They are around us, often invisible, and when we detect their presence it is usually too late… A little pain followed by itching, redness and a pimple? The mosquito has made a new victim and has added you to the long list of its favorite targets!
A very cumbersome surprise guest at our summer aperitifs, the mosquito does not only bite humans, far from it. According to its preferences, its first victims are either birds or land mammals, especially domestic animals such as cats or dogs. And some species even attack “cold-blooded” animals, such as frogs and snakes!
But how does this tiny insect bite even the toughest skin? And what consequences can his misdeed have on the organisms of his victims?
Blood, the assurance of well-born offspring
First of all, it should be remembered that only female mosquitoes bite, males much prefer flower nectar to our blood, or other sources of sugar (such as honeydew, a thick and viscous liquid excreted by certain insects which parasitize plants, such as aphids, whiteflies, scale insects, etc.). And for good reason: they do not have a pricking device...
In full flight, it is not easy to distinguish harmless males from females. Careful observation nevertheless reveals the presence of feathery antennae on the head of the former, absent from that of the latter. But anyway if a mosquito bites you, it is necessarily a female!
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Why then this appetite of females for blood meals? Quite simply because the blood collected is a very rich source of protein, used to complete the maturation of its eggs, after fertilization by the male. The blood is therefore not used to feed the mosquitoes, but to allow their offspring to see the light of day. Without a bite, no new mosquito!
The mosquito is well equipped
To bite us, the female mosquito has a formidable arsenal. It is composed of a trunk, called proboscis, which is itself made up of "vulnerable" mouth parts (capable of injuring), the stylets. These parts are enveloped by the labium, flexible, which folds up at the time of the bite. Unlike a stinger, the mosquito's proboscis is flexible, making it easier to get to the blood.
During the puncture, the stylets pierce the epidermis and cross it by "groping" in search of a blood vessel. Buccal appendages, called maxillae, allow the tube to stay in place. At the same time, via another appendage, the mosquito injects its saliva. The latter contains substances that prevent the blood vessels from contracting and thin the blood, preventing its coagulation and the aggregation of platelets, the initial stage of healing. The stylets thus delimit two channels: the alimentary canal, through which the blood is drawn in, and the salivary canal, through which the saliva is injected.
The mosquito takes, depending on the species, 0,001 to 0,01 milliliters of blood. A tiny amount of blood for us, but huge for him: it can be equivalent to the insect's own blood volume. And everything is ingested in less than 2 minutes!
If the "taken" victim does not have to worry about the quantity of blood stolen, the bite causes other problems, more or less serious...
A brief history of skin
To understand the consequences of mosquito bites, let's take a look at the body's largest organ: the skin. In direct contact with the outside world, it provides various essential functions to our body, relating in particular to its protection vis-à-vis the external environment and its perception.
The skin is organized into two main layers: the epidermis, the superficial part of the skin, and the dermis, thicker supporting tissue located under 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 suppleness, impermeability and resistance. Once on the surface of the epidermis, these cells form a layer of dead cells, the stratum corneum, which will eventually be eliminated by desquamation.
The dermis is made up of scattered cells within an extracellular material made up of fibers made of collagen and other proteins. The main cells of the dermis are the fibroblasts, which produce these fibers, but the dermis also contains nerve endings, blood vessels, as well as the sebaceous and sweat glands, which produce sebum and sweat respectively.
Given that it constitutes the interface between the body and the external environment, the skin is confronted with numerous aggressions, in particular mechanical or thermal. It is also at the forefront of attacks by micro-organisms, since it is a necessary gateway for the many pathogens that attempt to enter the body.
As such, it is a strategic outpost of immune defenses and is home to many sentinel cells, which ensure that it is protected against viral, bacterial or fungal attacks.
Mosquito saliva, the cause of our inconvenience
The mosquito bite is one of the many aggressions that the skin has to face. 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 that this insertion constitutes, it is above all the saliva injected during the operation that triggers a reaction from our skin. It 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, the “mast cells”, are the first to react. They secrete histamine, an inflammatory mediator that increases the diameter and permeability of blood vessels, thus causing edema: this is the famous mosquito button. It is also histamine which, by stimulating nerve fibers, causes itching and the irrepressible urge to scratch.
Many elements of mosquito saliva can be allergenic. For the tiger mosquito (Aedes albopictus), about fifteen salivary components that can cause an allergic reaction have been identified ! Also, the composition of saliva can vary from species to species (as can the number of allergens). Finally, the reactions vary from one person to another, because the individual sensitivity to the reaction vis-à-vis the sting is not identical in everyone.
