Fire Ant Bites 

Updated: Oct 04, 2016
Author: James P Ralston, MD; Chief Editor: Joe Alcock, MD, MS 

Overview

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. See the images below.

Red imported fire ant worker. From http://fireant. 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.
Fire ant mound in lawn. From http://fireant.tamu.e 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.

See Arthropod Envenomation: From Benign Bites to Serious Stings, a Critical Images slideshow, for help identifying and treating various envenomations.

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.[1] 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.[2, 3, 4]

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 two 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. See the image below.

Venom sac and stinger of a fire ant. From http://f 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.

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.[5] Four major allergenic proteins exist; Soli 1-4 induce immunoglobulin E (IgE) responses, including anaphylaxis, in patients who are allergic.[6] 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.

Epidemiology

US 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.

See the image below.

Imported fire ant national distribution map. From 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.

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.

Prognosis

Minor reactions have an excellent prognosis. Severe reactions have an excellent prognosis with early and appropriate treatment. However, 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.

Patient Education

Patient education is essential in preventing possible life-threatening reactions in patients who are allergic and in providing appropriate treatment of such reactions if they occur. This should include the following:

  • Identification of stinging insects

  • Knowledge of how to avoid being stung

  • Knowledge of how and when to self-administer epinephrine, if indicated

  • Carrying proper identification of stinging insect hypersensitivity (eg, Medic Alert bracelet)

For patient education resources, see the patient education articles Insect Bites, Allergy: Insect Sting, and Severe Allergic Reaction (Anaphylactic Shock).

 

Presentation

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.

See the images below.

Pustules and blisters formed following fire ant st 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 st 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. 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.

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.[7]

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[8]

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.

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.[9]

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.

 

DDx

Diagnostic Considerations

Local reaction (infection, punctures, foreign bodies, various other dermatoses)

Toxic reaction (chemical exposure/ingestion, intravenous drug abuse, environmental, plants)

Allergic reaction (medications, illicit drugs, foods, topical products, environmental, plants, chemicals)

Differential Diagnoses

 

Workup

Laboratory Studies

Laboratory studies are not necessary for most people with fire ant stings; however, in severe reactions, a CBC count, coagulation studies, and a urinalysis could be obtained for the following uncommon but possible manifestations:

  • Leukocytosis

  • Thrombocytopenia

  • Hypofibrinogenemia

  • Abnormal coagulation

  • Disseminated intravascular coagulation

  • Proteinuria

  • Hemoglobinemia

  • Hemoglobinuria

  • Myoglobinemia

  • Myoglobinuria

  • Azotemia

Other Tests

Skin testing, enzyme-linked immunosorbent assay (ELISA), and radioallergosorbent testing (RAST) can be used to confirm a clinical history of fire ant hypersensitivity.

A venom ELISA assay has demonstrated equivalent sensitivity to venom RAST and is less expensive.[10] Reagents containing venom proteins are required for these tests. Because pure venom vaccines are not commercially available, whole-body extracts are used.

Patients without a clinical history of allergic reactions to fire ants should not be tested because of the high degree of asymptomatic IgE production in an exposed population.

Histologic Findings

The histologic findings depend on the stage of evolution of the lesion. In early lesions, a perivascular infiltrate of lymphocytes, neutrophils, and eosinophils is found within the dermis. Later, an intraepidermal vesicle or pustule (containing mostly neutrophils) is usually present, often with a central focus of epidermal necrosis. Dermal edema is often present. Compared with other arthropod assaults, fire ant stings are far more pustular, with more neutrophils and fewer eosinophils.

 

Treatment

Medical Care

Local stings

Cool compresses and oral antihistamines are recommended for mild reactions. Corticosteroids can be used topically or intralesionally for anti-inflammatory effect.

Multiple stings

Systemic corticosteroid use is controversial in patients with extensive lesions who do not have systemic allergic reactions or generalized skin reactions. Large doses of corticosteroids and intravenous fluids may complicate the treatment of patients with preexisting cardiovascular disease. The immunosuppressive effect of corticosteroids may predispose patients to secondary infection.

