Richard D deShazo, MD
David F Williams, PhD
David B Golden, MD
Anna M Feldweg, MD
FIRE ANT OVERVIEW — Fire ants were accidentally imported into Mobile, Alabama in the 1930s from South America. Since then, their range has expanded throughout the southeastern, southern, and southwestern United States and into Puerto Rico. Regularly updated maps of their range and agricultural quarantine areas are available at file://www.aphis.usda.gov/plant_health/plant_pest_info/fireants/downloads/fireant.pdf.
Fire ants have a major impact because they prey on other insects and animals, sting humans and farm animals, and form large mounds of soil that damage farm machines and crops . Although fire ant stings are rarely fatal, people who live in areas where fire ants are common should know how to avoid being stung, how to treat a fire ant sting, and how to use insecticides to control the fire ant population.
WHAT ARE FIRE ANTS? — Fire ants are aggressive, venomous insects that have become a serious problem in many areas of the United States and around the world. The ants range in size; queens are often three-fourths of an inch long, while minor workers are approximately one-eighth inch.
The ants build dome-shaped nests (mounds) of soil that are 2.5 to 3 feet across. These tend to be flatter when built in sand. The queens are underground, protected by many workers, and continuously lay eggs to produce new ants. During dry or cold conditions, ant colonies move deeper into the mound or under paved areas and even into buildings. Colonies contain 100,000 to 500,000 worker ants.
The ants have pinching mandibles on the head and a sharp stinger connected to a large venom sac on the rear of the body. When attacking predators or disturbed by an object, the ant grasps the object with the mandibles, quickly inserts the stinger, and injects venom.
Once attached to the skin, fire ants continue to sting an average of three times before removal. Most stings occur on the feet or legs after stepping on a fire ant mound. Stings are largest and most painful during the summer, when the ants have the largest quantity of venom. Up to 50 percent of people who live in an area of fire ant infestation are stung per year.
REACTIONS TO FIRE ANT STINGS — There are three main types of reactions to fire ant stings.
- Local reaction — The most common reaction to fire ant stings is immediate intense burning, followed by itching, redness, and a 12 to 20 cm (5 to 9 inches) raised red welt on the skin at the sting site . These symptoms usually resolve over four to six hours. The next day, there is usually a sterile pustule at the sting site, which is best described as a small bump with a white blister on top. The pustule resolves over a week or so unless it is scratched off. Scratching is not recommended because it can allow an infection to develop in the skin.
- Large local reaction — A small percentage of people who are stung develop a large local reaction. Signs include extreme itching and a large raised red welt at the site of the sting. Over 6 to 12 hours, the swelling and intense itching increases and evolves into a large area of painful swelling. These reactions reach maximum size at 24 to 48 hours, when they are hot, itchy, and painful.Rarely, swelling can be large enough to interrupt the blood supply to toes or fingers. Fortunately, large local reactions do not cause a bodywide allergic reaction (anaphylaxis).
- Anaphylaxis — Between 0.6 and 16 percent of people who are stung by a fire ant have a severe, bodywide allergic reaction called anaphylaxis. This type of reaction is more common in people who have previously been stung or had an allergic reaction to a wasp (yellow jacket) sting. Anaphylactic reactions are very serious, and usually develop within 30 minutes of being stung. If left untreated, this reaction can be life-threatening. Signs of anaphylaxis include bodywide itching, welts, difficulty breathing or swallowing, weakness, or fainting. (See “Patient information: Anaphylaxis treatment and prevention”.)
TREATMENT OF A FIRE ANT STING — At the first sensation of a sting, the ant should be quickly killed with a slap. Then, wash the sting site with soap and cool (not hot) water.
Local reaction — If the sting site is itching severely, we recommend a non-sedating oral antihistamine, such as loratadine (Claritin®) or cetirizine (Zyrtec®) and/or a topical anti-itch ointment (1 percent hydrocortisone). Itching can last for many hours.
Diphenhydramine (Benadryl®) 50 mg is also effective and is available without a prescription, although it can cause a person to be very sleepy. Diphenhydramine should not be taken before driving or any other activity requiring alertness.
Keep the sting site clean and avoid scratching it. If the site becomes oozy and wet, a prescription antibiotic ointment (mupirocin/Bactroban®) is recommended to prevent infection. Avoid over-the-counter antibiotic ointments, such as Neosporin or Bacitracin, which can themselves cause an allergic reaction.
If the sting site appears infected, continues oozing, grows larger, or develops red streaks, it should be evaluated by a healthcare provider as soon as possible.
Large local reaction — Large local reactions are best treated with an oral antihistamine (loratadine/Claritin® or cetirizine/Zyrtec®) and 1 percent hydrocortisone ointment (or diphenhydramine/Benadryl®, see above). The ointment should be covered with first aid tape to increase absorption of the steroid.
Very large local reactions should be evaluated by a healthcare provider, and are usually treated with a prescription steroid ointment and/or an oral corticosteroid (eg, prednisone). Large reactions that surround a finger/toe or hand/foot should always be evaluated by a healthcare provider because of the risk that swelling could block blood flow.
Anaphylaxis — Anyone who develops difficulty breathing or swallowing, weakness, or fainting after being stung by a fire ant needs emergency medical treatment. The person (or a friend or family member) should call for emergency assistance, available in most areas of the United States by dialing 911. Do not wait to see if the reaction will get better. If available, the person should use an epinephrine autoinjector. (See “Patient information: Use of an epinephrine autoinjector”.)
If the person is alert and able to swallow, he or she should take an over-the-counter antihistamine (eg, diphenhydramine/Benadryl® 50 mg or cetirizine/Zyrtec® 10 mg for adults, half this dose for older children) while waiting for emergency assistance.
