pathophysiology. Venoms contain phospholipase A2, hyaluronidase and other allergens (Api m 1 in honeybee; Ves v 5 and Ves v 1 in wasp). After sensitisation, a sting triggers mast cell degranulation.

About 3% of adults have systemic reactions to stings, and large local reactions occur in up to 10% of the population. Risk factors for severe reactions are mastocytosis and raised baseline tryptase, older age, cardiovascular disease, beta blocker and angiotensin converting enzyme inhibitor use, and repeated stings.

Reactions are classified as large local (swelling over 10 cm lasting over 24 hours) or systemic (skin, respiratory, cardiovascular).

• Large local reaction: extensive swelling and redness lasting several days
• Systemic skin reaction: generalised urticaria, flushing, angio oedema
• Anaphylaxis: wheeze, stridor, hypotension, collapse
• Gastrointestinal symptoms: vomiting, cramps, diarrhoea
• Rapid onset, usually within 15 minutes of a sting
• Rarely delayed serum sickness like illness

• History: insect, circumstances, time to onset, features and treatment.
• Skin prick test and intradermal test with venoms (honeybee, wasp, hornet, bumblebee) and specific IgE to venoms and components (Api m 1, Api m 10, Ves v 1, Ves v 5): identifies the primary sensitiser and distinguishes double positivity caused by cross-reactive carbohydrates.
• Serum tryptase (baseline): a level over 11.4 microgram/L suggests mast cell disease and higher risk, and over 20 microgram/L makes systemic mastocytosis likely.
• Basophil activation test where available, for double positive patients.
• Mastocytosis assessment when tryptase is raised or hypotension occurs without skin signs.

Differential diagnosis.

• Large local reaction without systemic risk
• Idiopathic anaphylaxis
• Toxic reaction to multiple stings
• Systemic mastocytosis
• Cellulitis and infected sting site
• Insect bite hypersensitivity (non-hymenoptera)

• Acute sting: remove a honeybee sting by scraping without delay, and cool the site. For large local reactions use cold compresses, oral antihistamine and a short oral prednisolone course (40 mg daily for 3 days in adults).
• Anaphylaxis: adrenaline 0.5 mg intramuscularly (children 0.01 mg/kg, 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.
• Adrenaline auto injector for systemic reactions: 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.
• Venom immunotherapy for patients with systemic reactions beyond the skin (adults with generalised skin only reactions if risk is high or sting exposure is frequent) and a positive venom test. Use the venom of the primary sensitiser. Start with a build up (rapid or conventional) reaching a maintenance dose of 100 microgram of venom every 4 weeks, with 200 microgram for those still reacting.
• Duration: 3 to 5 years, and lifelong in mastocytosis, after a severe reaction, in those who stay at high risk, and in beekeepers.
• Sting avoidance: protective clothing, avoid sweet food and drinks outdoors, avoid strong perfume, and have nests removed.
• Review drugs: beta blockers and angiotensin converting enzyme inhibitors increase risk, and need review but are not always stopped.

Special groups.

• Beekeepers and outdoor workers: venom immunotherapy and an auto injector are especially important.
• Mastocytosis: lifelong immunotherapy with careful observation.

Monitoring. Review every 6 months during build up and yearly in maintenance. Check auto injector dose and expiry. Repeat tests at the end of therapy to guide stopping.

Referral and escalation. Refer to allergy for any systemic reaction. Emergency for anaphylaxis.