fire ants

If you live anywhere in the Southern United States from the Eastern seaboard to West Texas, your children may be exposed to stings of the fire ant.

Solenopsis invicta or imported fire ant is the chief culprit. There are four domestic species of fire ants, but the imported fire ant is most aggressive. This species of ants are a member of the Hymenoptera family, which includes social stinging insects such as honeybees, wasps and hornets. They are distributed throughout the southern United States, having entered US from South America in the 1920's and 1930's through the southern port of Mobile, Alabama. The ants need moisture and do not survive in the driest parts of the US except in well-watered lawns. They survive over the winter cold months burrowed under asphalt roads, which stay warm enough from the sun's rays to allow them to live.

Fire ants are small (about 1/16 inch or so long) and yellow-to-red or black with a large head, with large incurved jaws and a beelike stinger on the tail. When their nest is disturbed, they swarm out, emitting pheromones - chemical signals - that serve to make the ants even more aggressive. They are sensitive to vibration, which triggers the sting. Typically, a young child steps inadvertently in an ant bed, and his movement begins the stinging. The ants latch on by biting with their mouths, then sting repeatedly with their stinger as they move around in a circle. The ant may sting up to 20 times, making a neat little circle of stings with two central red jaw marks.

The venom is quite toxic. It immediately causes a temporary burning sensation, then an itchy wheal ("whelp"), and eventually forms a pustule in about 8 to 24 hours which is the result of sterile tissue breakdown from the venom and pus accumulation. They resolve in about 10 days. A late reaction consisting of a large surrounding area of red, hard, and very itchy skin is commonly seen, and can be confused with a bacterial infection (cellulitis).

Bites are treated with cool compresses, followed by application of a paste made with baking soda. Sarna® lotion (0.5%camphor + 0.5% menthol) is soothing, especially if it is refrigerated. Meat tenderizer, useful for jellyfish stings, is however of no value. Oral antihistamines such as Benadryl® (diphenhydramine) provide some relief. A short course of steroids is sometimes prescribed for severe local reactions.

Parents are sometimes tempted to prick open the characteristic little white pustules on the skin, thinking that they appear to be infected. However, the fire ant venom is toxic to bacteria as well, so the pustule that forms is sterile, and should not be opened but allowed to eventually flake off.

Fire ant stings can be no small matter. The venom is especially potent, containing little inert protein and almost all active toxic (whereas bee and wasp venom contains much more neutral protein material). Reactions to the sting range from hives, itching and redness to life-threatening breathing difficulty, or shock (anaphylaxis) and possible death. There may even be significant tissue loss requiring skin grafting.

Individuals prone to severe reactions can be desensitized with an imported fire ant vaccine. We desensitize to bee stings using a preparation of bee venom which can be "milked" from the bees, but the ants are just too small for this to be practical - they have about half as much venom per ant as a bee. For this reason, an extract of the whole ant is used for the vaccine. Enough of the venom proteins are present to make this work quite effectively.

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