The order hymenoptera includes bees, vespids (hornets and wasps), and fire ants. These insects cause one-third of all reported envenomations in the United States and an estimated 50–150 annual deaths. While hymenoptera venoms possess intrinsic toxicity, it is their ability to sensitize the victim and cause subsequent anaphylactic reactions that makes them so lethal.
Honeybees (Apis mellifera) are fuzzy insects with alternating black and tan body stripes. Not intrinsically aggressive, they usually sting defensively when stepped on. Like that of other hymenoptera, the honeybee's stinger is a modified ovipositor (only females' sting) that is connected to a venom sac. Since honeybees lose their barbed stinger after stinging and die, they generally only sting in defense when provoked.
Africanized honeybees, or “killer bees,” (Apis mellifera scutellata) (Fig. 134-6) are now found in Texas, Arizona, California, and most of the temperate southeastern and southwestern states.41 In the 1950s, African bees were imported into Brazil for breeding experiments designed to improve honey production and disease resistance. Many escaped and subsequently mated with previously imported European honeybees.42 These hybrids have since migrated northward along the coasts and temperate regions of the continent. Although the toxicity of their venom is equal to that of their native counterpart, they are far more aggressive. A hive can respond to a perceived threat with more than 10 times the number of bees than its typical native North American counterpart. Massive numbers of stings from an attack of Africanized bees can result in multisystem damage and death from severe venom toxicity. Most patients of massive envenomation suffer acute tubular necrosis or renal involvement with myoglobinuria.43 In swarms, these bees can overwhelm and kill even healthy nonallergic victims.44
Africanized honeybees, or “killer bees” (Apis mellifera scutellata), are dangerous because they sting in large swarms. (www.uni.uiuc.edu).
The most common hornets in the United States are the yellow jackets (Vespula pensylvanica). They are usually seen around garbage cans, beverage containers, and various foods. They are extremely aggressive and sting with little provocation. Wasps (Polistes annularis, the paper wasp) have thin, smooth bodies and a formidable sting. They build their nests in the eaves of buildings. These vespids are carnivorous and able to use their smooth stingers multiple times, unlike honeybees, which lose their stingers after a single sting.45,46
Hymenoptera venoms contain enzymes that directly affect vascular tone and permeability. Although their enzymes are similar, there is little immunologic cross-reactivity between bee and vespid venoms. While a bee sting may not sensitize a person to yellow jacket venom, a yellow jacket sting would more likely sensitize one to wasp venom.47 Four possible reactions are seen after hymenoptera stings: a local reaction, toxic reaction, systemic anaphylaxis, and a less common delayed-type hypersensitivity reaction.28,48
Local reactions are the most common reactions resulting from the vasoactive effects of the venom and are generally mild. The most common response includes pain, mild erythema, edema, and pruritus at the sting site. There are no systemic signs or symptoms, but a severe local reaction may involve one or more contiguous joints. Local reactions occurring in the mouth or throat can produce swelling that may lead to upper airway obstruction, especially in younger children.
Toxic reactions may occur when a patient suffers from multiple stings. Africanized bees are notorious for such attacks, but an aggressive native hive may elicit a similar response. The essential lethal dose is approximately 20 stings/kg in most mammals.45 Symptoms of a toxic reaction may resemble anaphylaxis, but gastrointestinal manifestations (nausea, vomiting, and diarrhea) and sensations of light-headedness and syncope may also occur. Headache, fever, drowsiness, involuntary muscle spasms, edema without urticaria, and convulsions may ensue. Although urticaria and bronchospasm are not always present, severe envenomations may lead to respiratory insufficiency and arrest. Hepatic failure, rhabdomyolysis, and DIC have been reported in both adult and pediatric victims. Toxic reactions are believed to occur from a direct multisystem effect of the venom.
