i70 Fire ant

Solenopsis invicta, the red imported fire ant, is one of the most notorious invasive ant species worldwide with a rapidly expanding geographical range, and occurs in high densities in urban, agricultural, and non-agricultural areas. Aggressive foragers, worker fire ants will rapidly swarm and repeatedly sting any human or animal with which they come into contact. In humans, injection of the venom results in the formation of a white fluid-filled pustule or blister at the site of the sting approximately one day after the attack, a pathognomonic symptom characteristic only of fire ants. Protein allergens within the venom can elicit reactions ranging from an immediate localized burning sensation with dermal wheal and flare responses, to painful large local reactions characterized by pruritus, erythema, and edema lasting 24‒72 hours. Serious systemic allergic reactions resulting in life-threatening anaphylaxis have been reported for between 0.6% to 16% of individuals stung by fire ants, resulting in at least 80 fatalities, most of which were caused by fewer than five stings. Four allergenic molecules have been characterized to date from S. invicta venom, with variable degrees of cross-reactivity to homologs in other Solenopsis species as well as with venom homologs from other Hymenoptera, insect, and arachnid species.

Living environment

S. invicta are omnivorous social insects occurring in high densities in urban, agricultural, and non-agricultural areas (4, 6, 9). Worker ants build a nest, care for the queen and brood (eggs, larvae, and pupae) ants, defend the colony, and forage outside the nest at soil surface temperatures between 12‒51°C (4). Foraging activity is temperature-dependent, with a drop in activity during cooler months and in shadier habitats (4).

Smaller nests may not be visible if they are constructed in soil or under objects such as timber or rocks, whereas dome-shaped mounds of soil up to 50 cm in diameter can be constructed in sunny or open areas with no cover such as lawns, gardens, parks, roadsides, schoolyards, and golf courses (4, 5, 9). A fully developed colony can contain between 200,000 to 400,000 individual ants (4). S. invicta breeds and spreads rapidly and can relocate a colony quickly if a nest is disturbed (4).

Of concern in terms of contact with humans, entire colonies of S. invicta can move into buildings or vehicles to seek favorable nesting sites, particularly during flooding or very hot dry conditions (4-6, 9). Additionally, S. invicta is attracted to electrical fields (11) and to pheromones released when other worker ants are electrocuted, and may construct nests in cable boxes, transformers, and other warm sheltered electrical equipment, which can cause mechanical failure and extensive damage to residential and commercial property (4, 6, 9).

Worldwide distribution 

S. invicta is native to central South America and, following accidental introduction, has become widely established across southern North America, the Caribbean, Asian mainland and island nations, Australia, and New Zealand (4, 5, 7). With a high potential for rapid colonization in a wide range of habitats (6, 12), areas considered at risk of invasion by S. invicta include northern South America and Central America, much of the region immediately surrounding the Mediterranean Sea, areas near the Black and Caspian Seas, and vulnerable island ecosystems worldwide (4, 12).

Key factors limiting the geographical range of S. invicta include low temperature and low levels of precipitation, however, reports of successful interbreeding between S. invicta and other ant species suggest distribution could expand if new hybrid ants display cold and/or drought resistance (4, 8, 11).

The lower body is the most frequent site of a stinging attack by S. invicta (7, 13). Infants, the elderly, and individuals with limited mobility may be at particular risk of being stung by numerous fire ants if they cannot move away from the threat (4, 9, 12, 14, 15).

One sting from S. invicta delivers between 0.04‒0.11 µl of venom and 10‒100 ng of protein (9, 11), however, in practice multiple stings are often received as worker ants swarm together in an attack (12). The venom of S. invicta is composed of 95% water-insoluble cytotoxic alkaloids and 5% aqueous proteins (7, 11). In humans, injection of the alkaloid component results in the formation of a white fluid-filled pustule or blister at the site of the sting approximately one day after the attack, a pathognomonic symptom characteristic only of fire ants (4, 11). Protein allergens can elicit reactions ranging from an immediate localized burning sensation with dermal wheal and flare responses, to painful large local reactions characterized by pruritus, erythema, and edema lasting 24‒72 hours (6, 7, 9).

Anecdotal reports suggest some individuals may be stung hundreds of times without developing systemic symptoms (11, 16), however, for hypersensitized individuals, a single sting can elicit anaphylaxis with flushing, generalized urticaria, angioedema, laryngeal edema, bronchospasm, or loss of consciousness (6, 7). Serious systemic allergic reactions resulting in life-threatening anaphylaxis have been reported for between 0.6% to 16% of individuals stung by fire ants, resulting in at least 80 fatalities (9, 11, 13-15). A survey of 29,300 physicians in the United States reported 83 fatal and two near-fatal fire ant sting reactions, with most deaths caused by fewer than five stings (13).

Stings can also trigger serum sickness, nephrotic syndrome, seizures, and worsening of pre-existing cardiopulmonary disease (7, 9, 11, 17, 18). Allergic reactions to fire ant stings have been reported by previously sensitized individuals as well as by non-fire ant-sensitized subjects who have documented sensitivity to yellow jacket (Vespula spp.) venom (11).

In the south-eastern USA, over 40 million people live in areas infested by S. invicta and an estimated 14 million people (35%) are stung every year (4). In 1998 in the state of South Carolina alone, one survey of 1,286 medical practitioners estimated that 600,000 cases of imported fire ant attack resulted in 33,000 people seeking medical treatment (94 people per 19,000 population), of whom 660 (1.9 per 10,000 population) were treated for anaphylaxis (19). In Taiwan, a recent observational study of 3,819 people stung by S. invicta reported that 21.8% (n=834) developed wheal and flare reactions, 7.5% (n=288) sought medical care, 2.78% (n=106) exhibited anaphylactic shock, and 0.55% (n=21) developed severe cellulitis and urticaria (6).

Author: RubyDuke Communications

Reviewer: Dr. Christian  Fischer 

Last reviewed:April 2022