Pimples, redness, itching… In summer, mosquitoes are a real nuisance for vacationers. While most of the time they are harmless, they can sometimes be deadly. They are considered to have killed more humans than any other insect or animal through the transmission of diseases such as dengue or malaria. Every year, they kill nearly three million people around the world. Why are they stinging us? How to prevent them? How to relieve a sting? All the answers with PasseportSanté.

Feed her eggs

As with wasps and bees, it is female mosquitoes that are responsible for bites in humans. After mating, the female goes hunting to collect the blood of mammals in order to find the source of protein necessary to nourish her eggs.

Forty-eight hours after taking this meal, the fertilized females deposit their eggs on the surface of stagnant water (ponds, streams, puddles, etc.). Water is absolutely necessary for the development of mosquito eggs: moist soil will not suffice. This “breeding-sting-laying” sequence is repeated several times during the mosquito’s life. The length and frequency of the cycle depends on the species, but the common mosquito is estimated to reproduce this cycle twice a week in summer. One bite is more than enough to cover the protein requirements of the eggs: it is therefore wrong to think that a mosquito can be responsible for several bites during the night. The bite, usually nocturnal (usually at dawn or dusk) and painless, lasts about three minutes if the mosquito is not disturbed.

Spot your prey

No need to turn off the light to avoid them, the mosquito locates its target thanks to its sense of smell. He is first able to sense the presence of carbon dioxide (emitted by the respiration and perspiration of living beings) at a distance of more than thirty meters. The closer it gets, the more it will detect human odors made up of fatty acids (sebum, butyric acid), lactic acids or other ammoniacal hints emitted by the skin, such as breath or urine. When it is very close to human skin, it uses thermoreceptors. These infrared heat detectors will allow him to find the venule, warmer, inside which he will draw the blood necessary for his laying. The visual system, especially sensitive to movements, is in fact very poorly performing.

How does an injection work?

When the mosquito is in place, it pushes its stylets through the skin into the human venule. Thanks to a first channel, formed by the hypopharynx, he injects saliva which prevents the blood from clotting in his tube. He can thus quietly suck, thanks to the second channel, the blood that has remained fluid. The amount of blood taken then varies from 5 to 10 mm³. When the mosquito has finished its meal, it leaves as it came: without worrying about what is going on in the human organism.

The immune system on alert

The mosquito therefore did not disperse an anesthetic throughout the body, as is often said. His saliva, which was only intended to stabilize the blood platelets, will have a completely different effect. Faced with the presence of this unknown substance, our immune system goes on alert and sends mast cells to the front line. These cells have the ability to detect foreign substances, and explode on contact to protect the body. When bursting, they release massivelyhistamine, which causes the redness and itching that we know. Other immune agents are then drawn in and cause a small red pimple to appear. It is a swelling of the skin filled with water: you should never try to pierce it, otherwise you risk infection.

This explains why the bites usually scratch much more in spring than in late summer, when the body has gradually become accustomed to the mosquito saliva.

We repeat it often enough: do not scratch your mosquito bites. Easier said than done… But why does scratching do so much good?

Two types of sensors

To understand this phenomenon, we must observe what happens to the skin during a bite. We know that the skin has many sensors capable of interpreting external stimuli. These sensors are connected to sensitive fibers which will transmit to the brain the information that the organism is in danger: burns, cuts, pinches… These are warning information. For example, when the skin is burned, the brain induces the perception of pain to alert the individual of the danger he is running. Here, in the case of a mosquito bite, the sensory fibers transmit the inflammation they detected and the brain induces the itching.

Some sensitive fibers are able to perceive simple touch signals, such as caresses for example. These stimuli will not alert the brain, but they still carry so-called contact information. Contact information and alert information will circulate within the same nervous network if they are located in a nearby region. Interactions can take place between the two, and one piece of information can interfere with another.

Parasite inflammation

So when we scratch around a mosquito button, the contact information is picked up by the brain and we bypass the alert information. For a short time, the brain no longer receives the message of inflammation and the itching goes away. This attenuation of pain perception can also be illustrated by taking a blow and massaging the injured area. These are the same mechanisms that come into play.

The more it itches me, the more I itch

However, we must avoid succumbing to temptation as much as possible. Scratching can allow bacteria to pass through the skin and cause secondary infections. And the more you scratch, the more it itches! How to explain this phenomenon ?

In fact, the mechanical action of scratching activates nerve endings and releases histamine molecules, which in turn will cause itching. It’s a real vicious circle.

Contrary to popular belief, mosquitoes are not sensitive to the amount of sugar in the blood. “Sweet skin” can therefore be reassured! In reality, there is two main reasons which explain this phenomenon.

As we have seen, the itching is due to an allergic reaction to the mosquito’s saliva. Some organisms, for reasons unknown, do not consider it harmful and the allergic reaction goes virtually unnoticed. They will then have the impression of being less stung than the others!

Favorite body odors

But recent research shows that this is probably not the only reason. Mosquitoes are attracted to body odors from organisms. These are linked to the pheromones that we give off, and some of which are more attractive than others. It can be related to the food, the amount of sweat, the use of a perfume but also the bacterial composition of our skin.

It is this last point that has caught the attention of researchers at the University of Wageningen (Netherlands). They studied 48 men to try to find out which smells and which bacterial populations were preferred by mosquitoes. Nine of them were particularly stung, while seven managed to slip through the cracks. In the most bitten people, we often find the same characteristic: a large amount of bacteria. We find in them very attractive bacteria that emit powerful volatile compounds: Leptotrichia sp., Delfia sp. et Actinobacteria Gp3 sp.

On the other hand, carriers of Variovorax sp. and Pseudomonas sp. are very little bitten …

Let us also add, that pregnant women are generally stung more because of the abundantly secreted hormones.