How mosquitoes bite us (and the consequences)
Ticks, mosquitoes, wasps, ants... In summer, the risk of encountering a small creature that stings, bites, or scratches increases significantly. Our series "A summer of bites" takes stock of the most common and worst bites, and ways to avoid them. In this first article, Yannick Simonin (University of Montpellier) and Sébastien Nisole (Inserm) introduce us to a serious contender for the title of king of bites: the mosquito!
Yannick Simonin, University of Montpellier and Sébastien Nisole, Inserm
They are all around us, often invisible, and when we detect their presence, it is usually too late... A slight pain followed by itching, redness, and a bump? The mosquito has claimed another victim and added you to its long list of favorite targets!
An unwelcome guest at our summer cocktail parties, mosquitoes don't just bite humans, far from it. Depending on their preferences, their first victims are either birds or land mammals, particularly domestic animals such as cats and dogs. And some species even attack "cold-blooded" animals such as frogs and snakes!
But how does this tiny insect manage to bite even the toughest skin? And what consequences can its actions have on the organisms of its victims?
Blood, the guarantee of well-bred offspring
First of all, it should be noted that only female mosquitoes bite; males much prefer flower nectar or other sources of sugar (such as honeydew, a thick, sticky liquid excreted by certain insects that parasitize plants, such as aphids, whiteflies, scale insects, etc.) to our blood. And for good reason: they do not have a stinging apparatus...
In flight, it is not easy to distinguish harmless males from females. However, careful observation reveals that males have feathery antennae on their heads, which females do not have. But in any case, if a mosquito bites you, it is definitely a female!
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Why do females have such an appetite for blood meals? Quite simply because the blood they collect is a very rich source of protein, which is used to complete the maturation of their eggs after fertilization by the male. Blood is therefore not used to feed mosquitoes, but to enable their offspring to be born. Without bites, there would be no new mosquitoes!
The mosquito is well equipped
To bite us, female mosquitoes have a formidable arsenal at their disposal. It consists of a proboscis, which is itself made up of "wounding" mouthparts (capable of causing injury), known as stylets. These parts are enclosed by the flexible labium, which folds back when the mosquito bites. Unlike a stinger, the mosquito's proboscis is flexible, which makes it easier for it to reach the blood.
When biting, the stylets pierce the epidermis and "feel" their way through it, searching for a blood vessel. Mouthparts called maxillae allow the proboscis to remain in place. At the same time, the mosquito injects its saliva through another appendage. The saliva contains substances that prevent the blood vessels from contracting and thin the blood, preventing it from clotting and platelets from aggregating, which is the initial stage of healing. The stylets thus form two channels: the alimentary canal, through which blood is sucked, and the salivary canal, through which saliva is injected.
Depending on the species, mosquitoes take between 0.001 and 0.01 milliliters of blood. This is a tiny amount of blood for us, but enormous for them: it can be equivalent to the insect's own blood volume. And it all gets swallowed in less than 2 minutes!
While the victim from whom the blood is taken has nothing to worry about in terms of the amount of blood removed, the puncture causes other problems, some more serious than others...
A Brief History of Skin
To understand the consequences of a mosquito bite, let's take a moment to consider the body's largest organ: the skin. In direct contact with the outside world, it performs various functions that are essential to our body, particularly in terms of protecting it from the external environment and enabling perception.
The skin is organized into two main layers: the epidermis, the surface layer of the skin, and the dermis, a thicker supporting tissue located beneath the epidermis.
The epidermis is mainly composed of keratinocytes, so named because they produce keratin, a hydrophobic protein that forms resistant filaments and gives the epidermis its flexibility, impermeability, and resistance. Once on the surface of the epidermis, these cells form a layer of dead cells, the stratum corneum, which is eventually eliminated through desquamation.
The dermis consists of cells scattered within an extracellular matrix made up of fibers composed of collagen and other proteins. The main cells of the dermis are fibroblasts, which produce these fibers, but the dermis also contains nerve endings, blood vessels, and sebaceous and sweat glands, which produce sebum and sweat, respectively.
As the interface between the body and the external environment, the skin is exposed to numerous aggressions, particularly mechanical and thermal ones. It is also on the front line when it comes to attacks by microorganisms, as it is the gateway for many pathogens attempting to enter the body.
As such, it is a strategic outpost of the immune system and home to numerous sentinel cells, which protect it against viral, bacterial, and fungal attacks.
Mosquito saliva, the source of our discomfort
Mosquito bites are one of the many aggressions that the skin has to deal with. When a mosquito bites, its proboscis penetrates the epidermis and then the dermis in search of a blood capillary.
In addition to the physical aggression caused by this insertion, it is mainly the saliva injected during the operation that triggers a reaction in our skin. This saliva contains a complex mixture of proteins, which is recognized by the skin's immune defenses as a foreign agent.
The reaction is almost immediate. Immune cells called "mast cells" are the first to react. They secrete histamine, an inflammatory mediator that increases the diameter and permeability of blood vessels, causing edema: this is the famous mosquito bite. It is also histamine that, by stimulating nerve fibers, causes itching and the irresistible urge to scratch.
