The usual reaction after an insect sting is mild erythema and swelling at the sting site. This reaction is transient and disappears within several hours. Little treatment is needed other than analgesics and cold compresses.
Insect stings, in contrast to insect bites, always cause pain at the sting site.

Extensive swelling and erythema, extending from the sting site over a large area, is a fairly common reaction. The swelling usually reaches a maximum in 24 to 48 hours and may last as long as 10 days. On occasion, fatigue, nausea, and malaise may accompany the large local reaction. Aspirin and antihistamines are usually adequate treatment. When severe or disabling, administration of steroids, such as prednisone, 40 mg daily for 2 to 3 days, may be very helpful. These large local reactions have been confused with infection and cellulitis. Insect sting sites are rarely infected and antibiotic therapy rarely indicated.

Most people who have had large local reactions from insect stings will have similar large local reactions from subsequent re-stings (5). The risk for generalized anaphylaxis is very low, less than 5%. Thus, people who have had large local reactions are usually not considered candidates for VIT (discussed later) and do not require venom skin tests. Recently, Kelly et al. have successfully used VIT to decrease local reactions in people frequently exposed (6). This decision to use immunotherapy for treatment of people with large local reactions should be dependent on the frequency and severity of the reaction and the therapeutic response to steroids administered shortly after the sting.

The most serious reaction that follows an insect sting is anaphylaxis. Retrospective population studies suggest that the incidence of this acute allergic reaction from an insect sting ranges between 0.4% and 3.0% (7–9). Allergic reactions can occur at any age; most have occurred in individuals younger than 20 years of age, and with a male-to-female ratio of 2:1. These factors may reflect exposure rather than any specific age or sex predilection. Several clinical studies suggest that about one-third of individuals suffering systemic reactions have a personal history of atopic disease. Stings around the head and neck most commonly cause allergic reactions, but reactions may occur from stings occurring on any area of the body (10–14).

In most patients, anaphylactic symptoms occur within 15 minutes after the sting, although there have been rare reports of reactions developing later. Clinical observations suggest that the sooner the symptoms occur, the more severe the reactions may be. The clinical symptoms vary from patient to patient and are typical of anaphylaxis from any cause. The most common symptoms involve the skin and include generalized urticaria, flushing, and angioedema. More serious symptoms are respiratory and cardiovascular. Upper airway edema involving the pharynx, epiglottis, and trachea has been responsible for numerous fatalities. Circulatory collapse with shock and hypotension also has been responsible for mortality. Other symptoms include bowel spasm and diarrhea and uterine contractions (12,15).

Severe anaphylaxis, including loss of consciousness, occurs in all age groups. In one large study (16), the incidence of severe anaphylaxis was similar throughout all ages. Most deaths from sting anaphylaxis occur in adults. The reason for this increased mortality rate in adults might be the presence of cardiovascular disease or other pathologic changes associated with age. Adults may have less tolerance for the profound biochemical and physiologic changes that accompany anaphylaxis (17–19).

There are no clinical, absolute criteria that will identify people at risk for acquiring venom allergy. Most people who have venom anaphylaxis have tolerated stings without any reaction before the first episode of anaphylaxis. Even individuals who have died from insect sting anaphylaxis usually had no history of prior allergic reactions (1). The occurrence of venom anaphylaxis after first known insect sting exposure is another confusing observation, raising the issue of the etiology of prior sensitization of the pathogenesis of this initial reaction. People who have had large local reactions usually have positive venom skin tests and often very high titers of serum venom-specific immunoglobulin E (IgE); thus, these tests do not discriminate the few potential anaphylactic reactors. Anecdotal observations suggest that the use of β-blocking medication, which certainly potentiates the seriousness of any anaphylactic reaction, may also be a risk factor for subsequent occurrence of anaphylaxis in people who have had large local reactions.

Many simultaneous stings (greater than 100) may sensitize a person, who then might be at risk for anaphylaxis from a subsequent single sting. Exposure to this large amount of venom protein can induce IgE production. This potential problem is now recognized more often because of the many stings inflicted by the so-called killer bees. After experiencing a large number of stings with or without a toxic clinical reaction, people should be tested to determine the possibility of potential venom allergy. After an uneventful insect sting, some people may develop a positive skin test, which is usually transient in occurrence. Currently, skin testing of people who do not have an allergic reaction from a single sting is not recommended.