Anyone who experiences anaphylaxis should be evaluated by an allergist, who can provide injections to desensitize the person to fire ant venom. These “allergy shots” can significantly reduce the risk of another anaphylactic attack if the person is stung again by a fire ant.
HOW TO AVOID BEING STUNG BY FIRE ANTS — The best ways to avoid being stung include the following:
- Know your environment. Walk around open areas and look for ant activity or mounds before using the area.
- Exterminate fire ants in frequently used areas with regular use of pesticides.
- In areas of infestation, wear lace-up shoes, thick socks, and long pants, and avoid sandals. When gardening or working in the soil, wear long sleeves and gloves.
- Insect repellents are not effective in preventing fire ant stings.
CONTROL OF FIRE ANTS — At present, the only effective methods available to the public to control fire ants are insecticides. There are no methods that permanently control or eliminate fire ants.
The chemical insecticides used today are only effective for short-term control (3 to 12 months), must be reapplied periodically, and are costly. They can be used effectively to control infestations . Disturbing mounds only makes the ants move short distances because the queen is not killed.
Outdoor treatments — The four basic strategies used for controlling fire ants with chemicals include:
- Broadcast bait or granular applications
- Individual mount treatments
- A combination of broadcast bait and individual mound treatments
- Barrier and spot treatment
Safety instructions for each product should be carefully followed.
Broadcast baits — Broadcast baits reduce fire ant populations by using a small amount of insecticide dissolved into an attractant food source (eg, soybean oil) . The oil containing the insecticide is then absorbed into a carrier (such as corn grits), which is dispersed over an area. This is considered the most effective and efficient method to control multiple colonies over a large area. Regular applications should be made in the spring and fall to reduce or prevent the ants from moving to nearby untreated areas.
Insecticides used in baits are usually slow acting so that the foraging worker ant can pick up the bait, extract the toxic oil, and feed it to the queen and other ants before dying. Depending upon the active ingredient, the queen either dies, becomes infertile, or does not produce mature ants, leading to the eventual death of the colony. There are now fast-acting baits that cause colony death in three days or less (compared to older baits that take two weeks or longer).
Broadcast bait applications eliminate the need to locate individual ant mounds, but instead depend upon foraging ants to take the bait back to the rest of the colony. In addition, there is no risk that the colony will simply relocate because the mound is not disturbed and the queen is eliminated. Because large areas can be treated, this method can result in slower reinfestation by colonies migrating from other, untreated areas.
Individual mound treatments — Individual mounds can be treated by applying an insecticide. The chemicals should be specifically labeled for use on imported fire ants.
Ideally, contact insecticides should kill worker ants quickly because slower-acting chemicals may allow the ants to relocate.
Use of gasoline or kerosene directly on mounds can be dangerous and is not recommended. Nonchemical methods of treating individual mounds, such as the use of hot water or nest removal, usually are not successful with mature colonies.
Combination treatments — Many experts prefer the use of a combination of treatments to kill fire ants. Broadcast baiting can be combined with individual mound treatments. Baits should always be broadcast first to efficiently reduce fire ant populations. After one or two days, the worker ants have spread the bait through the colony, which can then be treated with an individual contact insecticide to quickly eliminate the worker ants, which can sting.
Barrier and spot treatments — Barrier and spot treatments contain active ingredients that kill ants on contact and are usually sold as sprays or dusts. Some are mixed into latex paint. They may be applied in wide bands on and around building foundations, equipment, and other areas to create ant barriers. Barrier and spot treatments do not eliminate colonies but can prevent fire ants from foraging indoors or infesting electrical and electronic equipment.
Indoor treatments — On occasion, ants build colonies indoors that can be located by following foraging ant trails back to nesting areas . A professional licensed exterminator should be consulted to treat indoor colonies.
WHERE TO GET MORE INFORMATION — Your healthcare provider is the best source of information for questions and concerns related to your medical problem.
This article will be updated as needed every four months on our web site (www.uptodate.com/patients).
Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.
Patient Level Information:
Patient information: Anaphylaxis treatment and prevention
Patient information: Use of an epinephrine autoinjector
Professional Level Information:
Approach to the patient with pustular skin lesions
Bee, yellow jacket, wasp, and other Hymenoptera stings: Reaction types and acute management
Diagnosis of Hymenoptera venom allergy
Entomology and control of imported fire ants
Stings of imported fire ants: Clinical manifestations, diagnosis, and treatment
The following organizations also provide reliable health information.
- Texas Imported Fire Ant Research and Management Project (file://fireant.tamu.edu/)
- American Academy of Allergy, Asthma and Immunology (www.aaaai.org/patients/publicedmat/tips/fireants.stm)
- Nemours Foundation (www.kidshealth.org/kid/ill_injure/bugs/fire_ant.html, available in Spanish)
- Kemp SF, deShazo RD, Moffitt JE, et al. Expanding habitat of the imported fire ant (Solenopsis invicta): a public health concern. J Allergy Clin Immunol 2000; 105:683.
- Williams, DF, Oi, DH, Porter, SD, et al. Biological control of imported fire ants. Am Entomol 2003; 49:144.
- Williams, DF, Collins, HL, Oi, PH. The red imported fire ant (Hymenoptera:Formicidae): An historical perspective of treatment programs and development of chemical baits for control. Am Entomol 2001; 37:146.
- Rupp MR, deShazo RD. Indoor fire ant sting attacks: a risk for frail elders. Am J Med Sci 2006; 331:134.
- Goddard J, Jarratt J, deShazo RD. Recommendations for prevention and management of fire ant infestation of health care facilities. South Med J 2002; 95:627.