Most of the time we live in harmony with stinging insects - we at our summertime picnics and they busily sipping the nectar from one flower to the next. Such harmony is not always the case, as the majority of us who have been stung well know. Allergic reactions to insect stings can be severe, to the extreme of being fatal (40 reported fatalities each year). Fortunately, only between 0.5 - 5% of the United States (US) population is allergic to insect stings and thus at risk of developing reactions beyond the universal pain and local swelling.

The dreaded insects

Entomology is the field of science that studies insects and one that is largely unfamiliar to most practicing physicians. The medically important insects are found in a few of the over 16,000 species in North America making up the Order Hymenoptera. These stinging Hymenoptera insects do have some positive features, being either effective pollinators of plant life or predators of a wide variety of pest species. Insects with the capacity to elicit an allergic reaction include fire ant, yellow jacket, yellow hornet, white-faced hornet, paper wasp and honeybee. Bumble bees are felt to be of minor importance. In general terms, the winged insects are of greater concern in the northern US and the fire ant due to its cold intolerance is restricted to the southern US.

The fire ant is a year-round threat while the winged creatures are most active from early spring through fall. Fire ants build subterranean nests with as many as 230,000 insects in residence. When the nest is disturbed, the ants attack in mass and repeatedly sting their victim. The yellow jacket is the most aggressive of the flying insects, scavenges for meats and sweets, and nests in and around homes. Swatting at insects or bothering their nests will increase your chance of being stung, but in the autumn when food is scarce and their colonies declining, the yellow jacket may simply be out to get you. You may have noticed that it is usually one individual amongst a group of people who is attacked by a swarm of insects. This is because the first stinging insect releases a signal that alerts his mates to the location of his prey.

Reactions to stings

Allergic reactions are the result of a specific immune response to the venom injected into the skin. Previous venom exposure is necessary for one to become sensitized; therefore, people can almost always recall a well tolerated sting in the past. Large local swelling and discomfort at the site of a sting is a normal response to the toxins within the venom and should not be confused with an allergic reaction. Symptoms of an allergic reaction can be limited to the skin as hives or involve multiple organs, referred to as anaphylaxis (dizziness, weakness, swelling of nose, lips, tongue and throat, nausea, stomach cramps, breathlessness, or loss of consciousness). The allergic reaction usually begins within minutes, but can be delayed in rare situations for 20 minutes or more.

The chance of having a second allergic reaction to the same insect species is not 100%, but around 60%. This may sound encouraging, but after you have survived a frightening near-death experience, playing with 60% odds is not too comforting. In the Netherlands, people with a history of an allergic reaction are put in an Intensive Care Unit and intentionally stung to assess their ongoing sensitivity. Such an approach sounds a bit cruel and much too risky in the US malpractice climate. Our basis for considering someone at risk for repeated reactions is the type of reaction they experience. Reactions in children are often consistent, meaning that if hives occur the first time, nothing more serious than hives will develop with subsequent stings. Adults, on the other hand, are less predictable and successive stings can cause more serious reactions.

Treatment

Initial treatment depends of the nature of the reaction. The non-allergic local reaction should subside uneventfully over 48 hours, but on occasion will require the application of ice, elevation if possible, and, rarely, oral corticosteroids. Once the reaction is no longer local, i.e. swelling at distant sites from the sting or other anaphylactic symptoms, emergency help should be sought. An antihistamine will reduce the swelling and itching of hives, but do little for the more critical organ involvement (respiratory, heart and vascular). In an Emergency Room, epinephrine (adrenaline) will be given immediately to slow or stop the reaction. In some cases, further therapy with intravenous fluids, oxygen and medications will be necessary.

With the potential for repeated reactions hovering menacingly overhead prevention is paramount (see table). Epinephrine kits are available by prescription for self-administering epinephrine immediately after a second sting. Using this as directed will delay a reaction that otherwise may have progressed irreversibly beyond the response time of an emergency 911 call. Some individuals may have an adverse reaction to epinephrine (especially patients with heart trouble) and others will at some time forget to carry their kits with them. For these reasons, allergy immunotherapy, or shots, is the treatment of choice for children with a history of anaphylaxis and adults with hives or anaphylaxis following a sting.

In order to elect immunotherapy, the insect responsible for the sting must be identified. Beyond distinguishing the fire ant from his winged counterparts, features of the insects are too similar to reliably separate one from the other. Therefore, it is necessary for the allergist to perform skin testing to either the fire ant or the flying insects to confirm a patient's sensitivity. Skin testing is the same simple method used to identify allergies to dust mites, cats or pollens in patients with hay fever and asthma. With the combination of a true allergic reaction to a sting and a positive skin test, the patient is a candidate for allergy shots.

The objective of allergy immunotherapy is to "turn off" an individuals sensitivity to an allergen, in this case, insect venom. Injections of gradually increasing concentrations of venom (actually, the crushed whole body of fire ant is used rather than pure venom) are given on a weekly basis for 6-8 weeks to confer immunity. Maintenance injections of full-strength venom are then given every 4-6 weeks. Research studies with the flying insects suggest that shots given for 3-5 years provide lasting resistance to stings. These results are often extrapolated for the fire ant, although your allergist may choose to use a longer duration of treatment. Official recommendations on the proper length of effective fire ant immunotherapy should be available in the next few years.