Hymenoptera Venom Allergy
Anaphylactic reactions to Hymenoptera venom are relatively uncommon but can be life-threatening: venom immunotherapy is the treatment of choice. The primary allergen in honeybee venom is phospholipase A2 (Api m 1) and that of the vespid venoms (yellow jacket, hornet, wasp) is antigen 5 (Ves v 5). Another clinically important insect is the imported fire ant (also a member of the Hymenoptera family). Allergy to the imported fire ant is being reported increasingly often from the USA, Australia and South East Asia.
Risk assessment is based on the clinical allergy history background and measurement of venom-specific IgE. Those who have had systemic symptoms with a previous sting are at much greater risk of anaphylaxis than those who have only had large local reactions. The magnitude of the IgE response is not consistently correlated with the severity or pattern of reaction to a sting, since some patients who experience large local reactions have extremely high venom IgE levels, while other patients who suffer rapid vascular collapse and anaphylactic shock may have barely detectable levels of venom IgE.
The frequency of systemic reactions allergy in children and adults with a history of large local reactions and positive venom skin tests is in the range of 5–10%. The reported risk of another systemic reaction occurring in a patient with a previous systemic reaction varies from 30 to 60%, with lower risks in children and patients with milder reactions. The risk of systemic reaction in adults diminishes over 10–20 years towards 15–30% but does not seem to return to the background 3% prevalence in the general population. Children with a history of cutaneous systemic reactions had less than 5% risk of anaphylaxis during observation for 10–20 years. There is no test that accurately predicts the outcome of the next sting. Live sting challenge is a useful research procedure, but is generally not acceptable for clinical practice, not least because some patients who do not react to a first challenge sting may react to a subsequent sting.
The induction phase of venom SIT may be given weekly for 10 weeks, as a ‘semirush’ over 2–3 weeks, or as a ‘rush induction’ in hospital over 2–3 days. Once the maintenance dose is achieved, 95–98% of patients will have no systemic symptoms upon wasp-sting challenge (80–85% for honeybee venom SIT). Patients not fully protected by the conventional dose of 100µg may be more effectively treated with a 200µg maintenance dose. Protection is well maintained during subsequent maintenance therapy; maintenance injections are usually given every 4–6 weeks, but sometimes the interval is up to 8 weeks, especially in long-term maintenance therapy for venom allergy. Maintenance therapy is usually recommended for 3–5 years, with growing evidence that 5 years’ treatment provides more lasting benefit, although this has to be balanced against the inconvenience of extended duration of SIT. More prolonged treatment is offered to those with more severe previous reactions, those who had systemic reactions during venom SIT, and those allergic to honeybees (as opposed to wasps). The decision to stop therapy may also be based on a reduction in the skin test response to venom or a reduction in venom-specific IgE. Hymnoptera Venom-specific IgG rises in all patients as a result of therapy and a level of = 3µg/mL has been shown to be protective.
A low risk of systemic reaction to stings (10%) appears to remain for many years after discontinuing venom immunotherapy (VIT).Although post-SIT systemic reactions to stings have rarely been severe, a few fatalities have been reported, usually in patients with other risk factors (e.g. mastectomies, severe previous reactions, systemic reactions during VIT, honeybee allergy). In children who have received venom SIT, the chance of systemic reaction to a sting is lower, remaining below 5% for up to 20 years after discontinuing therapy. How much of this is due to SIT and how much to natural resolution of the sensitivity is uncertain, but the clinical message is clear: treatment is effective in the short term and patients are then at relatively low risk of reactions for many years thereafter.