Insect Sting Allergies

Insect stings and bites are common and usually cause minor reactions.  Mosquito bites generally cause a hive reaction and only rarely can precipitate a serious systemic reaction.  San flies, black flies, deer and horse files have been reported to cause a true allergic reaction, but usually they only  cause painful local reactions at the sting site.

Reactions to venom stinging insects (Hymenoptera) are of the greatest concern, as they can often cause a systemic and life-threatening allergic reaction.  These reactions are true allergy reactions referred to as IgE medicated reactions.  Approximately 4% of the population are at risk for developing a serious reaction to venoms.

Venom stings (Hymenoptera) include honey bees, bumblebees, hornets, yellow jackets, and wasps.  Fire ants, which inhabit the southern part of the United States, can also cause severe allergic reactions.  They belong to the insect family Formicidae.  Harvester ants, which are in the same family, have occasionally been reported to trigger systemic reactions as well.

Each of these venoms contain proteins and peptides which are responsible for the allergic reaction.  There is considerable cross-reactivity between the different vespids which include hornets, yellow jackets, and wasps. But, not with honeybees, wasp venom and honeybee venoms are distinct.

Reactions to the venom stings can be divided into local, large local, and systemic reactions.

Local reactions consist of sharp immediate pain at the time of sting, followed by redness and swelling for 2 to 3 hours.  Some discomfort remains for 24 for 48 hours.  Fire ants cause immediate pain and itching, followed by a blistering reaction with redness within 4 to 8 hours.  It may take 3 to 4 days for these reactions to clear.

Large local reactions are characterized by more swelling (more than 10 cm in diameter) and can last 5 days.  Despite these large reactions, the development of severe systemic reactions can not be predicted.

Systemic reactions range from mild hives to severe life threatening anaphylaxis.  These reactions are more common in children; however, more deaths are reported in adults.  Also, males are more at risk, probably because of exposure rates.  Most severe reactions occur within 30 to 60 minutes of the sting.  Upper respiratory tract obstruction is the most common cause of death.

The diagnosis of stinging insect sensitivity is made initially with clinical history and demonstration of IgE (allergy) antibodies.  Skin testing is the preferred method of testing.  Up to 20% of patients with positive skin tests have negative blood tests (RAST) for venom allergies.

A significant percentage of patients with systemic reactions will show diminished sensitivity over a period of time.  In children, up to 50% of patients will lose sensitivity within 10 years.  Patients with large local reactions have a low risk for developing systemic reactions with subsequent stings.  However, some patients have had repeat life-threatening reactions up to 20 years after the initial reaction.

Treatment of venom reactions initially consists of removing the stinger (if present) and applying ice at the reaction site.  Antihistamines can be given for itching.  All persons with a history of systemic reactions should carry injectable epinephrine such as EPI-PEN or ANAKIT.  Venom immunotherapy is indicated in patients with documented allergic reactions consisting of generalized hives and breathing difficulty.  This therapy can reduce the risk of having another serious reactions to less than  2%.  Children who have just had hives and not respiratory symptoms do not need this treatment.  If immunotherapy is started, the question always arises as to when this treatment can be stopped.  The general consensus is that after 5 years of therapy, most patients have shown a decrease or disappearance of IgE anitbodies and immunotherapy can be discontinued