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).
