SAN ANTONIO - Imported red fire ants entered the United States in the late 1920s and early 1930s via the port of Mobile, Ala., perhaps in the soil used as a ballast for ships originating from Paraguay, Uruguay and Southern Brazil. More than 60 years later, they pack an annual $3 billion punch on the U.S. economy - injuring livestock, damaging crops and farm equipment, and destroying native fauna and wildlife.
Medically, reactions to Hymenoptera venom can range from local pain and swelling to life-threatening anaphylaxis, with complications resulting in skin grafts, amputations or even death.
Labeled as aggressive, angry competitors, imported fire ants are sensitive to vibration or movement and will sting when the object they're on moves (i.e. a child running through the grass with bare feet who knocks an ant mound). The ants will swarm up a person's leg, and, if that person jerks or moves, can trigger other ants to sting in response.
There are several species of Hymenoptera, but the dominant species in the United States is Solenopsis invicta, or the red imported fire ant. There are at least four other species found in the United States, S. richteri (found in Alabama and Mississippi), S. xyloni, S. geminata and S. aurea.
Electron micrograph of a fire ant.
Photo courtesy of the USDA
The imported fire ant is confined mostly to moist areas, according to Ed Vargo, PhD, assistant professor, department of entomology, North Carolina State University in Raleigh. "The stings are reported throughout the Southeast and into the South Central United States. Far west into Texas it gets too dry for them, although they can occur in manicured lawns and golf courses."
There have been infestations reported in El Paso, Texas, southern New Mexico and areas around Phoenix, but imported fire ants cannot survive outside of well-watered areas in those regions. They've recently become established in southern California near Los Angeles and Bakersfield, where there are fairly large infestations. In addition, infestations have been reported throughout North Carolina, a good part of Virginia and the Virginia coast, up into southern Maryland and even the very southern tip of New Jersey.
According to Vargo, imported fire ants are more active in the warm weather and slow down when the weather cools. However, if it's too hot they become more active at night. "They go by the ground temperature," said Vargo. "When the ground temperature is between 68° F and 90° F, that's when they're most active."
Even in the dead of winter, black tar highways with the sun beating down on them keep the ground directly underneath warm enough for the fire ants to survive.
Imported fire ant mounds can mature for years if undisturbed, and a mature mound can contain more than 250,000 ants with several queens. Unlike other species, imported fire ant mounds allow vegetation to grow up through the mound.
"What happens when you disturb the mound is it will `boil over,'" said James Quinn, MD, assistant professor of medicine, Uniformed Services University of the Health Services and director of the allergy/immunology research laboratory, Wilford Hall Medical Center, Lackland Air Force Base, Texas. "Almost instantaneously if the weather is warm, thousands of fire ants will boil out of the mound. They're angry and aggressive and secreting a pheromone, which makes them more angry; so they're ready to attack and sting when they're able to get a hold of you."
A common misconception is that fire ants bite their victims, but they sting as well. "They do bite, but that's not what you feel and that's not what bothers you," said Quinn. "They anchor themselves with their mandible and their stinger is at the point of their abdomen. Then they arch their abdomen, sting you, and pivot in a semicircle until you're able to get them off. So they do actually bite and sting, but it's the sting that's the problem."
The imported fire ant venom sac contains 40 nL of venom, compared with 50 µg to 100 µg of venom in a flying Hymenoptera. Most of the venom, approximately 95%, is toxic piperidine alkaloids, which leads to tissue necrosis. However, it's bactericidal and nonantigenic.
Five percent of the venom is an aqueous protein with 10 ng to 100 ng of protein, where the antigenic components lie.
Victims usually encounter the imported fire ant outdoors and are usually stung multiple times. After being stung, there is an initial transient burn. The burning sensation stems from piperidine alkaloids going into the skin; 20 minutes later the victim will have pruritic wheal and flare that generally resolves within several hours.
Following onset of pruritic wheal and flare is the onset of the vesiculopustular lesion. The toxic invasive substance results in necrosis in the tissues that evolves to form a pustule usually over 24 hours.
"Later, as it heals over the course of the ensuing days, there will be an evolution of plasma cells. Eventually the epidermis will slough and it will heal from the base," Quinn said.
He warned that physicians may need to advise their patients to resist the temptation to think the pustule is an infection and to break it open. The pustule is actually sterile and protects the skin.
A large local reaction reaction site can be anywhere from 5 cm to 10 cm, must persist for more than 24 hours, cross a joint or be debilitating.
In a large local reaction, physicians should take a different approach to the problem, according to Quinn. "You will still have the initial wheal and flare and the pustule," said Quinn. "But if you're part of the approximately 3% to 50% who will have a large local reaction, what begins to happen at one to two hours is edema, erythema and induration extending over six to 24 hours.
A mild reaction to an imported fire ant sting consists of hives, itching and redness over the body. A moderate reaction may consist of all of these symptoms plus trouble breathing and tongue swelling. A more severe reaction may consist of all the mild and moderate symptoms, plus anaphylactic shock or severe asthma or respiratory distress, according to Richard Lockey, MD, professor of medicine, pediatrics and public health, University of South Florida School of Medicine in Tampa, Fla.
Lockey places all patients who have systemic reactions on allergen immunotherapy. He also vaccinates them with the imported fire ant whole-body vaccine.
"For someone with a bee or wasp anaphylactic-induced reaction, you would use the venom," said Lockey. "With those animals, they extract, or milk, the venom sacs to get the venom and make a vaccine. But the imported fire ant is so small that the process is too labor intensive. It's almost impossible to get the venom. So fortunately, the whole-body vaccine contains the venom proteins that account for the reaction; so when you immunize people it's an effective mode of therapy."
If a patient is stung and is admitted to the hospital because of anaphylactic shock, Lockey said he would test the patient with the imported fire ant whole-body vaccine to determine the level of sensitivity.
Depending on the sensitivity, he starts them on a very weak dilution of the vaccine and over a period of three to four months increases to a high dose of the vaccine.
"During that time patients become tolerant or desensitized to the proteins in the venom which cause the reaction," said Lockey. "Then if they get stung, they'll go back to having no reaction, not a systemic reaction. People usually stay on the injections for at least five years. We give the injections once every six weeks after one year, so it's about eight injections per year."
For patients who have experienced reactions, Lockey recommends they carry adrenaline-containing syringes in the event they are stung.
Lockey also advises his patients which chemicals to use and how to use them to decrease fire ant colonies in their yards.
For more information:
- Quinn J. Large local reactions to fire ant stings. Session 176. Presented at the 56th Annual Meeting of the American Academy of Asthma, Allergy and Immunology. March 3-8, 2000. San Diego.
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