Oral antihistamines and topical corticosteroids are recommended in most cases; nevertheless, some practitioners still use prednisone or other systemic steroids to treat patients with numerous lesions.

Anaphylaxis

Acute management of fire ant anaphylaxis is identical to treatment of anaphylaxis from other causes. Subcutaneous epinephrine is used and repeated every 10-15 minutes as needed to reverse the symptoms.

Further outpatient medications

Desensitization may be helpful to protect patients who are allergic from reactions to future stings. This type of immunotherapy has been used for almost 30 years to prevent the recurrence of anaphylaxis.[11, 12]

Treatment consists of weekly subcutaneous injections of increasing doses of whole-body vaccine until a predetermined maintenance dose is reached (usually 0.5 mL of a 1:10 dilution of the 1:10 weight/volume stock whole-body vaccine solution). Maintenance doses are typically administered every 4-6 weeks.

Immunotherapy for children with isolated skin reactions to fire ant stings is controversial because of a lack of data.[13] Most allergists do not routinely recommend immunotherapy for this population, but some do because of the great risk of stings in endemic areas.

Prescribe an anaphylactic kit (ANA kit) or Epi-Pen, if indicated.[13]

Consultations

An allergist/immunologist consultation for evaluation and possible skin or in vitro testing for fire ant hypersensitivity is appropriate for any patient who has a systemic reaction to a fire ant sting.[14] Consultation should be considered if the patient meets 1 of the following criteria:

  • Experiences anaphylaxis with a fire ant sting as a possible cause

  • Needs education regarding fire ant avoidance or emergency treatment

  • May need venom immunotherapy

  • Has a coexisting condition that may complicate treatment of anaphylaxis (eg, using beta-blockers, having hypertension or cardiac arrhythmias)

Diet

No dietary changes are recommended; however, patients should have nothing by mouth if experiencing a severe systemic reaction.

Activity

No restriction in activity is required; however, rest is recommended in severe cases to possibly slow the spread of the reaction.

Prevention

Avoidance of fire ants is important in the management of patients with fire ant hypersensitivity. Avoidance is facilitated by the following:

  • Having professionals evaluate the patient's home for stinging insect nests and fire ant mounds, and, if found, exterminating these nests and mounds

  • Not wearing brightly colored clothing or strongly scented lotions

  • Wearing shoes (not sandals) when walking outside

  • Being cautious around bushes, attics, picnic areas, or garbage containers

  • Keeping insecticides readily available

  • Wearing long pants, a long-sleeved shirt, socks, shoes, a hat, and work gloves when working outside

Attempts to control fire ant populations in endemic areas have included the use of chemical pesticides and novel biological control, including the use of decapitating flies. Decapitating flies (ie, Pseudacteon tricuspis, Pseudacteon curvatus, Pseudacteon littoralis) from South America have been released in the United States. These flies deposit an egg in the thorax of worker fire ants. The egg hatches and the larvae move toward the head, where they eat the ant's glands and muscles and release an enzyme that makes the ant's head fall off.[15]

 

Diphenhydramine (Benadryl, Benylin)

Diphenhydramine is used for symptomatic relief of symptoms caused by release of histamine in allergic reactions.

Ibuprofen (Ibuprin, Advil, Motrin)

Ibuprofen inhibits inflammatory reactions and pain by decreasing prostaglandin synthesis.

Prednisone (Deltasone, Orasone, Sterapred)

Prednisone may decrease inflammation by reversing increased capillary permeability and suppressing PMN activity. Many dosing regimens have been used.

Epinephrine (EpiPen, Adrenalin)

Epinephrine is the drug of choice for treating anaphylactoid reactions. It has alpha-agonist effects that include increased peripheral vascular resistance, reversed peripheral vasodilatation, systemic hypotension, and vascular permeability. Beta-agonist effects of epinephrine include bronchodilatation, chronotropic cardiac activity, and positive inotropic effects.