Fire Ant Bites Clinical Presentation

• 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 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. Fire ant bites on the foot.
• 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.

The fire ant prefers open, sunny areas, such as pastures, parks, lawns, playgrounds, golf courses, and fields. Colonies also occur in or around buildings. Mound building increases considerably during warm months of the year when soil is moist. Concentrations in some areas exceed 200 mounds per acre. Several risk factors have been identified:

• Immobility

  • Infants and elderly persons have an increased risk of fire ant stings, as do others with decreased mobility or an inability to defend themselves, such as persons who are inebriated and fall asleep on or near a mound.
  • Massive sting attacks by fire ants have occurred in nursing home residents.
  • Infants are unable to defend themselves from attacks.^[2]
  • Immobilized people are likely to have numerous stings when exposed to fire ants.
  • In these situations, determining the source of the fire ants and exterminating them are essential.
• Diabetes mellitus

  • Persons with diabetes are at an increased risk of secondary infection of a sting site because of potential circulatory or neurosensory compromise of the extremities.
  • Secondary infection of a sting site may lead to pyoderma or sepsis.
• Alcoholism

  • Several cases of severe fire ant stings have been reported in people who are alcoholics, often secondary to alcohol-induced unconsciousness.
  • One case involved a person with alcoholism who fell asleep in a ditch and apparently used a fire ant mound as a pillow. He was hospitalized hours later with about 5000 pustules from fire ant stings on his face, trunk, and extremities that eventually healed with scarring.
• Previous sensitization

  • Systemic reactions typically occur in patients previously sensitized to fire ant stings.
  • Individuals with no previous exposure can have anaphylactic reactions after their first sting. Most of these patients are previously sensitized to yellow jacket venom.