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Stings of imported fire ants: Clinical manifestations, diagnosis, and treatment
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Stings of imported fire ants: Clinical manifestations, diagnosis, and treatment
All topics are updated as new evidence becomes available and our peer review process is complete.
Literature review current through: Sep 2017. | This topic last updated: Sep 22, 2016.

INTRODUCTION — Imported fire ants (IFAs) are aggressive, venomous ants that sting with little provocation and are difficult to avoid in endemic areas. Their venom causes painful local reactions and induces a high rate of allergic sensitization. Patients who become allergic to IFA venom can experience a range of reactions, including life-threatening anaphylaxis.

The term "imported fire ant" refers to two members of the Solenopsis species [1]:

The red imported fire ant (RIFA), Solenopsis invicta

The black imported fire ant (BIFA), Solenopsis richteri

This topic review discusses the medical consequences of IFA stings and the treatment of IFA venom allergy. The entomology of IFAs and the chemical and biologic control of these pests are discussed separately. (See "Entomology and control of imported fire ants".)

TERRITORY — Regularly updated maps of the fire ant range and agriculture quarantine areas within the United States are available online.

Red imported fire ants — In the United States, the red imported fire ant (RIFA) is the most abundant, widespread, and damaging species. The RIFA is found in 14 states (Alabama, Arkansas, California, Florida, Georgia, Louisiana, Mississippi, New Mexico, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, and Virginia) with limited territory in Arizona, Maryland, and Delaware [2].

Infestations also occur in Mexico [3], as well as several Caribbean islands, including Puerto Rico, the Bahamas, the British and United States Virgin Islands, Antigua, and Trinidad. In addition, RIFAs have been detected in Australia, New Zealand, Taiwan, and China [4].

Black imported fire ants — The habitat of the black imported fire ant (BIFA) is relatively limited to a small area of northern Alabama, northern Mississippi, and southern Tennessee.

APPEARANCE AND BEHAVIORS OF IMPORTED FIRE ANTS — Worker ants vary in size from ⅛ to ¼ inches (3 to 6 mm) long and are reddish-brown to black in color. They all have stingers (figure 1) and will attack anything that disturbs them. A single ant can deliver multiple stings.

Fire ants are difficult to avoid in endemic areas. Stings of imported fire ants (IFAs) can be sustained both outdoors and indoors. Available data suggest that 50 to 89 percent of people residing in an IFA-infested area will be stung each year. In one report, 51 percent of 107 previously unexposed subjects were stung within three weeks of arrival in an endemic area [5].

Children are stung more often than adults. Patients in long-term care facilities are also at risk, as they may not be aware of their surroundings and may be immobilized by disease or otherwise unable to respond if they come into contact with ants.

When their nests are disturbed, IFAs swarm out and sting anything they encounter, including humans, pets, livestock, and wildlife [2]. Once foraging fire ants have made contact with a potential victim, a variety of stimuli, including simple movement of the patient, may trigger a stinging event, leading to multiple stings in a very short period of time.

Stings occur most frequently during summer, most commonly in children, and typically on the lower extremities. Off-season stings (eg, during winter months) often do not cause as much pain and may go unnoticed until the local reaction develops. This may reflect seasonal differences in IFA venom [6,7]. (See 'Protein allergens' below.)

During a sting, the ant uses its powerful mandibles to hold onto the skin, arches its body, and injects venom through the stinger located at the tip of its abdomen (picture 1). The ant will sting repeatedly if not quickly removed and often leaves a circular pattern of stings as it rotates its body. The human victim characteristically experiences an immediate, intense burning (the "fire" inspiring the name of the ant) and itching at the sting site.

Fire ant avoidance — IFAs will not sting through fabric, stockings, or tights [8]. Thus, wearing shoes (not sandals) with socks and long pants are helpful avoidance measures. Hands and arms should be covered with gloves and sleeves when gardening or doing outdoor work. Available insect repellents are not effective [8]. When a sting sensation is noted, the ant should be quickly killed with a slap to prevent multiple stings.

VENOM — The venom of imported fire ants (IFAs) is approximately 95 percent water-insoluble alkaloids by volume, with the remaining 5 percent being an aqueous solution of proteins. This differs markedly from the venoms of yellow jackets, hornets, wasps, and bees, which are largely aqueous protein solutions. The alkaloids cause the sterile pustule but are not allergenic (see 'Venom alkaloids' below). The allergenic portion of the venom is the protein component [9]. (See 'Protein allergens' below.)

The volume of injectate positively correlates with body size in S. invicta (red imported fire ant [RIFA]). Direct measurements suggest that the venom volume delivered per sting from a S. invicta worker is approximately 0.66 nanoliters, containing 0.56 mcg of venom alkaloid and 10 to 100 nanograms of protein [6].

Protein allergens — There are seasonal variations in the protein content of the IFA venom, with higher protein levels in summer than at other times [7]. S. invicta venom has four dominant allergenic proteins: Sol i 1, Sol i 2, Sol i 3, and Sol i 4 (table 1) [10].

Sol i 1, which has a molecular weight of 37 kD, comprises approximately 2 to 4 percent of venom allergen by weight and possesses both phospholipase A and B activities [11]. Some of the N-terminal sequences of Sol i 1 share sequences with the venom phospholipase of Vespula maculifrons, and sera from some patients with allergy to vespid venom contain immunoglobulin E (IgE), which binds to Sol i 1 by radioallergosorbent test (RAST) or immunoblot studies [10,12,13]. The close structural similarity of portions of the Sol i 1 molecule to portions of vespid phospholipase may explain this immunologic cross-reactivity.

Little is known about the clinical significance of the immunologic cross-reactivity between fire ant and vespid venom. However, it could explain the rare occurrence of anaphylaxis to the first recognized fire ant sting in isolated patients with existing allergy to vespid stings.

Sol i 2 comprises one-half to two-thirds of all venom protein [11,14]. No other proteins of similar structure have been found in the Protein Identification Resource or Swiss-Prot databases.