Anaphylactic reactions are generalized systemic allergic reactions that may occur after envenomation. Generalized systemic reactions to hymenoptera venom are thought to occur from an immunoglobulin E-mediated mechanism, leading to the release of pharmacologically active mediators within mast cells and basophils. Symptoms are often mild, but severe reactions can lead to death within minutes. Unlike the toxic reaction, there is no correlation between systemic allergic reactions and the number of stings. The majority of allergic reactions occur within the first 10–15 minutes and nearly all occur within 6 hours. Fatalities that occur within the first hour of the sting usually result from airway obstruction or hypotension. Initial symptoms typically consist of ocular pruritus, facial flushing, and generalized urticaria. Symptoms may intensify rapidly with chest or throat constriction, wheezing, dyspnea, abdominal cramping, diarrhea, vomiting, vertigo, fever, laryngeal stridor, syncope, and shock.
Delayed reactions, appearing 1–2 weeks after a sting, consist of serum sickness-like signs and symptoms of fever, malaise, headache, urticaria, lymphadenopathy, and polyarthritis. This reaction is believed to be immune complex mediated.48
If present, the embedded stinger should be removed manually. Previous sources recommended cautiously scraping the stinger off with lateral pressure, rather than grasping it, in order to avoid compression of the venom sac resulting in further release of venom. However, recent studies have demonstrated that this is erroneous because the venom has likely been completely released within seconds of envenomation.49 Treatment is symptomatic, with ice or cold compresses and an antihistamine. In more severe local reactions, there is a more sustained inflammatory response, and the swelling may spread to the entire extremity and persist for several days. A short course of prednisone (1 mg/kg/day for 5 days) may decrease the duration of symptoms.
Toxic reactions reflect the effects of multiple stings, usually 25–50 stings or more. Gastrointestinal symptoms are the principal features; urticaria and bronchospasm are not usually present. Treatment is supportive.
Systemic reactions occur in approximately 1% of hymenoptera stings. They range from mild, non–life-threatening cutaneous reactions to classic anaphylactic shock. In all but the mildest of systemic reactions, the mainstay of treatment is epinephrine. Epinephrine counteracts the bronchospastic and vasodilatory effects of histamine. Epinephrine can be given as a subcutaneous injection (0.01 mL/kg of 1:1000 solution; not to exceed 0.3 mL). In more severe reactions, the intravenous or endotracheal route is preferred (0.1 mL/kg of 1:10,000 solution). The dose may be repeated at 15-minute intervals as needed. Early intubation is indicated if there is evidence of severe laryngeal edema or stridor because airway obstruction is the leading cause of death in anaphylaxis. Antihistamines should be given early but not as a substitute for epinephrine. An H2-receptor blocker (e.g., cimetidine or ranitidine), in addition to an H1-receptor blocker (diphenhydramine), may aid in inhibiting the vasodilatory effects of histamine. Adjunctive therapy for bronchospasm might include inhaled β2-agonists (e.g., albuterol). When hypotension is present, vigorous isotonic fluid resuscitation should be instituted. Glucocorticoids should be given for their anti-inflammatory effects as well as their effect in preventing the late-phase response.
A delayed serum sickness-like reaction may appear 10–14 days following the initial sting. This immune complex disorder may be treated with a short course of prednisone.
Venom immunotherapy desensitization is very effective in preventing further systemic reactions, with 95–100% protection after 3 months of treatment. Referral to an allergist is indicated for any child who has experienced life-threatening respiratory symptoms or hypotension. Children less than 16 years old who have only urticaria or angioedema do not require venom immunotherapy. Only 10% of these children will have systemic reactions with subsequent stings.49
Essential to the treatment of any systemic reaction is the prevention of future reactions. Patients who have had a systemic reaction should be instructed to wear protective clothing and avoid hymenoptera-infested habitats. Portable epinephrine kits (Epi-Pen and Epi-Pen Jr) are available. They should be prescribed prior to the patient leaving the emergency department. The patient should be urged to carry the kit at all times and to use epinephrine for any systemic symptoms. Even if symptoms are mild, the patient should seek emergency care. The patient should also be instructed to wear a medical alert tag.