Ant bites and stings
What is an ant bite?
An ant bite occurs when an ant bites using their mandibles and mouth to pinch human skin. Bite differs from sting: ants sting with their stinger, the caudal-most part of their bodies. Fire ants grasp the skin (bite) then inject venom with their stinger (which is immediately painful). Other species of ants neither bite nor sting, but instead spray formic acid.
Who gets an ant bite?
Anyone in contact with ants is at risk of ant bites or stings, particularly if they are in an area where ants build their nests. Ant nests are in form of several mounds with varied diameter, sometimes reaching more than 0.5 metre and several centimetres high.
Ants tend to be more numerous in areas with:
- Compact soil
- Bare ground
- Little ground cover
- Few large trees
More about ants
Ants belong to Hymenoptera insect order, under the family of Formicidae. The order of Hymenoptera includes bees and wasps.
There are more than 12,000 species of ants. Although they can nearly all bite or sting, few cause significant local and/or systemic reaction in humans. Most ants are too small to effectively bite humans, and their sting is mild. However, the sting from harvester ants and fire ants can cause unpleasant symptoms and may lead to allergic reactions.
What are the clinical features of ant bites and stings?
The normal reaction to many forms of ant bite/sting is localised contact urticaria (illustrated), The normal reaction to a fire ant bite or sting is immediate pain and a red spot, followed a few hours later by a tender, itchy pustule that can last several days to weeks. It is common for a person to experience multiple stings.
Allergic reactions to ant bites and stings
Generally, allergic reactions to Hymenoptera bites are milder than to stings. Allergic reactions to Hymenoptera are classified into 4 categories:
Local reaction is the most common reaction to the venom injected by ants. It consists of localised pain, itch, redness, swelling and induration. The swelling usually less than 5 cm in diameter, and is sometimes urticarial (whealing). Local reaction lasts for less than 24 hours.
Large local reaction
A large local reaction (pain, erythema, blisters, swelling and itch) involves larger areas of skin (5 to 10 cm) in proximity with the bite/sting. In most cases, the severity peaks in 1 to 2 days, and it takes 7 to 10 days to resolve.
Mild systemic reaction
Mild systemic reaction involves skin and/or gastrointestinal system and affects less than 1% of ant bites/stings (0,4 – 0,8%). Skin manifestations include itch, angioedema, urticaria, and redness in areas distant from the bite/sting in more generalised distribution. Gastrointestinal system symptoms may consist of mild nausea, diarrhoea, and/or cramping.
Severe systemic reaction
A severe systemic response to ant venom is marked by clinical manifestations in 2 organ systems distant from the bite/sting, such as angioedema (especially of larynx), flushing, hoarseness, wheezing/bronchospasm, chest pain, hypotension, dizziness, severe abdominal pain, profuse vomiting, or uterine cramping.
How are ant bites/stings diagnosed?
The diagnosis of ant bites/stings is based on clinical findings and relevant exposure, or possible exposure, to ants. Referral for allergy testing is warranted only in cases presenting with severe systemic reaction. For systemic reaction isolated to the skin (large local reaction), skin testing is not required because the risk of having anaphylaxis after a subsequent ant bite/sting is extremely low.
What is the treatment for ant bites/stings?
Treatment for ant bites/stings depends on the type of reaction. First remove the ants from your skin and wash the affected area. Apply a cool compress to sooth the itching and reduce swelling.
Treatment for local and large local reaction
- Oral antihistamines for itch and urticaria
- Analgesics such as acetaminophen/paracetamol if pain is dominant
- Short-course of oral corticosteroids for large local reaction when inflammation is slow to resolve
- Education for insect avoidance: remove visible ant nests, avoid walking barefoot, wear trousers and long-sleeve shirt when engaged in outdoor activities
- Education for insect avoidance in areas where venomous ants are prevalent
- Wear shoes and socks when outdoors
- Wear work gloves when gardening
Treatment for mild and severe systemic reaction
- Adrenaline/epinephrine injection
- Basic life support for anaphylaxis
- Immediate referral to emergency room
- Antihistamine injection as an adjunct
Long term management:
- Epinephrine pen for patients with severe systemic reaction with education on how to use it
- Allergy specialist referral for skin testing and venom immunotherapy
- A systemic reaction confined to urticaria in children < 16 year-old does not need immunotherapy
What is the outcome for ant bites/stings?
- The chance of having severe systemic allergic reaction following an ant/insect bite is greater in adults (especially > 45 years old) than in children. Approximately 85% of adult having systemic allergic response are categorised as severe reaction, while it is only 40% of children.
- Less than 1% of ant bites result in more severe reaction after a re-bite.