Fire Ant Stings

November 1, 2006

Patients stung by fire ants instantly feel intense burning pain at the site, hence the name "fire" ants; the small pustular lesions typically develop about a half hour afterward and evolve over several days.

Red imported fire ants (Solenopsis invicta) were introduced into the United States in the 1930s by ships from South America that docked in Mobile, Ala. Fire ants are now active throughout the southeast Sunbelt states. Colonies are also found in California; one of the largest is in Orange County.

The 2- to 5-mm, reddish brown ants often build their mounds in grassy areas. Fire ants swarm over humans, as well as animals, that are perceived as a threat to the colony (eg, when a lawn mower passes over a mound or when a mound is intentionally disturbed by a child). Photo A shows the foot of a man who was swarmed by fire ants as he mowed his lawn. Photos B and C show the upper leg and torso and close-up of the arm of an intoxicated woman who was stung repeatedly after she fell asleep on a fire ant mound in a public park in Houston.

The bite of a fire ant is no worse than that of other similarly sized ants. The fire ant grabs hold with its mouthparts so that it can readily wheel its body around inflicting many stings. The stings from a single ant appear as a dotted circle. Patients stung by fire ants instantly feel intense burning pain at the site, hence the name "fire" ants; the small pustular lesions typically develop about a half hour afterward and evolve over several days.

The pustules that result from fire ant stings are commonly mistaken for an infectious process. In fact, these lesions are induced by venom alkaloids and are sterile. Antibiotic therapy is indicated only for secondary infection of the lesions, which may be related to intense scratching--indeed, broken pustules increase the likelihood of infection.

The lesions eventually resolve without intervention. In some cases, large local reactions with edema and erythema may develop; these can be easily mistaken for a bacterial infection. The early onset of such a reaction confirms that the cause is noninfectious; in the absence of fever or other systemic signs, antibiotic therapy is unwarranted. Pruritus and pain can be treated with antihistamines (oral or topical), topical corticosteroids, and analgesics; over-the-counter preparations are usually sufficient. Anaphylactic reactions in sensitized persons with fire ant stings require the standard treatment.1,2

References:

REFERENCES:1. deShazo RD, Butcher BT, Banks WA. Reactionsto the stings of the imported fire ant. N Engl J Med.1990;323:462-466.
2. Goddard J, Jarratt J, de Castro FR. Evolution ofthe fire ant lesion. JAMA. 2000;284:2162-2163.