Sol i 3 is a single chain molecule consisting of 212 amino acids that comprises approximately 15 to 25 percent of total venom protein that has almost 50 percent identity with vespid antigen 5 [11]. However, it lacks immunologic cross-reactivity with vespid antigen 5 [13]. Its complementary DNA (cDNA) has been successfully cloned.

Sol i 4 is a single chain 117-amino acid protein of molecular weight 13,340 and comprises between 8 and 10 percent of venom protein content [11]. Although it shares approximately 35 percent identity with Sol i 2, it lacks immunologic cross-reactivity. Sol i 2 and 4 are unrelated to any proteins found in venoms from other insect genera [11].

S. richteri (black imported fire ant [BIFA]) venom contains proteins designated as Sol r 1, 2, and 3, which are chemically and antigenically very similar to Sol i 1, 2, and 3. A protein corresponding to Sol i 4 has not been found in S. richteri venom (table 1).

Venom alkaloids — The alkaloids in IFA venom are toxic to human epithelium and induce a sterile pustule at the sting site. These compounds are alkaline 2-methyl-6-n-alkyl and alkenyl piperidines [6,15-17] and are classified as solenopsins and dehydrosolenopsins [18,19]. Venom alkaloids have hemolytic, cytotoxic, and antibacterial properties [20-24].

RIFA workers produce the most complex and toxic venom alkaloids of any of the North American Solenopsis species [25]. The relative composition of fire ant venom alkaloids varies with the size and age of the individual worker ant, as well as between species and among castes within a species [17,18,26].

Venom alkaloids have various other properties, as shown in animal and human studies. These include activation of platelets, neutrophils, and the alternate complement pathway, histamine release from peritoneal mast cells, irreversible inhibition of neuromuscular transmission, and inhibition of nitric oxide synthase [25,27-31]. Direct convulsant, cardiotoxic, and respiratory depressant actions of two synthetic venom alkaloids, solenopsin A and isosolenopsin A, have been demonstrated in rodents [32].

The data reviewed in the above section indicate that substantial pharmacologic activity is present in the alkaloid fraction of S. invicta venom. However, direct, nonallergic, toxic effects resulting from S. invicta envenomation in humans have not been reported. Although human systemic toxicity has not been convincingly demonstrated, the substantial pharmacologic activity of IFA venom alkaloids may underlie the poor clinical outcome seen in some cases of massive attacks.

Cross-reactivity with other Hymenoptera species — Cross-reactivity between antigens in IFA venom and antigens in the venoms of other Hymenoptera species is limited [13,33]. Thus, if a patient was able to identify an IFA as the insect which caused a reaction, there is no need to also evaluate the patient for allergy to other Hymenoptera species.

CLINICAL REACTIONS — Clinical reactions to imported fire ant (IFA) stings can range from mild discomfort to life-threatening anaphylaxis.

Local reactions — Local reactions are divided into three types: the wheal and flare reaction, the sterile pustule, and the large local reaction. By definition, local reactions involve signs and symptoms at or contiguous with the sting site.

Wheal and flare – Nearly all individuals stung by IFAs develop hive-like lesions with surrounding erythema at the sting site. This reaction develops within 20 minutes and is accompanied by painful burning and then pruritus (picture 1). The burning sensation is characteristic and helps differentiate the stings of IFA from those of other insects. Symptoms are more pronounced during the summer months when the ants have more venom. The initial symptoms are followed in the succeeding 24 hours by development of the sterile pustule.

Sterile pustule – A pustule filled with yellow fluid containing epithelial debris and few cells develops at the site of a sting within the first 24 hours. These pustules are sterile and caused by the venom alkaloids. The presence of these pustules with an appropriate history of a painful sting is relatively pathognomonic for the sting of the IFA (picture 2 and picture 3 and picture 4 and picture 5). The pustule peaks in size at 24 hours and can last for one week or more. It is not necessary to culture these pustules unless the patient has scratched the site and denuded the pustules and there is concern about superinfection with skin flora. The epidermis overlying the pustule usually remains intact for 48 to 72 hours before it erodes, which generally allows sufficient time for new epidermis to cover the base of the lesion.

Large local reaction – In individuals with venom-specific immunoglobulin E (V-IgE), large local reactions may develop at the sting site over 6 to 24 hours, simultaneously with the development of the sterile pustule. The pustule ultimately sits atop an area of intensely pruritic, painful inflammation (picture 1). Although usually self-limited, swelling can be severe, and vascular compromise can occur in extremities. This topic will refer to severe large local reactions as "extreme local reactions." Extensive and spreading edema can be a source of confusion in extreme local reactions. However, by definition, the swelling is contiguous with the site of the sting.

Histopathologic studies have demonstrated that large local reactions are IgE-mediated, late cutaneous allergic reactions like those that can occur after stings of flying Hymenoptera. On histologic examination, the lesions are composed of a dense fibrin gel and a mixed cellular infiltrate of eosinophils, neutrophils, and lymphocytes [34].

Systemic allergic reactions — Systemic allergic reactions may range from cutaneous (generalized urticaria, angioedema, pruritus, erythema) to life-threatening bronchospasm, laryngeal edema, or vascular collapse (table 2). Anaphylaxis may occur within minutes or may rarely develop hours after a sting. A single sting is sufficient to cause anaphylaxis [35]. Pathognomonic pustules can later identify the fire ant as the culprit.

Reliable estimates for the incidence of fire ant anaphylaxis are not available, because anaphylaxis is not a reportable disease. Published studies have reported rates ranging from 0.6 to 16 percent of those stung [36-38].

IFA stings represent a leading cause of anaphylaxis for populations residing in endemic areas [39-41]. One blinded survey of random adult blood donors in an IFA-endemic area demonstrated allergen-specific IgE to IFA if the reference population was 1.7 times more common than IgE for other allergens associated with fatal allergic reactions [41].

