How Mosquitoes Bite Us (and the Consequences)
Ticks, mosquitoes, wasps, ants… In the summer, the risk of encountering a small creature that stings, bites, or scratches increases significantly. Our series “A Biting Summer” takes a look at the most common and worst bites, and how 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 the bite: the mosquito!
Yannick Simonin, University of Montpellier and Sébastien Nisole, Inserm
They’re all around us, often invisible, and by the time we notice them, it’s usually too late… A slight sting followed by itching, redness, and a bump? The mosquito has claimed another victim and added you to its long list of favorite targets!
A rather bothersome uninvited guest at our summer gatherings, the mosquito doesn’t just bite humans—far from it. Depending on their preferences, their primary victims are either birds or land mammals, particularly domestic animals such as cats or dogs. And some species even prey on “cold-blooded” animals, like frogs and snakes!
But how does this tiny insect manage to bite even the toughest skin? And what effects might its bite have on its victims’ bodies?
Blood: the guarantee of a well-bred lineage
First of all, it’s important to remember that only female mosquitoes bite; males much prefer flower nectar or other sources of sugar (such as honeydew, a thick, sticky liquid secreted by certain plant-parasitic insects, like aphids, whiteflies, and scale insects) to our blood. And for good reason: they don’t have a proboscis…
While they’re flying, it’s not easy to tell the harmless males apart from the females. However, if you look closely, you’ll notice that the males have feathery antennae on their heads, which the females lack. But in any case, if a mosquito bites you, it’s definitely a female!
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Why, then, do female mosquitoes have such a craving for blood meals? Quite simply because the blood they collect is a very rich source of protein, which they use to complete the development of their eggs after fertilization by the male. Blood, therefore, does not serve to feed the mosquitoes themselves, but to enable their offspring to hatch. No bite, no new mosquito!
The mosquito is well-equipped
To bite us, the female mosquito has a formidable arsenal. It consists of a mouthpart called a proboscis, which itself is made up of “piercing” mouthparts (capable of causing injury), known as stylets. These parts are enclosed by the flexible labium, which folds back at the moment of the bite. Unlike a stinger, the mosquito’s proboscis is flexible, which makes it easier for it to reach the blood.
When the mosquito bites, its proboscis pierces the skin and moves through it, “feeling its way” in search of a blood vessel. Mouthparts called mandibles help the proboscis stay in place. At the same time, the mosquito injects its saliva through another mouthpart. This saliva contains substances that prevent blood vessels from constricting and thin the blood, preventing its clotting and platelet aggregation—the initial stage of healing. The stylets thus form two channels: the feeding channel, through which blood is sucked, and the salivary channel, through which saliva is injected.
Depending on the species, a mosquito takes between 0.001 and 0.01 milliliters of blood. That’s a tiny amount of blood for us, but a huge amount for the mosquito: it can be equivalent to the insect’s own blood volume. And it all gets swallowed in less than 2 minutes!
While the victim of the “blood draw” need not worry about the amount of blood taken, the needle prick 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 sensory perception.
The skin consists of two main layers: the epidermis, the outermost layer of the skin, and the dermis, a thicker layer of supportive tissue located beneath the epidermis.
The epidermis is composed mainly of keratinocytes, so named because they produce keratin, a hydrophobic protein that forms strong filaments and gives the epidermis its flexibility, impermeability, and strength. Once at the surface of the epidermis, these cells form a layer of dead cells, the stratum corneum, which is eventually shed through desquamation.
The dermis, on the other hand, 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.
Since it serves as the interface between the body and the external environment, the skin is exposed to numerous stresses, particularly mechanical and thermal ones. It is also on the front lines against attacks by microorganisms, as it is the primary entry point for many pathogens attempting to enter the body.
As such, it serves as a strategic outpost of the immune system and is home to numerous sentinel cells that protect it against viral, bacterial, or fungal attacks.
Mosquito saliva: the source of our troubles
A mosquito bite is one of the many threats the skin must contend with. When a mosquito bites, its proboscis penetrates the epidermis and then the dermis in search of a blood vessel.
In addition to the physical intrusion caused by the insertion, it is primarily the saliva injected during the procedure that triggers a reaction in our skin. This saliva contains a complex mixture of proteins, which the skin’s immune system recognizes as a foreign substance.
The reaction is almost immediate. Immune cells called “mast cells” are the first to respond. They release histamine, an inflammatory mediator that increases the diameter and permeability of blood vessels, thereby causing swelling: this is the well-known 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), about fifteen salivary components capable of triggering an allergic reaction have been identified! Furthermore, 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 mosquito bites is not the same for everyone.
