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Why Do Mosquito Bites Itch And Swell?

Mosquitoes: the vampires of nature

Mosquitoes

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There’s nothing more frustrating than stopping yourself from scratching an itching mosquito bite. Why do mosquito bites itch and swell? Let’s see how this tiny organism can cause so much bother.

How do mosquitoes feed and survive?

Mosquitoes might be one of the least tolerated insects out there. But before you decide to hate them, remember that all organisms on earth are part of a delicate ecosystem, where each organism is connected to another. As much as you’d like to remove mosquitoes from the world, the mosquito plays an important part in keeping the balance of nature.

mosquito life cycle
The Life Cycle of Mosquito, 0. Mosquito laying eggs on the surface of the water 1. Egg stage (individual or in groups) 2. Larvae stage (the larvae go through four substages known as instars) 3. Pupae stage 4. Adult mosquito. Image Credit : Mariana Ruiz Villarreal

The buzzing and biting aside, they’re pretty interesting creatures as they go through a process called complete metamorphosis within a couple of weeks. The life cycle of a mosquito is made up of four stages: egg, larva, pupa, and adult. The female mosquito deposits her eggs along the surface or edge of still water. This can be either a lake or any container that holds water near your house. The eggs can be placed individually or chained together in groups called rafts until they hatch. But how do mosquito larvae breathe underwater? Once the eggs hatch, the larvae hang suspended on the surface of the water.

A structure known as a siphon, which looks like a snorkel, helps them receive air from the outside. They continue to grow by filtering and feeding on microorganisms in the water until they reach the pupa stage. This stage is similar to the cocoon from which a butterfly will emerge. They surround themselves with a hard shell and float on the surface of the water. During this stage, the larvae undergo major transformations before they emerge as adult mosquitoes with legs, wings, and a proboscis, a tube-like structure that functions as their mouth.

The adult stage of the mosquito is when things start to get annoying for us. Interestingly enough, it’s only the female of the species that bite and feed on animal and human blood, while the male mosquitoes get their nutrients from flower nectar. Blood is an indispensable part of the female mosquito’s diet for egg production and the continuation of the life cycle. The female requires nutrients such as iron, protein, and different types of amino acids, that can only be found in animal and human blood.

What happens when a mosquito bites?

Some of us seem to be magnets for mosquito bites. You may find yourself asking why do mosquitoes bite me more than others? Mosquitoes rely on their sense of smell to find their source of fresh blood meal, with carbon dioxide as their primary indication of a living breathing thing. A few studies reported that mosquitoes prefer to feed on people with blood type O. Although, other variables also come into play, like body heat, pregnancy, and how much alcohol you had to drink. Odor and stink are other major attractions for mosquitoes, as they tend to target the smelliest parts of our bodies like our feet and ankles with a guaranteed supply of fresh blood. Some studies suggest that mosquitoes are less likely to get swatted away in those far away spots.

But why do mosquito bites itch more on your feet? That’s because feet have a different type of skin than the rest of your body and are filled with nerve endings making them very sensitive areas. As you go on with your daily tasks wearing socks and shoes, the healing process of the bumps becomes longer because of the friction and movement, making the bites and itching last for weeks.

As mentioned, only the females of the species require blood as part of their diet. Male mosquitoes don’t even have the appropriate mouthparts to pierce through the skin. Once a female mosquito lands on the skin, it readies its specialized proboscis made of six needle-like parts called stylets for a blood meal.

mosquito mouth parts
The Proboscis of the female mosquito. Image Credit : Choo, Buss, Tan and Leal. Creative Commons Attribution License. Front Physiol. 2015;6.306

1. The very sharp maxillae (Mx/red) saw through the skin

2. While the mandibles (M/dark green) hold the skin tissue apart.

3. Once a juicy blood vessel is found, the mosquito uses the hypopharynx (light green) to drool out saliva to make sure the blood doesn’t coagulate, before

4. It punctures through the blood vessel with the labrum (blue). She then lays the hypopharynx on the labrum to construct a suction tube to pump the blood into her abdomen.