Anaphylactic reactions to IFA stings usually occur in individuals who have been stung at least once before without eliciting anaphylaxis. However, some subjects have had anaphylactic reactions in response to their first stings [10]. Clinical and laboratory studies suggest that most of these subjects have been sensitized previously to Vespula (yellow jacket) venom [10]. Vespula venom cross-reacts with Solenopsis venom in vitro. There is also evidence that the venom of the common striped scorpion, Centruroides vittatus, may be cross-reactive with IFA venom and that this may be responsible for reported episodes of anaphylaxis upon an initial scorpion sting [42].

Toxic reactions — Conditions that have been associated with fire ant stings in humans include serum sickness, nephrotic syndrome, seizures, mononeuritis, and worsening of pre-existing cardiopulmonary disease [34,43-45]. The pathogenesis of these sequelae is unclear.

Massive attacks — Massive attacks by foraging fire ants have been increasingly reported in the medical literature and the lay press [46]. These attacks usually occur in the late summer months and frequently after it rains. Victims can be found in the early morning hours covered with ants. Massive attacks can occur indoors and generally affect debilitated or very young patients who cannot ward off the attack. Individuals can sustain hundreds to thousands of fire ant stings.

The reason for these massive attacks is unclear. Our group has hypothesized that IFA attacks on humans reflect normal foraging predatory behavior of the ants during periods of food scarcity, behavior that may reflect the increasing density of the ants in the environment. Examples of this behavior exist in the field. A Texas study reported 19 percent of cotton rats captured in traps in fire ant endemic areas were attacked by ants within a few hours [47]. Over one-half of the attacked rats were found dead or partially eaten.

It is unclear whether massive attacks can result in morbidity, secondary to relatively large doses of venom components. Such toxic reactions following 50 to 100 simultaneous stings by winged Hymenoptera species (bees, wasps, yellow jackets, and hornets) have been reported in humans, but equivalent reactions resulting from fire ant stings are not well-established. However, such reactions have been observed in small animals, such as dogs, rats, mice, and calves.

Variable clinical outcomes in humans occur after massive fire ant attacks. Some patients recover without sequelae [48,49]. In contrast, a five-day-old infant stung at home had a near-fatal response, and the death of a three-month-old infant subsequent to numerous fire ant stings in a home in Phoenix, Arizona was reported [50].

Several cases of massive fire ant attacks within health care facilities have been reported. Of the six patients included in one report, only one fared well [49]. That patient was in good general health except for Alzheimer disease and had no obvious untoward effects. In contrast, four of the six patients died within one week of the attack. Two had worsening of pre-existing cardiopulmonary insufficiency, one developed acute respiratory distress syndrome, and one had a cardiopulmonary arrest. The fifth patient, already bedbound with multiple health problems including severe heart failure, had progressive deterioration and died 13 months later.

INDICATIONS FOR REFERRAL — All patients with systemic allergic reactions should be referred to an allergy expert for further evaluation. Patients with extreme local reactions should also be referred to an allergy expert, since testing for venom-specific immunoglobulin E (V-IgE) may be indicated. (See 'Management of local reactions' below.)

DIAGNOSIS — The diagnosis of local reactions is based upon clinical signs and symptoms. The diagnosis of systemic allergic reactions requires specific testing for immunoglobulin E (IgE) antibodies specific to imported fire ant (IFA) venom.

Local reactions — The diagnosis of the various types of local reactions is clinical. The sterile pustule is relatively pathognomonic for the sting of the IFA (picture 1 and picture 2). (See 'Clinical reactions' above.)

Systemic allergic reactions: Testing for imported fire ant venom allergy — Patients who have experienced a systemic/anaphylactic reaction in the recent past should be examined for the development or presence of sterile pustules, as these lesions are an important clue that an imported fire ant (IFA) sting was responsible. On rare occasions, pustules are absent.

The diagnosis of IFA venom allergy requires the demonstration of venom-specific IgE (V-IgE) by skin testing or in vitro tests, in combination with a consistent clinical history. As with all forms of allergy, the presence of V-IgE alone does not predict systemic reactivity. For this reason, subjects without a history of systemic reactions to IFA stings should not be tested for V-IgE, as there is a high rate of asymptomatic production of V-IgE in populations exposed to fire ants. In studies of skin test reactivity to IFAs, positive skin tests were seen in up to 30 to 40 percent of history-negative control subjects [1,2,4,5]. Similarly, the level of V-IgE does not predict the severity of a sting reaction in those with systemic allergy, and patients with large local reactions may have equally high levels of V-IgE.

Skin testing is the preferred method of detecting V-IgE. Skin testing should only be performed by an allergy expert, both because of the skill required for interpretation and the potential risk of severe reactions in highly sensitive patients. It is general practice to allow at least six weeks to elapse between a reaction and IFA skin testing.

Laboratory studies — We suggest obtaining a baseline tryptase level to exclude underlying mast cell disorders in patients who have had hypotension with stings. Any elevation in baseline tryptase should be evaluated further. (See "Mastocytosis (cutaneous and systemic): Evaluation and diagnosis in adults".)

Skin testing — Whole body extracts (WBE) are commercially available for skin testing as well as for use in immunotherapy. To be useful, these preparations must contain adequate amounts of venom proteins, but this has not been consistently true in the United States in the past. United States manufacturers have since agreed to verify the content of Sol i 2 and Sol i 3 in their products. Despite these measures, we recommend confirming the venom content in extracts by skin testing an individual known to be skin test-positive [9]. We test all extracts on a volunteer with a history of large local reactions before using the extract in skin testing or immunotherapy.

WBE are available as 1:10 weight/volume stock solutions. We use WBE from the more widespread species, the red imported fire ant (RIFA), for testing and immunotherapy in all patients, as there is extensive cross-reactivity between the two species. We recommend that the 1:10 weight/volume stock solutions be diluted 10-fold (to 1:100 weight/volume) and used for epicutaneous (prick/puncture) testing. If the patient has had a severe anaphylactic reaction, further dilution for the first prick/puncture test may be prudent.

