The toxicology of fire ant venom is relatively well studied. The venom plays a central role in the biology of Red imported fire ants, such as in capturing prey, and in defending itself from competitors, assailants, and diseases. Some 14 million people are stung annually in the United States, suffering reactions that vary from mild discomfort, to pustule formation, swelling, and in rare cases, systemic reactions followed by anaphylactic shock. Fire ant venoms are mainly composed (>95%) of a complex mixture of insoluble alkaloids added to a watery solution of toxic proteins. For the Red imported fire ant Solenopsis invicta Buren there are currently 46 described proteins, of which four are well-characterised as potent allergens.
Venom
Venom plays an important role in the biology of fire ants, being used to capture prey items, nest defense, and antimicrobial action. On average, however, a worker stores very little venom (only about 0.5 μg at any given time). Newborn workers contain little to no venom within their reservoirs, but workers that are only one day old can produce 1.17 μg/day. However, workers that are 17 days old only produce 0.3 μg/day. Workers deliver 0.66 nl of venom when they sting, which amounts to 3.1% of their supply. Older workers deliver less venom when they sting, but middle-aged workers and nest-defenders deliver much higher quantities. Like all fire ant species, venom is secreted by the venom gland and is stored in the poison sac. When in use, it is ejected through the stinger's main duct. Capacity is between 20 and 40 nl, but this depends on the worker's size. The American entomologist Justin O. Schmidt described it as being "sharp, sudden, mildly alarming", therefore ranking at "1" in the Schmidt sting pain index, a pain scale which ranks the pain intensity of an insect's sting from 0 to 4. Over 95% of the venom components are water-insoluble piperidine alkaloids. Piperidines include trans-2-methyl-6-n-undecylpiperidines, trans-2-methyl-6-n-tridecylpiperidine, trans-2-methyl-6-(cis-4-tridecenyl) piperidines, trans-2-methyl-6-n-pentadecylpiperidine, trans-2-methyl-6-(cis-6-pentadecenyl)piperidine and 2,6-dialkylpiperidines (the ants' venom is dominated by the trans- stereoisomers of this specific ingredient). trans-2-Methyl-6-n-undecylpiperidine (solenopsin) has been shown to have cytotoxic, hemolytic, necrotic, insecticidal, antibacterial, antifungal, and anti-HIV properties. As well as that, the alkaloid has shown antiangiogenic activity. These components are responsible for the formation of hives, and also for the development of sterile pustules on areas where the ant has stung. Experiments indicate that the median lethal dose (LD50) on tested female rats is 0.36 mg/kg. Approximately 46 proteins have been identified in the red imported fire ant's venom, although scientists have long believed the venom only contained alkaloids. This assumption was mostly due to the difficulties in obtaining sufficient venom for analysis because of its low protein content, which is only 0.1% of the venom's total weight. These proteins are experimentally suggested to directly account for the anaphylactic reactions seen in humans sensitive to the venom. Whilst including a number of neurotoxins and potential allergens, not all of these proteins are involved with venom function. At least four protein allergens have been characterised, named Sol i 1–4. Of these, Sol i 3, is part of the antigen 5 family, and Sol i 1 is a phospholipase A1B; Sol i 1 shows a close relation with wasp venom phospholipases. Sol i 2 and 4 are unique, odorant-binding proteins of poorly understood function. Other proteins found in the venom may benefit the colony; some of these proteins can kill off bacteria, which may explain why workers spray venom around their nests by vibrating their gasters. Other proteins also bind pheromones which may assist a worker to lay chemical trails to communicate with other nestmates.
Incidence In the United States, more than 40 million people live in areas infested with fire ant populations and 14 million people are stung by them annually. A quarter of all victims stung by red imported fire ants are expected to develop sensitivity to the venom, and approximately 6,000 will suffer anaphylaxis. 51% of people who relocated themselves to infested areas report getting stung within three weeks after arrival. In a survey conducted in South Carolina, 33,000 people (or 94 per 10,000 population) received medical attention due to red imported fire ants, and 660 people (1.9 per 10 000 population) were treated for anaphylaxis. In Texas, 79% of participants in a survey stated they had been stung by red imported fire ants, while 20% had not. 61% of West Texans state they had been stung by the ants before, compared to 90% in central Texas, 89% in east Texas, 86% in the gulf coastal regions, 78% in the south and 72% in the north. In separate survey, 87% of individuals classed their reactions as mild, 12% as moderate and 1% as severe. In Australia, 64,000 homes are within red imported fire ant infested areas, and 140,000 consultations and 3,000 anaphylactic reaction cases are predicted every year by 2030 if government efforts to eradicate the ant fail. A survey conducted in China shows that one-third of participants in infested areas were victims of red imported fire ant stings. Studies suggest that the rate of systemic reactions to stings may be associated with seasonal variations in the venom's potency. 51% of allergic reactions occurred in summer, and 19% in spring. However, A survey reported a higher incidence during spring (39.9%) than summer (31.9%). Younger people, usually those under 20 years, experience the highest rate of sting attacks (50%), but the rate declines with older people. Among men and women, the rate varies as some studies report more women being attacked than men and vice versa. Deaths from red imported fire ant stings are rare, but may become common the more the ant spreads. Many cases have also been reported in the past. It is reported that more than 80 deaths have been recorded; of these, 22 cases were recorded in Florida and 19 in Texas. However, when duplicate reports are excluded, four deaths were recorded in Alabama, 10 in Florida, two in Georgia and Louisiana, and 14 in Texas. People can be educated and be made aware of the dangers of red imported fire ants.
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