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Fire Ant Envenomations

A review of the diagnosis and treatment of envenomations of fire ants.

Fire ants floating in flood water

Introduction

This article will review the diagnosis and treatment of envenomations of venomous creatures - fire ants. Whether an individual clinician will commonly treat these envenomations depends on where in the country they are practicing.


Fire Ants

Fire ants first were found in the United States in the 1930s. Now there are five times more fire ants per acre in the U.S. than in South America, as they escaped their natural enemies and thrived in the southern U.S.[1] There are two types of fire ants; the red fire ant named Solenopsis invicta and the black named Solenopsis richteri.[2] Red fire ants are typically found in the southeast from Texas to North Carolina. There are areas of New Mexico, Arizona and California that are also infested.[1] The black fire ant is found in northern Alabama, northern Mississippi, and southern Tennessee.[2] Fire ants are remarkably hardy and can even form floating life rafts made of thousands of ants in times of flooding.[3]


Fire ants floating in flood water

Fire ant bites usually occur when their nest is disturbed, and occur mostly in the summer months.They can swarm with multiple fire ants stinging the victim, and if a fire ant mound is disturbed hundreds to thousands of fire ants may respond.[4] In addition, each ant can sting repeatedly within a very short time. When a fire ant stings its mandible locks onto its prey and venom is injected through a stinger located on the abdomen. It typically stings an average of seven to eight times while rotating its body in a circular pattern repeatedly stinging. Since fire ants hold on to the skin with their mandibles, they are not easily brushed off and often have to be pulled off individually.[4]


Fire Ant Venom

Fire ant venom is normally used to immobilize or kill prey the ants want to eat. The venom is a 95% water-insoluble alkaloid, with the rest being an aqueous protein solution. The alkaloid part of the venom has cytotoxic and hemolytic properties, while the protein portion may contain allergens.

Fire ant stings
Fire ant stings

Fire Ant Envenomation

In humans the initial response to the venom is a severe burning sensation. In a few minutes, after that subsides, a dermal flare and wheal occur and then within two hours papules form. Vesicles develop within four hours and by 24 hours these become sterile pustules. One way of identifying a fire ant envenomation is by these pustules that form after the sting. There is also a local reaction, which is characterized by erythema and edema which can be larger than ten centimeters in diameter, and can be very painful and pruritic, lasting from 24 to 72 hours. The skin reactions are IgE-mediated, and contain a dense fibrin gel that contains eosinophils, neutrophils, and lymphocytes. Anaphylaxis, may occur due to the allergens in the aqueous protein solution, more frequently in persons sensitized by a previous sting.[2]


Treatment

Most patients stung by fire ants just require supportive care. Hydrocortisone cream, antihistamines and over the counter analgesics are typically all that is required initially. Ice packs may be helpful to relieve pain and swelling. The sterile pustules should not be opened. However, if they do open up they should be cleaned with soap and water and an antibiotic ointment may be used to prevent secondary infection. If a secondary infection does occur oral antibiotics may be needed. Occasionally, if there is a large local reaction a one-time dose of prednisone may be helpful. Anaphylactic reactions are treated in standard fashion.[2]


People who are severely allergic to fire ant stings can receive whole body extract immunotherapy, which contains the entire body of the ant, not just the venom as is typical treatment for other venomous insects. In whole body extract immunotherapy gradually increasing doses are administered which can reduce the risk of a future allergic reaction to fire ant venom.[4]

 

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References


[1] Fire Ants. CDC. Last reviewed: May 31, 2018. Retrieved from: https://www.cdc.gov/niosh/topics/insects/fireants.html


[2] Kruse B, Anderson J, Simon LV. Fire Ant Bites. [Updated 2020 Aug 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan. Retrieved from: https://www.ncbi.nlm.nih.gov/books/NBK470576/


[3] Zhang S. Yes, That's a Huge Floating Mass of Live Fire Ants in Texas. The Atlantic. August 29, 2017. Retrieved from: https://www.theatlantic.com/science/archive/2017/08/fire-ants-flooding-hurricane-harvey/538365/


[4] Insect Sting Allergy. American College of Allergy, Asthma, and Immunology. Last updated 2/5/2018. Retrieved from: https://acaai.org/allergies/types/insect-sting-allergy


Originally published October 2021

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