INSECT STINGS AND BITES

Insects that sting belong to the order Hymenoptra.  The families within this group include Vespidae (e.g. yellow jackets, wasps, & hornets), Apidae (honeybees & bumblebees), and Formicidae (fire ants).  Insects tend to sting primarily as self-defense or to protect their nests, however, members of the Vespidae family as well as Africanized honeybees are occasionally more aggressive and might sting to obtain food. 

The stinging apparatus, called the aculeus, is found only on females.  The aculeus has a variable number of barbs—depending on the species—that results in its becoming stuck in the flesh of the animal into which it was impaled, allowing for a prolonged release of venom from the attached sac.  Once stuck in its victim, the detachment of the aculeus results in the death of the insect.  Species with fewer barbs, however, can sting multiple times.  Fire ants can grasp onto their victims with their pinchers, allowing them to sting repeatedly.  Removing the aculeus after it has been impaled is necessary to both minimize venom injection and to prevent foreign body reactions; ideally removal should be done in less than 2 seconds, as this is the length of time it takes for venom to empty from the attached sac.   

The reaction to a sting is due to changes in permeability of blood vessels and to the reaction produced to the introduction of protein antigens that may lead to the production of IgE.

Reactions to Stings

1. Localized reactions 

     a. Can be divided into uncomplicated and large local reactions

i. Uncomplicated local reactions usually cause pain at the site and localized swelling and redness 1-5 cm in diameter and resolve within approximately 2 days; these should be treated with cold compresses

ii. Large local reactions slowly enlarge over 48 hours and can reach over 10 cm in diameter and resolve over 5-10 days; these are often well treated with cold compresses and NSAIDs, anthistamines, topical steroids, and even a single oral dose of 40-60 mg prednisone have been shown to be helpful for patients in whom the reaction is especially worrisome or in a sensitive area

iii. Fire ant stings might present with pustules, these should not be opened to prevent secondary infection. 

     b. May represent an IgE mediated reaction; individuals with large reactions often have positive skin tests. 

2. Anaphylaxis 

     a. There are no reliable predictors of who will react with an anaphylactic reaction.  50% of deaths occur in individuals with no past history of severe reactions to stings.   The incidence of anaphylaxis is rare in subsequent stings, and immunoprophylaxis is not recommended.

     b. Often there is a history of atopy, and there is a higher incidence in young males, which may reflect an increased exposure. 

     c. There is little cross reactivity between Vespid and Apis venom

     d. Manifestations

          i. Diffuse urticaria

         ii. Flushing

        iii. Angioedema

         iv. Laryngeal edema

          v. Bronchospasm

         vi. Circulatory collapse

     e. Diagnostic tests for reactions to insect stings include skin tests with venom specific or mixed vespid venom.  Systemic reactions to skin tests are rare.  Radioallergosorbent tests, a measure of specific IgE, are available but are less sensitive and more expensive than skin testing.

     f. Treatment

        i. Subcutaneous 1/1000 epinephrine given immediately. 

       ii. Antihistamines 

      iii. Other treatment may be necessary if there is bronchospasm, hypoxia, circulatory collapse, and hypotension.  If there is no response to initial treatment, IV steroids may be necessary.

Prevention of Stings

1. Decrease exposure with clothing, wearing dark colors, decrease use of fragrances when outside, and use of insect repellents

2. Be careful when cooking outside because of attraction of insects

3. Immunotherapy

     a. If there has been a severe (anaphylaxis, serum sickness, or toxic reaction) reaction following an insect sting, skin tests should be performed. 

     b. If positive, immunotherpy should be initiated.  The mechanism is to increase the amount of IgG specific anti-venom and lower the amount of IgE. This is highly effective and repeated stings have lead to anaphylaxis in only 2% of patients.

     c. Immunotherapy may be given at intervals for up to three years.  Reversion to a negative skin test is a good marker of successful treatment

     d. Extensive local reaction, pain, swelling, and hives usually do not require immunotherapy

4. Epi-Pen kits

5. Bracelets to identify individual as reactors to stings.

Bites

1. Unlike stings, bites from insects such as mosquitoes, cause only localized swelling and pruritis.  Anaphylactic reactions are very unusual.

References

1. Reisman, Robert. Current Concepts: Insect Stings.  NEJM August 25, 1994. Vol 331, No. 8 

2. Golden D et al. Outcomes of Allergy to Insect Stings in Children, with an without Venom Immunotherapy.  NEJM Vol 351 No. 7 August 12, 2004

3. Freeman T. Hypersensitivity to Hymenoptera Stings. NEJM Nov 4, 2004

4. Booker G. Insect Stings.  Pediatrics in Review. October 2005

5. Visscher PK, Vetter RS, Camazine S. Removing bee stings. Lancet. 1996; 348 (9023): 301.

6. Severino M, Bonadonna P, Passalacqua G. Large local reactions from stinging insects: from epidemiology to management. Curr Opin Allergy Clin Immunol 2009; 9:334.