by Kristy Fleming, MD
An otherwise healthy 14-month old boy from Texas developed bilateral edematous hands. Examination revealed numerous pustules with an erythematous base scattered over the hands and arms. His parents noted the child had been playing in the grass with his older brother prior to the eruption. The skin lesions first presented as 1 mm to 2 mm red bumps, then quickly evolved into edematous vesicles and within hours became pustular. The child was fussy and appeared to be in discomfort, but had no other signs of illness. His older brother did not have similar lesions. What’s your diagnosis?Submit your answer, and then read the full explanation below. If you like this activity or have a suggestion, tell us about it in the comment box at the bottom of the page.
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Two species of fire ants, black Solenopsis richteri and red S. invicta, are native to South America, but have invaded the southern United States with a vengeance, becoming a formidable agriculture pest and health hazard.1 Named for the intense burning pain induced by its sting, the fire ant has a venom more potent than any other member of the order Hymenoptera, which includes wasps and bees.
Fire ants attack without provocation and without warning. They tend to build their mounds in open sunny areas, such as household lawns, parks and golf courses. They are also destructive. Reports exist of fire ants damaging farm equipment, electrical systems, irrigation systems and air conditioning units.
Each fire ant can produce many stings in a matter of seconds. The ant clamps onto its victim with its mandibles, then whips its tail in a circular fashion, producing 6 to 8 stings in a ring formation.2
Fire ant venom contains dialkylpiperidine hemolytic factors, which induce the release of histamine and other vasoactive amines from mast cells. The stings cause sudden intense pain, immediate whealing and flare response. Within hours of the initial sting vesicles develop, which evolve to exquisitely pruritic sterile pustules.3
Systemic reactions range from generalized skin manifestations including erythema, pruritus, angioedema and urticaria to potentially life-threatening anaphylaxis. Bronchospasm, laryngeal edema, and/or hypotension may occur immediately or several hours after an attack. These may increase in severity with each successive attack, and cases of anaphylaxis-related fatalities have been reported.3
Diagnosing fire ant stings usually depends clinical appearance and history. Skin biopsy indicates features of an arthopod assault, such as endothelial swelling, spongiosis with intraepidermal vesicles, perivascular inflammation (neutrophils, lymphocytes or eosinophils) and dermal edema. Fire ant stings are often more pustular and have neutrophils compared with other types of arthopod assaults.4
Limited local reactions are self-limited and require only symptomatic relief with cool wet dressings, topical antipruritics and oral antihistamines. More widespread cutaneous reactions may warrant a short course of oral prednisone.
For severe systemic reactions, epinephrine 0.3 mL (1:1000 aqueous solution) is injected intramuscularly. Susceptible individuals should carry an emergency epinephrine response kit for immediate response.
Clinicians should encourage patients to avoid excoriation to prevent secondary bacterial infection.
This patient received oral dyphenhidramine (Benadryl, McNeil Pharmaceuticals) and cool compresses. He was closely monitored at home for signs of systemic involvement. His lesions resolved within a few days with no complications.
Kristy Fleming, MD, is a third year dermatology resident at Baylor College of Medicine.
1. James WD, Berger TG, Elston DM. Andrew’s Diseases of the Skin Clinical Dermatology. 10th ed. Pennsylvania: Saunders Elsevier; 2006: 450.
2. Paller AS, Mancini AJ. Hurwitz Clinical Pediatric Dermatology. 3rd ed. Elsevier; 2006:493-494.
4. Rapini R. Practical Dermatopathology. Elsevier; 2005: 206.