Note that histamine does not only intervene during a mosquito bite. This molecule also plays an important role during allergic reactions, especially in reaction to pollen, latex or certain foods. It is thus responsible for allergic symptoms such as runny nose, tears, redness on the skin, itching... Which can, in very rare cases, have certain consequences after a mosquito bite.
The mosquito bite usually fades in a few days, so it is not really dangerous in itself and is mostly itchy. Remaining localized in the skin, they can sometimes be significant in the most sensitive among us.
Although complications are rare, some people nevertheless have a higher risk of a strong allergic reaction or even anaphylactic shock, in particular children who have not yet acquired a natural tolerance to mosquito bites.
In very rare cases, the bites cause a 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.
There are many products on the market and many “homemade” recipes to reduce the sting and the associated itching. These include the use of a cloth soaked in hot water on the bite or, conversely, an ice cube or even alcoholic compresses and certain essential oils.
Antihistamine ointments or oral antihistamines are often very effective. Ointments based on corticosteroids are also used. They help to reduce the itching and swelling resulting from the inflammatory reaction.
But the main problem with the bite is not the rare complications or the inconvenience caused by the itching, even if the resulting discomfort is undeniable.
Indeed, in some cases, the mosquito does not come alone. Depending on the regions of the globe and the species considered, it can carry cumbersome partners, viruses or parasites that are more or less dangerous for humans. But at the time of the bite, these stowaways can enter our body.
Beyond pimples, the risk of infection
The problem of viruses transmitted by arthropods (arboviruses, from English ARthropod-BOrne VIRUSes) and therefore by insects such as mosquitoes, is not new. But if it has mainly concerned tropical and subtropical regions for a long time, it has moved, and has now settled in our latitudes, at the same time as the mosquito vectors of these diseases.
Indeed, the geographical distribution of certain species of mosquitoes, in particular the tiger mosquito (Aedes albopictus), has increased considerably in recent decades. Yesterday confined to Southeast Asia, this invasive species has spread all over the planet: today only Antarctica is still preserved from this mosquito and the viruses it transmits, such as Dengue, the Zika, or the Chikungunya virus. Consequence: autochthonous cases of viral diseases transmitted by mosquitoes are increasing in new latitudes, including around the French Mediterranean.
These viruses can be dangerous. After replicating in the skin following the bite (mainly after infection of immune cells in the skin), they can enter the bloodstream, from where they can reach many organs. Liver, spleen, kidneys… The affected organs are numerous. But the most serious occurs when some of these viruses reach the brain.
Indeed, the immune system being naturally relatively weak there, they can multiply at will, if they manage to cross the barriers protecting the brain, and induce various serious pathologies: encephalitis (inflammation of the brain), meningitis (inflammation meninges) or meningoencephalitis (inflammation of the meninges and brain).
Protect yourself from stings
The best way to avoid the inconvenience and potential risks associated with bites is to ensure that the mosquito does not manage to add us to the long list of its victims.
In addition, avoiding the bite not only means protecting yourself, but also slowing down the reproductive cycle of the female, who will have to look for a new victim in order to lay her eggs. However, a female mosquito being able to lay several hundreds of eggs with each laying, and the females of certain species being able to carry out several layings during their life (around 5 generally), the effort is not insignificant!
All precautions are therefore good to take: loose and covering clothing, mosquito nets, repellents... We must also ensure that we eliminate all stagnant water in our environment, which can be conducive to the spread of mosquitoes, by emptying the cups of flower pots. , watering cans, covering rainwater containers, etc.
On the other hand, it is absolutely necessary to avoid parapharmaceutical gadgets such as “mosquito repellent bracelets”, because these accessories are at best ineffective, at worst, harmful. ANSES (National Agency for Food, Environmental and Occupational Health Safety) alerted in April 2020 on the risks of these devices, which can lead to irritation or burns.
It is therefore better to favor the good old natural methods, whose effectiveness has been proven, such as candles and other mosquito coils, which give off aromas of citronella, geranium or lavender... Mosquitoes are indeed very sensitive to odors.
Note also that the notion of "mosquito skins" is justified. Indeed, the odors released by our skin as well as by the bacteria which colonize it (odors amplified in particular by perspiration), make our epidermis more or less attractive to mosquitoes. We are therefore not all equal when it comes to the risk of bites. Fortunately, the means of preventing and combating their spread work for everyone!
Yannick Simonin, Virologist, lecturer in surveillance and study of emerging diseases, University of Montpellier et Sebastien Nisole, Researcher, Inserm
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.