James Gathany, CDC / Wikimedia Commons
It’s important to note that histamine isn’t only released in response to a mosquito bite. This molecule also plays an important role in allergic reactions, particularly in response to pollen, latex, or certain foods. It is thus responsible for allergic symptoms such as a runny nose, watery eyes, skin redness, and itching… Which, in very rare cases, can have certain consequences following a mosquito bite.
Rare complications
A mosquito bite usually fades within a few days, so it isn’t really dangerous in itself and mostly causes itching. Since the reaction is confined to the skin, it can sometimes be quite severe in 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 can cause Skeeter syndrome, a systemic inflammatory reaction associated with fever, sometimes accompanied by vomiting and respiratory distress. This syndrome results from a hypersensitivity reaction caused by the production of antibodies (immunoglobulin E, or IgE, and immunoglobulin G, or IgG) directed against certain components of mosquito saliva.
Relieve itching
There are many over-the-counter products and “home remedies” available to relieve the sting and associated itching. These include applying a cloth soaked in warm water to the sting, or conversely, using an ice cube, alcohol-soaked compresses, or certain essential oils.
Antihistamine creams or oral antihistamines are often very effective. Corticosteroid creams are also used. They help reduce the itching and swelling caused by the inflammatory reaction.
But the main issue with the bite isn’t the rare complications or the discomfort caused by the itching, even though the resulting discomfort is undeniable.
In fact, in some cases, mosquitoes don’t come alone. Depending on the region of the world and the species in question, they can carry along unwanted passengers—viruses or parasites that pose varying degrees of danger to humans. And when they bite, these stowaways can enter our bodies.
Beyond the pimples: the risk of infection
The problem of viruses transmitted by arthropods (arboviruses, short for ARthropod-BOrne VIRUSes)—that is, by insects such as mosquitoes—is not a new one. But while it has long been confined mainly to tropical and subtropical regions, it has spread and is now established in our latitudes, along with the mosquitoes that transmit these diseases.
In fact, the geographic range of certain mosquito species, particularly the Asian tiger mosquito (Aedes albopictus), has expanded significantly over the past few decades. Once 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 regions, including the French Mediterranean coast.
However, these viruses can be dangerous. After replicating in the skin following a bite (primarily after infecting immune cells in the skin), they can enter the bloodstream, from where they can reach many organs. The liver, spleen, kidneys… Many organs can be affected. But the most serious consequences occur when some of these viruses reach the brain.
In fact, because the immune system is naturally quite weak there, these pathogens can multiply unchecked—if they manage to breach the barriers protecting the brain—and cause various serious conditions, such as encephalitis (inflammation of the brain), meningitis (inflammation of the meninges), or meningoencephalitis (inflammation of both the meninges and the brain).
Protect yourself from bites
The best way to avoid the inconvenience and potential risks associated with mosquito bites is to make sure that mosquitoes don’t add us to their long list of victims.
Furthermore, avoiding a bite not only protects you but also slows down the female’s reproductive cycle, forcing her to seek out a new victim in order to lay her eggs. Since a female mosquito can lay several hundred eggs in a single clutch, and females of certain species can lay several clutches during their lifetime (usually around five), this effort is no small matter!
It’s therefore a good idea to take every precaution: wear loose-fitting, long-sleeved clothing, use mosquito nets and repellents… You should also make sure to eliminate all standing water in your surroundings—which can serve as breeding grounds for mosquitoes—by emptying flowerpot saucers and watering cans, covering rainwater containers, and so on.
On the other hand, you should absolutely avoid over-the-counter gadgets such as “mosquito repellent bracelets,” as these accessories are, at best, ineffective and, at worst, harmful. In April 2020, ANSES (the French Agency for Food, Environmental and Occupational Health & Safety) issued a warning about the risks posed by these devices, which can cause irritation or burns.
It’s therefore best to stick to good old-fashioned natural methods that have proven effective, such as candles and mosquito coils, which release scents like citronella, geranium, or lavender… Mosquitoes are, in fact, very sensitive to smells.
It’s worth noting, by the way, that the term “mosquito bait” is accurate. In fact, the odors emitted by our skin and by the bacteria that colonize it—odors that are amplified, in particular, by sweat—make our skin more or less attractive to mosquitoes. We are therefore not all equally at risk of being bitten. Fortunately, methods to prevent and control their spread work for everyone!
Yannick Simonin, virologist and associate professor of surveillance and research on emerging diseases, University of Montpellier and Sébastien Nisole, Virologist – Head of the “Viral Traffic, Restriction, and Innate Immunity” team, Montpellier Institute for Research Infectious Diseases (IRIM), Inserm
This article is republished from The Conversation under a Creative Commons license. Readthe original article.