If prick testing is negative, intradermal testing is then performed. For intradermal testing, we start at 1:1,000,000 weight/volume and increase 10-fold until 1:500 to 1:1000 weight/volume is reached or a positive result is obtained [51,52]. One center reported that 9.5 percent of patients undergoing skin testing to IFA venom developed systemic allergic reactions, which is higher than the reported rates (2 to 4 percent) with other types of allergens, including other Hymenoptera venoms [35].

Skin testing with fire ant WBE is considered diagnostic for the presence of V-IgE with a positive reaction (ie, a wheal measuring at least 3 mm greater than the negative control) at a concentration of 1:500 weight/volume or lower [52]. A general discussion of the techniques of skin testing is presented separately. (See "Overview of skin testing for allergic disease".)

Skin testing is our preferred method of diagnosis. The sensitivity of testing with WBE preparations has been estimated to be between 76 and 94 percent, depending upon the concentration used [53,54]. Approximately 20 percent of patients with positive skin tests will have negative in vitro tests. Conversely, up to 20 percent of patients with negative skin tests can have positive in vitro tests, depending on the assays used [55,56].

Therefore, negative skin tests in patients with highly suggestive histories of systemic reactions should be followed by in vitro testing. If in vitro testing is negative in such patients, then we recommend repeat skin testing with another manufacturer's reagent.

In vitro tests — There are multiple in vitro tests available to detect V-IgE, including enzyme-linked immunosorbent assays (ELISA), radioallergosorbent tests (RAST), and Phadia CAP solid-phase assay). All of these tests use WBE as the allergen:



WBE Phadia ImmunoCAP solid-phase assay (flow enzyme immunoassay or CAP-FEIA)

Assays using venom are available only in research laboratories and may be more sensitive than those using WBE [37]. The sensitivities of the three commercially available assays appear to be quantitatively similar to those of WBE intradermal skin tests [57]. The authors use the WBE Phadia ImmunoCAP assay.

The absolute titer of a positive in vitro test does not predict the severity of the clinical reaction. Patients with severe anaphylaxis can have low-positive titers.

There are specific circumstances in which in vitro testing would be the initial modality of choice, such as in patients taking significant doses of beta-adrenergic blocking medications or with extensive skin disease. A discussion of situations in which in vitro testing would be preferred can be found elsewhere. (See "Overview of in vitro allergy tests", section on 'Indications'.)


Management of local reactions

Wheal and flare reactions and sterile pustules – No specific treatment is required for wheal and flare reactions and sterile pustules. Application of topical anti-itch preparations, such as pramoxine hydrochloride (1%) with hydrocortisone acetate (1%) cream, and/or a topical antihistamine (doxepin cream 5%) and/or oral antihistamine treatment, provide comfort until the itching stops. Itching can last for many hours. The lesions should be left intact as scratching can lead to infection [52]. Excoriated lesions should be kept clean with soap and water. Infected lesions require appropriate antimicrobial therapy.

Large local reactions – Large local reactions at the site of stings often require treatment. Pruritus can be treated with the topical agents listed above or, if refractory, by oral antihistamines and high potency topical corticosteroids. We prefer cetirizine and fluocinonide 0.05% or clobetasol 0.05% ointment. Topical corticosteroids are applied every four hours until the itching subsides.

In individuals with predictable large local reactions, we have found that immediate application of high potency topical corticosteroids and administration of a single dose of 20 mg of prednisone attenuates the response.

Patients who experience large local reactions uniformly have venom-specific immunoglobulin E (V-IgE); however, they are not believed to be at significantly higher risk for subsequent systemic reactions. Thus, testing for V-IgE and immunotherapy is not routinely offered.

Extreme local reactions – Extreme local reactions may merit skin testing and venom immunotherapy for the prevention of recurrent extreme local reactions. This approach is similar to that used for large local reactions to winged Hymenoptera stings [58]. (See "Hymenoptera venom immunotherapy: Efficacy, indications, and mechanism of action", section on 'Patients with large local reactions'.)

Potential complications – In some patients with large or extreme local reactions, there may be enough induration, pain, and heat at the site to suggest cellulitis. However, fire ant venom alkaloids have both antibacterial and antifungal properties, and cellulitis rarely occurs in nondiabetic patients. The presence of a sterile pustule is a clear indicator that these are late-phase reactions to an imported fire ant (IFA). However, if the clinician is unsure, a course of oral antibiotics may be appropriate. (See "Cellulitis and skin abscess in adults: Treatment".)

Management of allergic reactions — Treatment of an IFA-allergic patient involves acute management, education, and possible immunotherapy for prevention of recurrent systemic reactions. Patients should also obtain a medical identification bracelet or necklace stating that they have a life-threatening allergy to IFA.

Emergency treatment — Acute management of fire ant-induced anaphylaxis is identical to the treatment of anaphylaxis from other causes, with epinephrine being the drug of choice. (See "Anaphylaxis: Emergency treatment", section on 'Immediate management'.)

Epinephrine autoinjectors — All patients with IFA venom allergy should be equipped with an epinephrine autoinjector and advised to have it immediately available at all times. Epinephrine must be accompanied by clear instructions for proper use of the device as well as instructions about which symptoms should prompt its use. Epinephrine should be injected intramuscularly in the thigh, preferably with the patient supine. It should be administered without delay for the following symptoms:

Difficulty breathing/wheezing

Throat tightness/hoarse voice

Lightheadedness/altered mental status

Widespread hives

Information about prescribing epinephrine autoinjectors, as well as patient information about the optimal use of these devices, is provided separately. (See "Prescribing epinephrine for anaphylaxis self-treatment" and "Patient education: Use of an epinephrine autoinjector (Beyond the Basics)".)

Venom immunotherapy — Immunotherapy is recommended for the treatment of IFA allergy.

Efficacy — The efficacy in preventing recurrent systemic reactions cannot be defined, because the natural history of IFA allergy has not been adequately studied. Specifically, it is not known with what frequency an IFA-allergic individual will exhibit systemic symptoms upon subsequent stings [59].

