Insect bite: how can it be recognised?

With the summer heat, insects become more numerous and interactions with humans increase. In most cases, they bite or sting in self-defence, particularly when they feel threatened, for example if one comes too close to their nest or makes sudden movements. This defensive reaction is common in venomous insects such as bees, wasps or hornets. Conversely, some insects do not sting to protect themselves, but to feed. This is the case for mosquitoes or ticks, which require blood to ensure their development or reproduction. In mosquitoes, only the females bite, as blood is essential for the maturation of their eggs.

The manifestations associated with stings are variable.

Some go unnoticed at the time, such as mosquito bites, while others cause immediate pain. In all cases, the skin reaction most often results from the introduction of foreign substances into the skin, whether saliva, enzymes or venom, which trigger a local inflammatory response and potentially an allergic reaction. The vast majority of bites and stings are mild and lead only to limited discomfort, such as itching and redness. However, certain situations require increased vigilance, particularly in people with allergies and especially if the bites or stings are located in the throat area, which, in rare cases, can be life‑threatening.

On the skin, the appearance of a sting or bite varies according to the insect or arthropod involved. Certain signs can provide guidance, although the circumstances in which the lesion appears remain essential for refining the identification.

• Mosquitoes : Mosquito bites generally present as small, round, slightly raised papules, sometimes pale in the centre and surrounded by a red area. They appear a few minutes to a few hours after the bite and are accompanied by itching, which may be intense. They are most often isolated or few in number, on exposed areas such as the arms or legs. The two species that bite most frequently are the tiger mosquito and the common mosquito. The tiger mosquito is more dangerous than the common mosquito because it can transmit viruses such as chikungunya, dengue and Zika.

• Bedbugs : Bedbug bites appear as small red lesions, often grouped together or aligned in a line or triangular pattern. They cause pronounced itching and generally occur on areas exposed during the night, such as the arms, shoulders, neck, or face. Their appearance upon waking is a highly suggestive feature.

• Fleas : Flea bites present as small, very itchy red papules, often grouped in clusters. They are typically located around the ankles and lower legs. A central point corresponding to the bite site may sometimes be visible.

• Bees : A bee sting is immediately painful and is accompanied by a marked local inflammatory reaction. The area becomes red, warm and swollen, with a central point where the sting is often still visible. The swelling may extend around the sting site over the following hours. Bee stings produce a puncture wound or a laceration of the skin, due to the venom that causes a local toxic reaction at the site of the sting. The area is raised and a black dot is often visible, corresponding to the insect’s sting. It is then important to disinfect the area with an antiseptic solution. If the sting is still present in the skin, it must be removed gently without pressing on the venom sac, by scraping with the back of a knife or another sharp object. Tweezers are not recommended, as they may compress the venom sac and thereby increase the amount of venom released into the wound.

• Wasps and hornets : Stings from wasps and hornets also cause sharp, immediate pain, followed by redness and local swelling (oedema). Unlike bees, these insects do not leave a sting in the skin and can sting multiple times. The local reaction may be more extensive, especially in the case of multiple stings.

• Horseflies : Horseflies inflict a bite rather than a sting, often described as very painful, with a burning sensation. The lesion is generally larger than a mosquito bite, red and inflamed, sometimes accompanied by marked swelling, or even a small bruise.

• Ticks : Tick bites are often painless and go unnoticed at the time they occur. They appear as a small black dot attached to the skin, corresponding to the body of the tick, while its head is anchored in the epidermis. After removal, redness may develop. In some cases, a red patch that gradually expands over the following days or weeks may appear, which is suggestive of an infection such as Lyme disease. Indeed, tick bites can transmit various diseases, including Lyme disease, which is caused by infection with the bacterium Borrelia. After spending time in nature, it is therefore important to carefully examine one’s body and skin to detect any ticks and remove them completely, making sure not to leave the head behind.

• Harvest mites : Harvest mites cause very small red lesions, often numerous, associated with intense itching. They usually appear in areas where the skin is thin or compressed by clothing, such as behind the knees, around the waist or at the ankles.

• Spiders : Spider bites, often overestimated, are in fact rare. When they do occur, they most commonly result in localised redness, slight swelling and sometimes two small central marks corresponding to the spider’s fangs. In the majority of cases, they remain harmless.

Note : Certain bites can appear very similar, even though they may progress differently depending on individual sensitivity. The context in which they occur (environment, time of day, exposure) is therefore a key factor in guiding the diagnosis.

The vast majority of insect stings and bites are harmless and resolve spontaneously within a few days, causing itching and localised redness.

However, certain signs should be cause for concern. If the stung area becomes increasingly red, warm, painful, or if the swelling spreads significantly, it is advisable to consult a doctor or a pharmacist. These symptoms may indicate an excessive inflammatory reaction or a secondary infection, particularly in the event of repeated scratching. It is therefore preferable to avoid scratching the lesion, even if the itching is intense, in order to limit the risk of skin infection.

In addition, certain situations constitute an emergency. A sting located in the mouth, throat or on the tongue requires immediate medical attention, as the swelling may interfere with breathing. Likewise, the onset of general symptoms such as shivering, sweating, feeling unwell, vomiting or a drop in blood pressure should lead to calling the emergency services without delay. In some individuals, particularly those who are allergic to bee or wasp venom, a sting may trigger a severe allergic reaction, known as anaphylaxis. This may present with rapid swelling of the face or neck (Quincke’s oedema), breathing difficulties, widespread urticaria (hives), or a feeling of faintness. Finally, after a tick bite, the appearance in the following days or weeks of a red patch that gradually spreads should also prompt a medical consultation, due to the risk of infection such as Lyme disease.

In all cases, if there is any doubt or an unusual change, it is preferable to seek medical advice.

• RUËFF F. & al. Insect stings: Clinical features and management. Deutsches Ärzteblatt Info (2012).

• SCHMIDT J. Pain and lethality induced by insect stings: An exploratory and correlational study. Toxins (2019).

• HEMMER W. & al. Allergy to stings and bites from rare or locally important arthropods: Worldwide distribution, available diagnostics and treatment. Allergy (2023).