Many components of mosquito saliva can be allergens. In the case of the tiger mosquito (Aedes albopictus), around fifteen saliva components that can cause an allergic reaction have been identified! In addition, the composition of saliva can vary from one species to another (as can the number of allergens). Finally, reactions vary from person to person, as individual sensitivity to the bite is not the same for everyone.
James Gathany, CDC / Wikimedia Commons
It should be noted that histamine is not only involved in mosquito bites. This molecule also plays an important role in allergic reactions, particularly in response to pollen, latex, or certain foods. It is responsible for allergic symptoms such as a runny nose, watery eyes, skin rashes, itching, etc. In very rare cases, this can have certain consequences after a mosquito bite.
Rare complications
Mosquito bites usually fade within a few days, so they are not really dangerous in themselves and mainly cause itching. Remaining localized on the skin, they can sometimes be significant for those of us who are more sensitive.
Although complications are rare, some people are nevertheless at higher risk of severe allergic reactions or even anaphylactic shock, particularly children who have not yet developed a natural tolerance to mosquito bites.
In very rare cases, bites cause Skeeter syndrome, a systemic inflammatory reaction associated with fever, sometimes accompanied by vomiting and respiratory problems. This syndrome results from a hypersensitivity reaction due to the production of antibodies (immunoglobulins E, or IgE, and G, or IgG) directed against certain components of mosquito saliva.
Relieve itching
There are many commercial products and homemade remedies available to relieve the sting and associated itching. These include applying a cloth soaked in hot water to the sting, or conversely, an ice cube, or even alcohol compresses and certain essential oils.
Antihistamine creams or oral antihistamines are often very effective. Corticosteroid creams are also used. They help reduce itching and swelling caused by the inflammatory reaction.
However, the main problem associated with the bite is not the rare complications or the discomfort caused by itching, even though the resulting discomfort is undeniable.
In fact, in some cases, mosquitoes do not come alone. Depending on the region of the world and the species in question, they can carry with them cumbersome partners, viruses, or parasites that are more or less dangerous to humans. When they bite, these stowaways can enter our bodies.
Beyond the pimples, the risk of infection
The problem of viruses transmitted by arthropods (arboviruses, from the English ARthropod-BOrne VIRUSes), i.e. by insects such as mosquitoes, is not new. But while it has long been confined mainly to tropical and subtropical regions, it has now spread to our latitudes, along with the mosquitoes that carry these diseases.
Indeed, the geographical distribution of certain mosquito species, particularly the tiger mosquito (Aedes albopictus), has increased considerably in recent decades. Previously confined to Southeast Asia, this invasive species has spread across the globe: today, only Antarctica remains free of this mosquito and the viruses it transmits, such as dengue, Zika, and Chikungunya. As a result, indigenous cases of mosquito-borne viral diseases are on the rise in new latitudes, including around the French Mediterranean coast.
However, these viruses can be dangerous. After replicating in the skin following the bite (mainly after infecting immune cells in the skin), they can enter the bloodstream, from where they can reach many organs. Liver, spleen, kidneys... Many organs are affected. But the most serious consequences occur when some of these viruses reach the brain.
In fact, since the immune system is naturally less active in this area, they can multiply at will if they manage to cross the barriers protecting the brain, causing various serious conditions such as encephalitis (inflammation of the brain), meningitis (inflammation of the meninges), or meningoencephalitis (inflammation of the meninges and brain).
Protecting yourself from bites
The best way to avoid the discomfort and potential risks associated with bites is to ensure that mosquitoes do not add us to their long list of victims.
Furthermore, avoiding being bitten not only protects you, but also slows down the female's reproductive cycle, as she will have to find a new victim in order to lay her eggs. However, since a female mosquito can lay several hundred eggs each time, and females of certain species can lay several times during their lifetime (around five times in most cases), this is no small feat!
It is therefore important to take every precaution: wear loose-fitting, covering clothing, use mosquito nets and repellents, etc. It is also important to eliminate any standing water in our environment that could encourage the spread of mosquitoes, by emptying flower pot saucers and watering cans, covering rainwater containers, etc.
However, parapharmaceutical gadgets such as "mosquito repellent bracelets" should be avoided at all costs, as these accessories are ineffective at best and harmful at worst. In April 2020, ANSES (the French Agency for Food, Environmental and Occupational Health & Safety) issued a warning about the risks associated with these devices, which can cause irritation or burns.
It is therefore better to opt for good old-fashioned natural methods that have proven their effectiveness, such as mosquito repellent candles and coils that give off aromas of citronella, geranium, or lavender. Mosquitoes are very sensitive to smells.
It should be noted that the concept of "mosquito skin" is justified. The odors emitted by our skin and the bacteria that colonize it (odors that are amplified by perspiration) make our skin more or less attractive to mosquitoes. We are therefore not all equally vulnerable to bites. Fortunately, the means of preventing and combating their spread work for everyone!
Yannick Simonin, Virologist, Associate Professor in Surveillance and Study of Emerging Diseases, University of Montpellier and Sébastien Nisole, Virologist – Head of the "Viral Traffic, Restriction, and Innate Immunity" team, Montpellier Research Institute (IRIM), Inserm
This article is republished from The Conversation under a Creative Commons license. Readthe original article.