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How mosquitoes get under your skin

To understand how they spread disease, scientists have studied just how the mosquito bites. “Gee-Whiz Science” columnist David Ropeik gives you the lowdown.
/ Source: contributor

Public health officials on the East Coast are rushing around trying to kill mosquitoes that carry West Nile virus. To understand how they spread disease, scientists have studied just how the mosquito bites. And you have to give the buggers their due: They are fabulously sophisticated at getting to our blood.

Ecologists may argue, but most of us firmly believe there is no such thing as a good mosquito. That being said, the complexity of their mechanical and chemical systems for finding us, finding a nice juicy spot once they land, and then manipulating our body chemistry while they draw their nourishment, deserves respect as a remarkable example of evolution.

Let’s start with how they find you. They have antennae that can detect carbon dioxide, which we exhale. They can also detect heat, given off by warm-blooded animals. They can even detect lactic acid on our skin, a byproduct of muscular exertion. The species that spreads malaria, Anopheles gambia, is particularly attracted by the chemicals that give foot odor and limburger cheese their distinctive odor. Yummm!

Then, once they land, they have chemical receptors that find a likely spot to go drilling for blood by detecting heat and chemicals on our skin. They also have mechanoreceptors that push ever so gently against the skin to detect differences in resistance that indicate arterioles and venules in certain spots.

Then the mouth goes to work. It’s a complex seven-piece set of devices for doing a variety of jobs. One of those parts is a tube that allows the mosquito to secrete its saliva on you. It’s gross, but remarkable. That saliva contains a drug that acts like an anesthetic. The little buggers numb us out so we don’t know they’re probing around in our skin!

The attack begins
Meanwhile, the other parts begin an intricate pattern of coordinated movements that let them hack into your flesh. The mandibles, on the edges, are serrated on the tip, like tiny steak knives, to carve their way in. Just below the surface, the whole package of nasty pointy parts bends and takes a 90-degree turn. Then the attack really begins.

That set of needlelike parts begins to randomly poke away, anywhere between 12 and 18 times in just several seconds. It’s not actually looking for a venule or arteriole just yet. It’s indiscriminately poking around to try and poke into anything in the area that may bleed. (Which is actually rare in the top layers of skin, where there are few blood vessels.)

That tube in the middle of their mouth assembly that does the spitting is actually two straws in one, one inside the other. As they continue to spit out their anesthetic, the other wider tube in the straw is sucking in samples of fluid to see if it’s hit paydirt. Mosquitoes are the only known creatures that can spit and suck at the same time.

If they don’t find blood, they draw their probing mouth parts back a little, not all the way out, and dig into another spot. This process can last up to two minutes, if somehow you remain unaware that the bugs are there. (Only females feed this way, and in fact it’s not for food that they do all this work. The blood carries protein and is passed directly on to the eggs the mother-to-be mosquito is carrying in her abdomen.)

If an insect does taste blood, it turns on more powerful pumps in its mouth, increasing the suction on its “drinking” straw. That draws on the tiny cloud of blood that has formed under your skin. The nearest venule or arteriole, even if it’s not the one already wounded, is pulled over to the needlelike tip of the whole apparatus, where it’s impaled. That opens up the hole through which the mosquito starts to drink. (Eeewww!)

Then more highly evolved chemistry comes into play. The mosquitoes inject chemicals that act as vasodilators, expanding the blood vessel to increase the flow. They also inject anti-clotting chemicals, to keep the tiny wound from sealing up before they’ve had its fill.

Mixed in with these chemicals are parasites that the mosquitoes have picked up while feeding on other animals, often birds. The parasites are only hitching a ride and live in the mosquitoes’ salivary glands, waiting for the bugs to do their thing on its next victim. When the insects squirt their chemical payload into our skin, out come the parasites. That’s how mosquitoes spread disease.

Immune system's role
If it’s your first mosquito bite ever, nothing will happen. You get one bite, free. Our immune system has not yet developed antibodies to recognize and flag the foreign chemicals.

But after just one dose of mosquito spit (Eeeewww!), the familiar itchy bump left behind by subsequent bites is caused by an allergic reaction to all those chemicals the bugs are squirting into you.

The immune system releases histamines, chemicals that make the cells in the lining of our blood vessels spread apart. That lets fluid leak out into the skin. That fluid is carrying components of our immune system that attack and destroy the foreign chemicals from the mosquito. The extra fluid from the leaking blood vessels causes a bump, which irritates other receptors in the skin, leading to the itching.

After we’ve been bitten enough, and after our immune systems have matured a bit more, we have a different reaction to mosquito saliva. Instead of triggering histamines, another part of the immune system grabs onto the foreign chemicals and drags them away to macrophages, which are like microscopic garbage disposers in our bloodstream. They chew up and destroy the “Jus de Mosquito.” If you’re bitten enough, like the mosquito lab people who have figured all this out, you stop developing bumps and itching completely.

Kids get bigger bumps from mosquito bites than adults because their immature immune systems respond with larger amounts of histamines. More leaking blood vessels equal bigger bumps, equal more itching and scratching. And more reason to think, despite how fascinating they are, that there is no such thing as a good mosquito.

David Ropeik is a longtime science journalist and currently serves as Director of Risk Communication at the Harvard Center for Risk Analysis.