History
Fire ants can inflict several painful burning stings within seconds. The severity of symptoms varies with the size of the ant and the allergic response of the patient.
Patients often present with a history of an immediate intense burning sensation (the "fire" associated with the ant's name) and itching at the sting site.
Stings occurring during the winter months are often less severe and may go unnoticed until a local reaction develops. This reflects the seasonal variation in venom protein concentration.
Physical Examination
Physical findings from fire ant bites and stings can be subdivided into local and systemic reactions.
Local reactions
Skin lesions produced by fire ants typically occur in clusters. The attachment site of the ant's mandibles makes 2 small, hemorrhagic puncta. The initial reaction to the sting is the development of a wheal, followed within 24 hours by a sterile vesicle.
The fluid in the vesicle becomes cloudy; after 8-10 hours, the typical lesion is an umbilicated, sterile pustule on a red, edematous base. The pustule may last for several days and is characteristic for fire ant stings. The pustule then ruptures, forms a crust, and heals several days later, sometimes leaving small scars. Excoriation and open erosions may lead to secondary infection.
See the images below.
Pustules and blisters formed following fire ant stings on the arm. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
Pustules and blisters formed following fire ant stings on the hand. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
Systemic reactions
Systemic reactions range from skin manifestations (eg, generalized urticaria, angioedema, pruritus, erythema) to potentially life-threatening bronchospasm, laryngeal edema, or hypotension.
Anaphylaxis may occur immediately or hours after a sting. These reactions are similar to those caused by venom of other Hymenoptera insects, except for the characteristic pustule.
Seizures, mononeuritis, serum sickness, nephrotic syndrome, and worsening of preexisting cardiopulmonary disease have also occurred.
The reactions may increase in severity with successive attacks, and fatal allergic reactions are becoming more common.
Complications
Systemic allergic reactions are a potential complication of fire ant stings. One report describes a healthy 21-year-old man who developed renal failure due to hemolytic uremic syndrome after fire ant bites. [11]
Secondary infection of the sting site with possible pyoderma or sepsis can occur.
Fatal toxic reactions from ant stings have been reported in small animals, but no human fatalities from toxic reactions have been reported. Toxic reactions have been considered as possible factors in deaths occurring in immobilized, chronically ill subjects stung by fire ants, but toxicologic studies of fire ant venom effects in humans have not been performed. It seems unlikely that the venom toxicity alone explains these deaths because patients who are not allergic have endured thousands of stings with no complications other than pustules.
Seizures and mononeuropathy are rare but have been reported.
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Imported fire ant national distribution map. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Red imported fire ant worker. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Fire ant mound in lawn. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Venom sac and stinger of a fire ant. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Fire ant worker biting and stinging. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Pustules and blisters formed following fire ant stings on the arm. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Pustules and blisters formed following fire ant stings on the hand. From http://fireant.tamu.edu. Reproduced with permission from B.M. Drees, Texas Imported Fire Ant Project Coordinator, Texas A&M University, College Station, Texas.
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Fire ant bites on the foot.
