eMedicine Specialties > Emergency Medicine > Environmental

Fire Ant Bites

Author: James P Ralston, MD, President, Dermatology Center of McKinney
Coauthor(s): Ronald P Rapini, MD, Professor and Chair, Department of Dermatology, Professor of Pathology, University of Texas Medical School at Houston, MD Anderson Cancer Center
Contributor Information and Disclosures

Updated: Jun 11, 2007

Introduction

Background

The fire ant is a wingless member of the order Hymenoptera, which includes wasps and bees. It is a potentially lethal environmental hazard in the United States, infesting more than 310 million acres of land. Fire ants are resistant to control efforts and can overwhelm an environment. They damage farm equipment, electrical systems, irrigation systems, and land. They build mounds in sunny, open areas (eg, lawns, playgrounds, parks, golf courses) and aggressively attack anyone who disrupts their mound.

Fire ants are thought to have arrived in the United States between 1918 and the 1930s from South America by ships that docked in Mobile, Alabama. They are now found throughout the Southeast and are migrating rapidly. One contributing factor to this expansion is progressive urbanization in the United States, which creates the type of disturbed habitat that the fire ants prefer. Their mobility and ability to establish colonies in diverse habitats makes the detection of new infestations difficult. Sometimes, colonies exist several years before detection.

Each year, fire ants sting more than one half of the population in endemic areas of the Southeast. They cause a variety of medical problems, including increasing numbers of hypersensitivity reactions, secondary infections, neurologic complications, and even death.

Pathophysiology

The fire ant uses its mandibles to grasp its victim. It arches its body and drives an abdominal stinger into the skin to release venom. If not quickly removed, it then pivots around its mandibles and inflicts further stings in a circular pattern.

The stinger is a modified ovipositor that consists of a dorsal stylet and 2 ventrolateral lancets. These structures surround the venom canal, which connects to the venom sac. A pair of coiled glands produces the venom that discharges into the venom sac.

Fire ant venom differs from bee and wasp venom, which are mostly proteinaceous solutions. About 95% of fire ant venom is water-insoluble, is nonproteinaceous, and contains dialkylpiperidine hemolytic factors. These hemolytic factors induce the release of histamine and other vasoactive amines from mast cells, resulting in a sterile pustule at the sting site. These alkaloids are not immunogenic, but their toxicity to the skin is believed to cause the pustules to form.

The venom also contains several allergenic proteins, measuring about 1.5% by dry weight. Four major allergenic proteins exist; Soli 1-4 induce immunoglobulin E (IgE) responses, including anaphylaxis, in patients who are allergic. Antigenic similarity exists between these proteins and bee and wasp venoms.

Many patients have venom-specific IgE-mediated wheal and flare reactions that develop over hours into pruritic edematous, indurated, and erythematous lesions that persist for up to 72 hours. These lesions may involve an entire extremity. They histologically resemble late-phase mast cell–dependent reactions and show an infiltrate of eosinophils, neutrophils, and fibrin deposition. Large, local reactions rarely can cause edematous tissue compression, leading to vascular compromise of an extremity.

Frequency

United States

Because most fire ant stings are not severe enough to cause the victim to seek medical attention, estimating the frequency of stings is difficult; however, annually, more than one half of the population in endemic areas is stung, and the incidence appears to be increasing.

Mortality/Morbidity

Fire ants are becoming an increasingly important public health concern in the United States. More than 80 fatalities have been reported from fire ant-induced anaphylaxis.

Race

Fire ant stings may occur in people of any race. No race has been shown to have an increased risk of being stung or to have a higher predisposition to complications.

Sex

Fire ants sting both males and females without discrimination.

Age

Fire ants sting people of all ages, but children are overrepresented, probably because of greater environmental exposure.

Clinical

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

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.
• 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.

Causes

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.
  • 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.

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