The best evidence for the efficacy of IFA immunotherapy is provided by unblinded, nonrandomized studies.

In one study of 65 subjects with previous anaphylaxis who were treated with immunotherapy, 47 (72 percent) unexpectedly sustained IFA stings and only 1 had anaphylaxis [60]. In addition, 31 subjects underwent fire ant stings under direct observation, and none had anaphylactic reactions while receiving maintenance immunotherapy. In contrast, there were 11 fire ant-allergic subjects in the control group who declined immunotherapy, of whom 6 sustained unexpected field stings. All 6 had anaphylactic reactions.

Three reports of successful rush fire ant immunotherapy protocol (an accelerated form of immunotherapy) have been published [61-63]. In a two-day program with 58 adult subjects, 3 (5.2 percent) experienced systemic reactions during the protocol, which were reported to be "mild" (table 3) [62]. This approach is typically more costly because it must be performed under continuous observation, but it can be useful if protection is required quickly, such as in military personnel. Sting challenges with IFA were performed 20 days later on 56 of the subjects with more than 112 ant stings among the group. One mild systemic reaction occurred, and the clinical efficacy of this rush protocol was reported to be 98 percent.

Another report described a one-day rush protocol that was successfully administered to three children under three years of age [64]. No systemic reactions occurred during the procedure.

A third study observed a trend toward reduction of systemic reactions during a one-day rush protocol that included premedication with antihistamines and prednisone (9.5 percent in the premedicated group versus 24.3 percent in the nonpremedicated group) [63].

Indications — Immunotherapy to prevent recurrent IFA-induced systemic reactions for:

Adults who have experienced a systemic reaction to IFA stings.

Children who have had systemic reactions that extend beyond the skin.

The value of immunotherapy in children with systemic reactions limited to the skin (ie, generalized urticaria, angioedema, pruritus, erythema, but without symptoms in other organ systems) in response to fire ant stings has not been well-studied. With flying Hymenoptera hypersensitivity (bees, wasps, yellow jackets, and hornets), a very low risk for future systemic reactions is observed in pediatric patients younger than 17 years who experience systemic cutaneous reactions. Such patients are not routinely treated with immunotherapy. However, it is not clear that children with these reactions in response to fire ant stings are similarly at low risk for anaphylaxis. Most allergists do not routinely recommend immunotherapy for such children in the absence of respiratory or vasomotor symptoms. This approach is supported by a retrospective analysis of fire ant stings in children under 16 years [65]. Nevertheless, approximately one-third of responding clinicians in an endemic area did recommend immunotherapy because of the greater risk of re-stings compared with nonendemic areas [5,66]. In the authors' clinic, we offer immunotherapy for children with respiratory or vascular features but not for children with systemic cutaneous reactions, with the acknowledgment that there are inadequate data in these areas.

Protocols — Whole body extracts (WBE) of IFA contain adequate amounts of venom and are used in immunotherapy, in contrast to whole body preparations of flying Hymenoptera. "Whole body vaccine" (WBV) is a term used for the solution used in immunotherapy that is prepared from WBE.

Pure fire ant venom is unavailable commercially and has not been studied for immunotherapy. We recommend confirming the venom content of each lot of WBE prior to use. (See 'Skin testing' above.)

Conventional immunotherapy protocols with fire ant WBV consist of weekly subcutaneous injections of gradually increasing doses of WBV. The stock is available as 1:10 weight/volume extract. We start with 0.05 mL of a 1:100,000 weight/volume solution, depending on the severity of the reaction and the degree of skin test positivity. We proceed stepwise to 0.5 mL before moving to the next 10-fold more concentrated dilution (table 4). Our desired maintenance dose is 0.5 mL of a 1:100 weight/volume extract, given monthly. This dose contains the same amount of venom as approximately 19 fire ant stings [67]. We continue maintenance injections for three to five years. The optimal maintenance dose has not been established. One large survey of 1091 prescriptions for fire ant immunotherapy submitted by over 300 practices serving personnel in the United States military found that the most commonly prescribed maintenance dose was 0.5 mL of a 1:200 weight/volume extract [68].

We do not mix WBV with other extracts in immunotherapy. The compatibility of WBV with other allergen extracts has not been extensively investigated, but one study demonstrated that WBV degraded the proteins in timothy grass extract [69]. There was no apparent degradation of proteins in extracts of cat, ragweed, or dust mite.

Safety — The safety of conventional immunotherapy for fire ant allergy was reviewed in a retrospective series of 77 patients who had received 1887 injections. Systemic allergic reactions to the immunotherapy itself occurred in 9 percent of patients and were mild [35]. Risk factors for a systemic reaction to immunotherapy included large local reactions or a history of systemic allergic reactions during diagnostic skin testing.

Rush protocols — A one-day rush protocol consisting of 10 injections has been described, which was successfully completed in 28 of 37 (76 percent) patients [70]. Those who developed allergic symptoms during the protocol were transitioned to a conventional immunotherapy protocol. No premedications were administered prior to the injections. Of the 9 patients who developed reactions, 7 had generalized pruritus or urticaria, and 2 had systemic reactions.

Duration of immunotherapy — We offer IFA immunotherapy for three to five years. However, if a patient has been receiving immunotherapy and is stung in the field without developing symptoms, we would allow the patient to stop after three years. There are no controlled trials evaluating the optimal length of treatment with fire ant WBE.

We do not repeat skin testing in preparation of discontinuing treatment. If the patient is reluctant to stop immunotherapy after an appropriate interval, we offer a sting challenge under controlled circumstances in the hospital. Patients should continue to carry epinephrine autoinjectors if they have histories of anaphylaxis.

Immunotherapy failure — The most common reason for immunotherapy "failure" is patient noncompliance. Even patients who have experienced anaphylaxis need attention, encouragement, and follow-up to assure compliance [71].

