The Curious Case of Mosquito Bites

by Manya Awal | SQ Blogger | SQ Online (2021-2022)

The leaves crackled under the soles of our shoes as we made our way from one part of the island to the other. A few beams of light pierced through the thick overhead canopy as my family and I trudged through the jungles of Seychelles. Although we didn’t choose the best time to traverse this terrain, I was grateful to have the company of three others. As I climbed uphill, however, I realized we were not actually limited to three. They had stealthily been there all the while, but I realized only as I felt a few red, itchy bumps on my skin. The buzzing noise clearly indicated their presence. There they were, a scourge of mosquitoes buzzing around, searching for their next prey. Hours later, as we reached our destination, my hands did not seem to get enough of the itching.While my family left the jungle unscathed, I returned with more bites than I could count. This only made me wonder why I was the human mosquito magnet. Why was it that some people were more likely to be bitten by mosquitos than others?A mosquito bite is caused by a female mosquito that feeds on the blood using its proboscis, or an intricate mouthpart system. It consists of six needle-like mouthparts called stylets that help the mosquitos suck blood easily by piercing the skin and finding blood vessels.

Image: A closeup of the proboscis of the Aedes aegypti mosquito, sucking on blood (Source)

What is really intriguing is that male mosquitoes don’t have the ability to suck blood, as male proboscis are not strong enough to pierce the skin. They can’t feed on blood, and they don’t need to as they don’t produce eggs nor need the protein that they derive from humans. I think I speak for everyone when I say that mosquito bites are not pleasant at all, to say the least. While sucking blood, female mosquitoes inject their saliva into our skin. The saliva contains proteins and an anticoagulant that stops blood from coagulating or clotting. These proteins are recognized as foreign to the body and trigger the immune system. Histamines, or organic nitrogenous compounds, are released as an immune response to help the white blood cells reach the affected area. This release of histamines is what causes itchiness, inflammation, and swelling—symptoms that plagued me and only seemed to worsen as I walked around with my family in Seychelles.

The next day, as I sat by the beach and watched the waves crash onto the shore, I immediately thought of supplementing the layer of sunscreen on my body with a thick film of bug spray. Growing up, I had heard the phrase “prevention is better than cure” so many times that it was best to just take my mum’s advice and (try) keep the mosquitos at bay. Despite my best efforts, I was, unfortunately, bitten yet again! While trying not to lose my calm, my mum came up to me and said that “mosquitos like you, you have the sweetest blood.” Though I did not believe there was any scientific basis to that, it did make me wonder why I would always take one for the team and be the involuntary shield, consistently walking out of an area with the most mosquito bites relative to the people in my vicinity. 

Sometimes even clothes and bug sprays have failed to protect me against the wrath of these insects. Only later did I learn that genetics plays a significant role in determining how attractive one is to mosquitoes. A study done with ​​Aedes aegypti mosquitoes revealed that mosquitos showed a different response to pairs of identical twins and pairs of fraternal twins. Identical twins, who are genetically the same, were similar in how attractive they were to mosquitos, while fraternal twins, who are genetically different, varied in their level of attractiveness. This study suggests that there is a genetic influence with regards to mosquito attraction.

Additionally, statistics show that relative to men, women are more likely to be attractive to mosquitos and report both a more severe itch and bigger bites. This was found to be associated with a genetic locus, rs201452941, in the Human Leukocyte antigens (HLA) region on chromosome 6. The HLA are found on most cells in the body and are inherited from our parents. These antigens are substances that help in the regulation of our immune system by producing an immune response against a foreign substance. HLA genes also help in specifically regulating human body odor by producing skin volatiles, and thus affect how attractive humans are to mosquitoes. People carrying the HLA gene Cw*07 are more likely to be attractive to mosquitoes, and it is mostly women who carry this gene. This is because this gene variation results in the production of skin volatiles that mosquitos are more attracted to. These skin volatiles are volatile organic compounds released from skin glands and microbiodata residing on the skin. Higher amounts of L-lactic acid, 2-methylbutanoic acid, tetradecanoic acid and octanal are associated with individuals highly attractive to An. gambiae mosquitoes while limonene, 2-phenylethanol and 2-ethyl-1-hexanol were associated with individuals that were poorly attractive to An. gambiae mosquitoes.

Just like me, you may be wondering how such tiny creatures have the incredible ability to know just where to bite. They are able to do so thanks to the more than 150 receptors that help them scout for victims by picking up on the skin volatiles on our bodies. Skin volatiles aren’t the only thing that they rely on. One species of mosquitoes, the Anopheles, detect the carbon dioxide that we exhale. Thus our metabolic rate, or our output of carbon dioxide as our bodies utilize energy, is significant in understanding the attractiveness of mosquitoes to certain individuals. Anything that increases our metabolic rate makes us more attractive to mosquitos, so, the next time you’re out for a run and come back with itchy bumps, you now know why.

If one were to combine the genetic and metabolic rate factors, one would find that pregnant women are the perfect victims for mosquitoes. As females, they are genetically predisposed to relatively more bites (due to the HLA region). One study also shows that pregnant women exhale 21% more carbon dioxide compared to their non- pregnant counterparts. They also tend to have higher body temperatures, which attracts mosquitoes. All these factors together lend to why pregnant women are extremely prone to mosquito bites.

Even though we now know a lot about bites, the scientific community is still searching for exact answers. And while I haven’t been able to decipher all the reasons why mosquitos love me, I now know that the next time my mum blames it on my “sweet blood,” I will actually be able to blame it on my parents’ genes.

Sources:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5390679/ 

https://search.creativecommons.org/photos/d8775880-2df6-4ce0-bb23-4b7a6c92c948 

https://www.npr.org/sections/health-shots/2015/04/22/401469931/why-do-mosquitoes-like-to-bite-you-best-its-in-your-genes 

https://www.scientificamerican.com/article/do-mosquitoes-bite-some-people-more-than-others/ 

https://www.sciencedirect.com/science/article/pii/S1567134813001986