That’s not all, evolution has also optimized this whole feeding process to allow the mosquito to ingest more blood in less time. Inside the abdomen, specialized structures simultaneously separate the red blood cells from water, which is expelled from the rear end. It might not be a pretty image, but scientists have spent decades trying to understand this procedure.

Video of Labrum probing and finding a blood vessel

Why do mosquito bites itch and swell?

Ironically, the bumps and itches that you experience following a mosquito bite are the response of the body’s immunity coming to the rescue. Most of us undergo a small allergic reaction due to the presence of foreign materials in our bodies. The immune system is composed of different types of cells that the body uses to protect and fight against pathogens and foreign compounds that enter our bodies from outside. Under normal circumstances, the body reacts to an injury by constricting the blood vessels, recruiting platelets at the site of trauma to immediately stop bleeding, and sending out immune cells to fight off pathogens. In the case of a mosquito bite, the saliva that the mosquito injects into your skin is made of a mixture of proteins that inhibit these reactions and allow blood flow to continue smoothly straight up into the mosquito’s abdomen.

This becomes even more dangerous in the case of some species of mosquitoes that are vectors for serious human infections like malaria, dengue, and zika. The presence of these proteins triggers the immune system to send out chemicals called histamines to attack and destroy the foreign compounds. Once at the site of injury, the histamines cause the blood vessel to dilate, which leads fluids to leak out and accumulate under the skin causing a bump. The bump would in turn trigger an inflammatory response that leads to itching. And this is when it’s important not to give in to that itching urge. By scratching the bumps, you irritate the skin even more, which leads to more inflammation and itching responses.

mosquito bite
The reaction of a mosquito bite 1. Mosquito injects saliva triggering the body’s immune system 2. Histamines gather at the site of the bite causing leakage and the bump 3. The bump triggers an inflammatory response 4. The inflammation induces the itching sensation

Why do mosquito bites itch more at night?

 You often experience the itching sensation getting more intense during the night. Besides the fact that you’re more distracted during the day by morning tasks and routine, there’s another biological reason why this happens. As covered in the previous paragraph, the itching sensation is a response to the inflammatory response at the site of the bite. And this is where a hormone called cortisol comes in. This chemical regulates anti-inflammation and stress in your body and coordinates your body clock (when to wake up and when to sleep). Cortisol levels tend to be very high in the morning to stimulate alertness, but they gradually decrease as the day goes by and become very low during the night. With low cortisol levels, the anti-inflammatory signal is reduced, leading to renewed inflammation under the bite and another sleepless night.

It all comes down to MAKING THE ITCHING STOP. An interesting home remedy can be found in the science of proteins. Most proteins become disabled at high temperatures. Applying hot water as soon as you get bitten can be an effective way to defuse the proteins found in the saliva of the mosquito and try to circumvent the entire nightmare of scratching and itching.

Glossary

Ecosystem: A geographic area where plants, animals, and other organisms, as well as weather and landscape, work together to form a bubble of life.

Complete metamorphosis: A kind of metamorphosis in which insects undergo complete physical change

Pupa: A pupa is the life stage of some insects undergoing transformation between immature and mature stages

Siphon: A siphon is a tubular organ of the respiratory system of some insects that spend a significant amount of their time underwater, that serves as a breathing tube.

Proboscis: The mouth part of mosquitoes used to puncture the skin and gather blood from humans and other mammals.

Platelets: tiny blood cells that help your body form clots to stop bleeding.

Histamines: A chemical found in some of the body’s cells associated with allergic symptoms.

Cortisol: An essential hormone that affects almost every organ and tissue in your body.

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Author

  • Joseph Sahakian has a Master's in Biology and a minor in Psychology from Notre Dame University. Joseph is a strong advocate of scientific communication and he tries his best to break the jargon barrier to make scientific progress accessible to all. In his free time, Joseph enjoys reading and creative writing.

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