In our experience, the most frequent technical reason for recurrent systemic reactions to fire ants stings, despite having received immunotherapy with IFA WBE, is use of an extract containing little or no IFA venom. This is a problem unique to fire ant immunotherapy, as fire ant extracts are the only WBE in clinical use, and the venom content of these preparations is variable. For this reason, we test each lot of IFA WBE by using it to skin test an individual with known reactivity to IFA, before the extract is used for skin testing and immunotherapy in other patients. (See 'Skin testing' above.)

The possibility that the IFA WBE used for immunotherapy contains minimal fire ant venom can be investigated by skin testing the patient with the extract in question at the concentration used for routine skin testing. If the patient has a negative result, then the problem is revealed. Under these circumstances, we recommend that IFA immunotherapy be reinitiated with an active extract. We prefer administering a new extract composed of two different lots of extracts known to induce positive skin test reactions in such a patient.

If the above approach fails, increasing the volume of the maintenance extract using two separate injections up to 0.5 mL of the maintenance concentration in two separate sites may be of benefit.

In patients with systemic reactions to IFA immunotherapy, in whom higher maintenance doses might pose a risk, omalizumab has been reported to be helpful [72,73]. These practices are based on clinical experience. There are no published data on the optimal management of such patients.

Massive attacks — If any fire ants are found on a debilitated patient, clinical evaluation is needed, with possible transport (depending upon findings) to the nearest emergency department. We have made recommendations on how to proceed with patient care and facility management if this occurs [46].

There is no evidence that individuals who experience massive fire ant stings, in the absence of systemic allergic reactions or generalized late-phase dermal reactions, require parenteral glucocorticoid or antibiotic therapy. Treatment with large doses of glucocorticoids and intravenous fluids may complicate the management of those with pre-existing cardiovascular disease.

Treatment with oral nonsedating antihistamines and topical corticosteroids appears to be adequate in most cases. We use the medications mentioned previously for either uncomplicated local or large local reactions, depending on the cutaneous response.

Measures to prevent indoor infestation by IFAs are discussed separately. (See "Entomology and control of imported fire ants".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Stinging insect allergy".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topic (see "Patient education: Insect allergy (The Basics)")

Beyond the Basics topic (see "Patient education: Imported fire ants (Beyond the Basics)")


Imported fire ants (IFAs) are aggressive, venomous ants that sting with little provocation and are difficult to avoid in endemic areas (throughout the southern United States). (See 'Territory' above.)

Fire ant stings produce an immediate, intense burning (the "fire" inspiring the name) and itching at the sting site. Stings occur most frequently during summer, most commonly in children, and typically on the lower extremities. (See 'Clinical reactions' above.)

Most people develop only local reactions, which may take several forms, including wheal and flare reactions, sterile pustules, and large local reactions. Local reactions are contiguous with the sting site. (See 'Local reactions' above.)

Treatment of local reactions is symptomatic and not always necessary (see 'Management of local reactions' above):

For patients with uncomfortable wheal and flare reactions, we suggest the application of low-potency topical corticosteroids and/or oral antihistamines (Grade 2C).

Large local reactions often require treatment with topical agents and oral antihistamines. High-potency topical corticosteroids may be required. For severe swelling, we suggest administration of oral prednisone (60 mg given as a single dose) and elevation of the affected extremity (Grade 2C).

Some individuals develop systemic allergic reactions, which can range from cutaneous symptoms (generalized urticaria, angioedema, pruritus, erythema) to life-threatening anaphylaxis. Anaphylaxis may occur within minutes after a sting or on rare occasions can develop hours after. The pathognomonic sterile pustules can later identify the fire ant as the culprit. (See 'Systemic allergic reactions' above.)

Patients with systemic allergic reactions must be treated emergently for anaphylaxis and equipped with an epinephrine autoinjector for self-treatment of possible future reactions. (See "Anaphylaxis: Emergency treatment".)

Patients with allergic reactions should be referred to an allergy expert. The diagnosis of IFA venom allergy requires the demonstration of venom-specific immunoglobulin E (V-IgE) by skin testing or in vitro tests. Skin testing is the preferred method, but in vitro testing is available if the skin is compromised or if there is a medical contraindication to skin testing. (See 'Diagnosis' above.)

In patients with confirmed IFA venom allergy, we recommend three to five years of venom immunotherapy with commercially available whole body IFA vaccines (Grade 1B). Various protocols have been established. (See 'Treatment' above.)

Massive attacks, in which individuals sustain hundreds or thousands of stings, have been reported. Multiple stings can result in a variety of toxic reactions, such as serum sickness, nephrotic syndrome, seizures, mononeuritis, and worsening of pre-existing cardiopulmonary disease. Treatment should be individualized to the patient's symptoms, and we suggest against empirically administering large doses of glucocorticoids and intravenous fluids, particularly in debilitated patients and those with pre-existing cardiovascular disease (Grade 2C). For most patients with multiple stings, the interventions suggested for local reactions are sufficient. (See 'Toxic reactions' above.)

