The true stinging insects that account for the majority of allergic reactions belong to the order Hymenoptera (Box 425.1). The females of each species have a modified ovipositor stinger through which an injection of venom is delivered. Biting insects such as mosquitoes, flies, and bugs only rarely produce systemic reactions and are not considered in this discussion.

Honeybees are the most common members of the apid family. They are small, fuzzy, relatively docile insects that usually live in domestic hives and often are seen gathering nectar and pollinating clover and flowering plants. They usually sting only when sat on or caught underfoot, and they leave their barbed stinger embedded in the victim. Bumblebees are large, slow-flying, yellow-and-black-striped bees that are usually solitary and only rarely sting. Honeybees and bumblebees survive the winter and are present throughout the summer.

The vespid family includes yellow jackets, hornets, and wasps. In most areas of the United States, these insects account for the majority of stings. Yellow jackets are common in the Northeast, whereas wasps are dominant in the South and Southwest. Yellow jackets are small, black-and-yellow-striped insects that usually nest in the ground or in decaying logs. They scavenge for food, are often seen around picnics and garbage, and become particularly aggressive late in the summer, when their nests are crowded. White-faced hornets are large black insects with white faces that build teardrop-shaped paper nests suspended in trees. The thin-bodied brown-and-yellow-striped Polistes wasp typically creates open-faced nests under the eaves of buildings.

Imported fire ants are less common members of the order. They inhabit the coastal areas of the Southeast and live in large dirt mounds. They attach themselves to the skin and deliver multiple stings that result in sterile pustules. Although their stings are a cause of systemic reaction, their venom has been less well studied. Current diagnosis and immunotherapy uses whole body extract, which appears quite successful.

After a sting, 90% of children experience a normal reaction consisting of transient redness, swelling, and pain localized to the sting site, usually less than 2 in. in diameter and lasting for less than 24 hours (Box 425.2). Hymenoptera venoms contain a variety of enzymes (phospholipase A, hyaluronidase), cytotoxic proteins (apamine, mellitin), and vasoactive compounds (histamine and kinins) that, in the normal individual, induce local vasodilatation, edema, and tissue damage.

In 10% of children, the sting results in a large local reaction that is extensively swollen and tender, is larger than several
inches in diameter, and peaks in 3 to 7 days. Although the exact mechanism of this reaction is unknown, 75% of these individuals demonstrate venom-specific IgE, suggesting that immediate hypersensitivity may play some role in this exaggerated sting response.

True systemic anaphylactic reactions are less common. Estimates of their incidence in the general population range from 0.5% to 5.0%. Anaphylaxis is caused by the activation of mast cells sensitized by venom-specific IgE. This triggers the release of large quantities of vasoactive mediators, including histamine, leukotrienes, and other cytokines, and leads to vasodilatation and increased vascular permeability. Most of these reactions (70% to 80%) are not life-threatening. They begin several minutes to several hours after the sting and consist of simple generalized urticaria, erythema, pruritus, and angioedema. Life-threatening reactions begin within 5 to 10 minutes. Airway obstruction may occur secondary to laryngeal edema (tickle in the throat, gagging, difficulty in swallowing, or voice change) or bronchospasm (chest tightness or wheezing). Hypotension (dizziness or fainting) and frank cardiovascular collapse are accompanied by metabolic acidosis, clotting abnormalities, and evidence of complement activation. Although approximately 50 deaths per year are attributed to insect allergy, almost all of these occur in adults, particularly the elderly. Fatal outcome in children is extremely rare, averaging only one death per year in the United States.

Several types of non–IgE-mediated reactions include serum sickness, renal disease, neurologic manifestations, and delayed hypersensitivity phenomenon. Their pathophysiology remains unknown. When a child is stung many times simultaneously, a toxic, nonallergic reaction consisting of delayed fever, nausea, vomiting, and other systemic symptoms sometimes occurs. With an extremely large number of stings, such as may occur with Africanized honeybees or “killer bees,” this type of nonallergic reaction occasionally is fatal.