The natural history of insect sting anaphylaxis has now been well studied and is most intriguing. People who have had insect sting anaphylaxis have an approximate 60% recurrence rate of anaphylaxis after subsequent stings (20). Viewed from a different perspective, not all people presumed to be at risk react to re-stings. The incidence of these re-sting reactions is influenced by age and severity of the symptoms of the initial reaction. In general, children are less likely to have re-sting
reactions as compared to adults. The more severe the anaphylactic symptoms the more likely it is to reoccur. For example, children who have had dermal symptoms (hives, angioedema) as the only manifestation of anaphylaxis have a remarkably low re-sting reaction rate (20,21). On the other hand individuals of any age who have had severe anaphylaxis have an approximate 70% likelihood of repeat reactions (16,20). When anaphylaxis does reoccur, the severity of the reaction tends to be similar to the initial reaction. No relationship has been found between the occurrence and degree of anaphylaxis and the intensity of venom skin test reactions. Thus, factors other than IgE antibodies modulate clinical anaphylaxis.

Serum sickness type reactions, characterized by urticaria, joint pain, malaise, and fever, have occurred approximately 7 days after an insect sting. On occasion these reactions have also been associated with an immediate anaphylactic reaction. People who have this serum sickness type reaction are subsequently at risk for acute anaphylaxis after repeat stings and thus are considered candidates for VIT (22).

There have been isolated reports of other reactions such as vasculitis, nephritis, neuritis, and encephalitis occurring in a temporal relationship to an insect sting. The basic etiology for these reactions has not been established (23).

Toxic reactions may occur as a result of many simultaneous stings. Insect venom contains a number of potent pharmacologic agents, and as a result of the properties of these substances, vascular collapse, shock, hypotension, and death may occur (24). The differentiation between allergic and toxic reactions sometimes can be difficult. As noted, after toxic reactions, individuals may develop IgE antibody and then be at risk for subsequent allergic sting reactions following a single sting.

The risk of insect stings may be minimized by the use of simple precautions. Individuals at risk should protect themselves with shoes and long pants or slacks when in grass or fields, and should wear gloves when gardening. Cosmetics, perfumes, and hair sprays, which attract insects, should be avoided. Black and dark colors also attract insects; individuals should choose white or light colored clothes. Food and odors attract insects; thus garbage should be well wrapped and covered and care should be taken with outdoor cooking and eating. Insect repellents are not effective against stinging insects (34).

Acute allergic reactions from the insect stings are treated in the same manner as anaphylaxis from any cause. See Chapter 14 for specific recommendations. Patients at risk are taught to self-administer epinephrine and are advised to keep epinephrine and antihistamine preparations available. Epinephrine is available in preloaded syringes (Twinject, Verus Pharmaceuticals, San Diego, CA; EpiPen, Dey LP, Napa, CA) and can be administered easily. Consideration should be given to having an identification bracelet describing their insect allergy.

At the present time, studies are being done with a fast-disintegrating tablet of sublingual epinephrine (35). Plasma concentrations of epinephrine 40 mg sublingually are similar to those obtained after intramuscular injection of 0.3 mg of epinephrine, which is the usual therapeutic dose. With further development, sublingual epinephrine tablets may replace intramuscular injections providing a more feasible alternative for the medical treatment of anaphylaxis.

Venom immunotherapy (VIT) has been shown to be highly effective in preventing subsequent sting reactions (36,37). Successful therapy is associated with the production of venom-specific IgG, which appears to be the immunologic corollary to clinical immunity. Current recommendations are to administer VIT to individuals who have had sting anaphylaxis and have positive venom skin tests. As discussed previously, recent studies of the natural history of the disease process in untreated patients have led to observations that modify this recommendation. The presence of IgE antibody in an individual who has had a previous systemic reaction does not necessarily imply that a subsequent reaction will occur on re-exposure. Observations relevant to the decision to use VIT include age, interval since the sting reaction, the nature of the anaphylactic symptoms, and anticipated exposures. Immunotherapy guidelines are summarized in Tables 15.1 through 15.3.