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  1. Lofgren CS, Banks WA, Glancey BM. Biology and control of imported fire ants. Annu Rev Entomol 1975; 20:1.
  2. 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.
  3. Sanchez-Pena SR, Patrock RJW, Gilbert LA. The red imported fire ant is now in Mexico: documentation of its wide distribution along the Texas-Mexico border. Entomol News 2005; 116:363.
  4. McCubbin KI, Weiner JM. Fire ants in Australia: a new medical and ecological hazard. Med J Aust 2002; 176:518.
  5. Tracy JM, Demain JG, Quinn JM, et al. The natural history of exposure to the imported fire ant (Solenopsis invicta). J Allergy Clin Immunol 1995; 95:824.
  6. Haight KL, Tschinkel WR. Patterns of venom synthesis and use in the fire ant, Solenopsis invicta. Toxicon 2003; 42:673.
  7. Hannan CJ Jr, Stafford CT, Rhoades RB, et al. Seasonal variation in antigens of the imported fire ant Solenopsis invicta. J Allergy Clin Immunol 1986; 78:331.
  8. Goddard J. Personal protection measures against fire ant attacks. Ann Allergy Asthma Immunol 2005; 95:344.
  9. Baer H, Liu TY, Anderson MC, et al. Protein components of fire ant venom (Solenopsis invicta). Toxicon 1979; 17:397.
  10. Hoffman DR. Fire ant venom allergy. Allergy 1995; 50:535.
  11. Hoffman DR. Allergens in Hymenoptera venom XXIV: the amino acid sequences of imported fire ant venom allergens Sol i II, Sol i III, and Sol i IV. J Allergy Clin Immunol 1993; 91:71.
  12. Hoffman DR. Allergens in hymenoptera venom. XXVI: The complete amino acid sequences of two vespid venom phospholipases. Int Arch Allergy Immunol 1994; 104:184.
  13. Hoffman DR, Dove DE, Moffitt JE, Stafford CT. Allergens in Hymenoptera venom. XXI. Cross-reactivity and multiple reactivity between fire ant venom and bee and wasp venoms. J Allergy Clin Immunol 1988; 82:828.
  14. Schmidt M, Walker RB, Hoffman DR, McConnell TJ. Nucleotide sequence of cDNA encoding the fire ant venom protein Sol i II. FEBS Lett 1993; 319:138.
  15. Blum MS, Roberts JE, Novak AF. Chemical and biological characterization of venom of the ant Solenopsis xyloni McCook. Psyche, Camb 1961; 68:73.
  16. Brand JM, Blum MS, Fales HM, MacConnell JG. Fire ant venoms: comparative analyses of alkaloidal components. Toxicon 1972; 10:259.
  17. Deslippe RJ, Guo YJ. Venom alkaloids of fire ants in relation to worker size and age. Toxicon 2000; 38:223.
  18. MacConnell JG, Blum MS, Fales HM. The chemistry of fire ant venom. Tetrahedron 1971; 26:1129.
  19. Leclerq S, Braekman JC, Daloze D, et al. Biosynthesis of the solenopsins, venom alkaloids of the fire ant. Naturwissenschaften 1996; 83:222.
  20. ADROUNY GA, DERBES VJ, JUNG RC. Isolation of a hemolytic component of fire ant venom. Science 1959; 130:449.
  21. Koch RB, Desaiah D, Foster D, Ahmed K. Effect of piperidines and fire ant venom on ATPase activities from brain homogenate fractions and characterization of Na+-K+ ATPase inhibition. Biochem Pharmacol 1977; 26:983.
  22. Koch RB, Desaiah D. Sensitivity of ATPase activities to fire ant venom and abdomen preparations. Life Sci 1975; 17:1315.
  23. Cheng EY, Cutkomp LK, Koch RB. Effect of an imported fire ant venom component on respiration and oxidative phosphorylation of mitochondria. Biochem Pharmacol 1977; 26:1179.
  24. Jouvenaz DP, Blum MS, MacCONNELL JG. Antibacterial activity of venom alkaloids from the imported fire ant, Solenopsis invicta Buren. Antimicrob Agents Chemother 1972; 2:291.
  25. Rhoades RB. Medical aspects of the imported fire ant, The University Presses of Florida, Gainesville, FL 1977.
  26. Vander Meer RK. Behavioral and biochemical variation in the fire ant, Solenopsis invicta. In: Interindividual behavioral variability in social insects, Jeanne RL (Ed), Westview Press, Boulder 1988.
  27. Javors MA, Zhou W, Maas JW Jr, et al. Effects of fire ant venom alkaloids on platelet and neutrophil function. Life Sci 1993; 53:1105.
  28. Read GW, Lind NK, Oda CS. Histamine release by fire ant (Solenopsis) venom. Toxicon 1978; 16:361.
  29. Lind NK. Mechanism of action of fire ant (Solenopsis) venoms. I. Lytic release of histamine from mast cells. Toxicon 1982; 20:831.
  30. Yeh JZ, Narahashi T, Almon RR. Characterization of neuromuscular blocking action of piperidine derivatives. J Pharmacol Exp Ther 1975; 194:373.
  31. Yi GB, McClendon D, Desaiah D, et al. Fire ant venom alkaloid, isosolenopsin A, a potent and selective inhibitor of neuronal nitric oxide synthase. Int J Toxicol 2003; 22:81.
  32. Howell G, Butler J, Deshazo RD, et al. Cardiodepressant and neurologic actions of Solenopsis invicta (imported fire ant) venom alkaloids. Ann Allergy Asthma Immunol 2005; 94:380.
  33. Rhoades RB, Schafer WL, Newman M, et al. Hypersensitivity to the imported fire ant in Florida. Report of 104 cases. J Fla Med Assoc 1977; 64:247.
  34. deShazo RD, Griffing C, Kwan TH, et al. Dermal hypersensitivity reactions to imported fire ants. J Allergy Clin Immunol 1984; 74:841.
  35. La Shell MS, Calabria CW, Quinn JM. Imported fire ant field reaction and immunotherapy safety characteristics: the IFACS study. J Allergy Clin Immunol 2010; 125:1294.
  36. Adams CT, Lofgren CS. Red imported fire ants (Hymenoptera: Formicidae): frequency of sting attacks on residents of Sumter County, Georgia. J Med Entomol 1981; 18:378.
  37. Stafford CT. Hypersensitivity to fire ant venom. Ann Allergy Asthma Immunol 1996; 77:87.
  38. Caldwell ST, Schuman SH, Simpson WM Jr. Fire ants: a continuing community health threat in South Carolina. J S C Med Assoc 1999; 95:231.
