Chapter 195. Insect Allergy

Insect stings and bites, including toxins are discussed further in Chapter 120.

Epidemiology

Up to 3% of the general population has bee sting allergy.1 While fatal reactions are uncommon, most of these deaths can be avoided with appropriate treatment, including venom immunotherapy. The frequency of significant reactions to stings from hymenopterids is related to the degree of potential exposure, with up to 35% of beekeepers sensitized to hymenoptera venom and rural areas in general having much higher rates than urban areas.2

The different families of hymenopterids have different behaviors and degrees of aggressiveness. Honeybees (Apis mellifera) are minimally aggressive and will only sting if attacked, such as being accidentally grabbed or stepped on. In contrast, yellow jackets and hornets (Vespula, Dolichovespula, and Vespa spp) are more aggressive predators and will sting with less provocation. The wasps (Polistes) are less aggressive than the vespids, but tend to build their nests near buildings and under eaves, making them more likely to encounter people and so still contribute to a large number of stings.

Four types of reactions to hymenopterid stings occur. Most people, when stung, will develop swelling, redness, and pain at the site of the sting, with a rapid onset and resolution within hours. This reaction is expected and may be managed with ice, antihistamines, and pain relief. More significantly, some patients may develop a second type of reaction, known as large local reactions, which extend beyond the site of the sting to involve the entire limb. These reactions, with swelling, erythema, and tenderness, develop slowly, over about 48 hours, and resolve over days. Large local reactions are frequently confused with cellulitis, but they rarely become infected and they do not respond to antibiotic treatment.

Direct toxic reactions to Hymenoptera venom can also occur with large doses. Though it is difficult to predict the number of stings at which a toxic reaction will occur, these reactions have been reported to occur in as few as 50 stings. Anyone who has sustained more than 50 stings should be monitored for the complications of Hymenoptera venom overdose, which includes rhabdomyolysis and myocardial infarction. Africanized honeybees (Apis mellifera scutellata), which are indistinguishable from ordinary honeybees and do not have more potent venom, are much more aggressive about defending their nest and territory than ordinary honeybees, making toxic envenomations much more likely. Stings from this subspecies are treated as any other toxic Hymenoptera envenomation, with supportive care and monitoring for rhabdomyolysis.

The final type of reaction is a systemic, IgE-mediated reaction, which causes anaphylaxis. This reaction is responsible for almost all Hymenoptera sting deaths. Systemic reactions develop rapidly, usually within 30 minutes of the sting, and may consist of urticaria, angioedema, wheezing, laryngeal edema, hypotension, tachycardia, and diarrhea or vomiting. Systemic reactions can be distinguished from large local reactions by involvement of sites or organs distant from the site of sting and by the rapidity with which the reaction develops.

Most children (60%) have symptoms limited to the skin, in contrast to adults, 75% of whom have organ involvement as well. Systemic reactions to Hymenoptera stings should be managed as with other anaphylaxis, by securing a stable airway, early use of epinephrine, and a short (2–3 days) course of corticosteroids and antihistamines. While delayed, late-phase reactions are less common in insect stings, but they do still occur, and patients should be monitored for late-phase anaphylactic reactions for at least 4 hours after the sting. In addition, all patients with a significant insect sting reaction should be given epinephrine autoinjectors with instruction on proper use and a referral to an allergist for evaluation and possible immunotherapy.

Perhaps because of the relatively low rate of referrals to allergists for sting allergy, venom immunotherapy is an underused therapy. Venom immunotherapy is one of the most effective therapies available for managing allergic disease and greatly improves the outcome of subsequent stings. Venom desensitization is also now rapid, and progression to the maintenance dose can usually be accomplished safely in less than a day.3,4 Thereafter, maintenance doses of venom are given every 1 to 2 months for at least 5 years. Therapy can be discontinued at that point, and most people will maintain their desensitized state for up to 10 years after cessation of therapy. About 10% of patients will have a reaction after discontinuing therapy, but these are usually mild. Consequently, venom desensitization imposes relatively little logistical burden on the patient, is low-risk compared to the alternative of managing anaphylaxis after the fact, and is highly effective. Even patients on beta-blockers or angiotensin-converting enzyme inhibitors, not ordinarily candidates for desensitization, should be offered this treatment, because the risk of a sting outweighs the risk of therapy.

While all adults with systemic reactions should be offered venom immunotherapy, children with only cutaneous manifestations may not require desensitization. Children with only mild cutaneous reactions appear to be unlikely to go on to have more significant reactions, though treatment may reduce the rate of subsequent cutaneous reactions. Children with reactions that are not limited to the skin, though, benefit greatly from immunotherapy, and the benefit is even greater than that seen in adults.1

Venom immunotherapy is only effective for IgE-mediated disease, and so is not recommended in large local reactions or for those that do not have evidence of IgE-mediated reactions.4 Consequently, establishing the presence of IgE-dependent reactivity to the individual venoms by skin prick and/or radioallergosorbent testing is required prior to planning an immunotherapy course. Reactions to Hymenoptera tend to be specific to the particular family of insect, so that those who are sensitized to yellow jacket venom will not necessarily react to honeybee or wasp venom, but 30% of patients will have more than one sensitization.