  39. Rhoades RB, Stafford CT, James FK Jr. Survey of fatal anaphylactic reactions to imported fire ant stings. Report of the Fire Ant Subcommittee of the American Academy of Allergy and Immunology. J Allergy Clin Immunol 1989; 84:159.
  40. Prahlow JA, Barnard JJ. Fatal anaphylaxis due to fire ant stings. Am J Forensic Med Pathol 1998; 19:137.
  41. Caplan EL, Ford JL, Young PF, Ownby DR. Fire ants represent an important risk for anaphylaxis among residents of an endemic region. J Allergy Clin Immunol 2003; 111:1274.
  42. Nugent JS, More DR, Hagan LL, et al. Cross-reactivity between allergens in the venom of the common striped scorpion and the imported fire ant. J Allergy Clin Immunol 2004; 114:383.
  43. deShazo RD, Williams DF, Moak ES. Fire ant attacks on residents in health care facilities: a report of two cases. Ann Intern Med 1999; 131:424.
  44. Swanson GP, Leveque JA. Nephrotic syndrome associated with ant bite. Tex Med 1990; 86:39.
  45. Fox RW, Lockey RF, Bukantz SC. Neurologic sequelae following the imported fire ant sting. J Allergy Clin Immunol 1982; 70:120.
  46. Rupp MR, deShazo RD. Indoor fire ant sting attacks: a risk for frail elders. Am J Med Sci 2006; 331:134.
  47. Flickinger EL. Observation of predation by red imported fire ants on live-trapped wild cotton rats. Texas J Sci 1989; 41:223.
  48. Diaz JD, Lockey RF, Stablein JJ, Mines HK. Multiple stings by imported fire ants (Solenopsis invicta), without systemic effects. South Med J 1989; 82:775.
  49. deShazo RD, Kemp SF, deShazo MD, Goddard J. Fire ant attacks on patients in nursing homes: an increasing problem. Am J Med 2004; 116:843.
  50. More DR, Kohlmeier RE, Hoffman DR. Fatal anaphylaxis to indoor native fire ant stings in an infant. Am J Forensic Med Pathol 2008; 29:62.
  51. Triplett RF. Sensitivity to the imported fire ant: successful treatment with immunotherapy. South Med J 1973; 66:477.
  52. deShazo RD, Butcher BT, Banks WA. Reactions to the stings of the imported fire ant. N Engl J Med 1990; 323:462.
  53. Stafford CT, Wise SL, Robinson DA, et al. Safety and efficacy of fire ant venom in the diagnosis of fire ant allergy. J Allergy Clin Immunol 1992; 90:653.
  54. Bahna SL, Strimas JH, Reed MA, Butcher BT. Imported fire ant allergy in young children: skin reactivity and serum IgE antibodies to venom and whole body extract. J Allergy Clin Immunol 1988; 82:419.
  55. Golden DB, Kagey-Sobotka A, Norman PS, et al. Insect sting allergy with negative venom skin test responses. J Allergy Clin Immunol 2001; 107:897.
  56. Reisman RE. Insect sting allergy: the dilemma of the negative skin test reactor. J Allergy Clin Immunol 2001; 107:781.
  57. Ford JL, Dolen WK, Feger TA, et al. Evaluation of an in vitro assay for fire ant venom-specific IgE. J Allergy Clin Immunol 1997; 100:425.
  58. Golden DB, Moffitt J, Nicklas RA, et al. Stinging insect hypersensitivity: a practice parameter update 2011. J Allergy Clin Immunol 2011; 127:852.
  59. Brown SG, Heddle RJ, Wiese MD, Blackman KE. Efficacy of ant venom immunotherapy and whole body extracts. J Allergy Clin Immunol 2005; 116:464.
  60. Freeman TM, Hylander R, Ortiz A, Martin ME. Imported fire ant immunotherapy: effectiveness of whole body extracts. J Allergy Clin Immunol 1992; 90:210.
  61. Duplantier JE, Freeman TM, Bahna SL, et al. Successful rush immunotherapy for anaphylaxis to imported fire ants. J Allergy Clin Immunol 1998; 101:855.
  62. Tankersley MS, Walker RL, Butler WK, et al. Safety and efficacy of an imported fire ant rush immunotherapy protocol with and without prophylactic treatment. J Allergy Clin Immunol 2002; 109:556.
  63. Arseneau AM, Nesselroad TD, Dietrich JJ, et al. A 1-day imported fire ant rush immunotherapy schedule with and without premedication. Ann Allergy Asthma Immunol 2013; 111:562.
  64. Judd CA, Parker AL, Meier EA, Tankersley MS. Successful administration of a 1-day imported fire ant rush immunotherapy protocol. Ann Allergy Asthma Immunol 2008; 101:311.
  65. Nguyen SA, Napoli DC. Natural history of large local and generalized cutaneous reactions to imported fire ant stings in children. Ann Allergy Asthma Immunol 2005; 94:387.
  66. Moffitt JE, Barker JR, Stafford CT. Management of imported fire ant allergy: results of a survey. Ann Allergy Asthma Immunol 1997; 79:125.
  67. Hoffman DR, Jacobson RS, Schmidt M, Smith AM. Allergens in Hymenoptera venoms. XXIII. Venom content of imported fire ant whole body extracts. Ann Allergy 1991; 66:29.
  68. Haymore BR, McCoy RL, Nelson MR. Imported fire ant immunotherapy prescribing patterns in a large health care system during a 17-year period. Ann Allergy Asthma Immunol 2009; 102:422.
  69. Rans TS, Hrabak TM, Whisman BA, et al. Compatibility of imported fire ant whole body extract with cat, ragweed, Dermatophagoides pteronyssinus, and timothy grass allergens. Ann Allergy Asthma Immunol 2009; 102:57.
  70. Dietrich JJ, Moore LM, Nguyen S, et al. Imported fire ant hypersensitivity: a 1-day rush immunotherapy schedule without premedication. Ann Allergy Asthma Immunol 2009; 103:535.
  71. Stokes SC, Quinn JM, Sacha JJ, White KM. Adherence to imported fire ant subcutaneous immunotherapy. Ann Allergy Asthma Immunol 2013; 110:165.
  72. Tartibi HM, Majmundar AR, Khan DA. Successful use of omalizumab for prevention of fire ant anaphylaxis. J Allergy Clin Immunol 2010; 126:664.
  73. Tille KS, Parker AL. Imported fire ant rush desensitization using omalizumab and a premedication regimen. Ann Allergy Asthma Immunol 2014; 113:574.
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