Mosquitoes and the diseases they spread have killed more people than all wars in history combined.
In fact, according to statistics, the mosquito is by far the deadliest creature in the world to man.
In 2018 alone, this insect was responsible for around 725,000 deaths.
That same year, the second position was occupied by humans themselves, which caused the death of 437,000 congeners.
Followed (quite far) by the combined attacks of snakes, dogs, poisonous snails, crocodiles, hippos, elephants, lions, wolves and sharks.
The obviously worrying situation prompted the World Health Assembly to endorse the “Global Vector Control Response (GVCR) 2017-2030” in 2017.
This is an action aimed at strategically guiding countries for the urgent development of strengthening vector controlamong which mosquitoes stand out.
This approach is essential to prevent disease and respond to emerging infectious epidemics.
Mosquitoes can transmit diseases as diverse as West Nile fever, Zika, dengue fever, yellow fever, chikungunya, St. Louis encephalitis, lymphatic filariasis, La Crosse encephalitis, Pogosta, Oropouche fever, Tahyna virus disease, Rift Valley virus, Semliki forest virus infection, Sindbis fever, Japanese encephalitis, Ross River fever, Barmah forest fever or malaria (causing 627,000 deaths in 2020 alone).
Hence the importance of understanding what drives mosquitoes to choose to bite us and not who we have next to us.
Carbon dioxide and body odor
Mosquitoes, male and female, could live without biting other animals.
But females need blood to complete the reproductive cycle.
Almost a century ago, carbon dioxide (CO₂) was identified as a attractive to mosquitoes.
In addition, this gas has been used to trap female mosquitoes looking for blood necessary to acquire nutrients for egg generation, oogenesis.
However, no evidence is available suggesting that CO₂ mediate differential attraction.
Or what amounts to the same thing, carbon dioxide emission levels don’t explain why mosquitoes consistently prefer one person over another.
What is it then?
There are other physical and chemical cues that determine the mosquito’s attraction to certain people.
Notably heat, water vapor, humidity, visual cues and, more importantly, odors emanating from the skin.
Although it is not yet fully understood which scents are most attractive to mosquitoes, several studies point to molecules such as indole, nonanol, octenol and lactic acid as the main suspects.
A team of researchers led by Matthew DeGennaro from Florida International University (USA) has identified a unique odorant receptor, known as ionotropic receptor 8a (IR8a), which enables the Aedes aegypti mosquito to detect acid lactic.
This mosquito, by the way, is a transmitter of dengue fever, chikungunya and zika.
When scientists mutated the IR8a receptor, which is found on the antennae of insects, they discovered that mosquitoes were unable to detect lactic acid and other acidic odors emitted by humans.
The “perfume” that attracts them
On the other hand, recent research suggests that dengue and zika viruses change the smell of mice and humans those they infect to make them more attractive to mosquitoes.
This is an interesting strategy, as it encourages them to bite the host, draw its infected blood and then carry the virus to another individual.
They achieve this by modifying the emission of an aromatic ketone, acetophenone, particularly attractive to mosquitoes.
Normally, the skin of humans and rodents produces an antimicrobial peptide that limits bacterial populations.
However, it has been shown that in mice infected with dengue fever or Zika the concentration of this peptide decreases and certain bacteria of the genus Bacillus proliferate, triggering the production of acetophenone.
Something similar happens in humans: odors collected under the armpits of patients with dengue fever contained more acetophenone than those in healthy people.
What’s interesting about this is that it can be fixed.
Some of the dengue-infected mice were treated with isotretinoinwhich has led to a reduction in emissions of acetophenone and therefore reduces the attraction to mosquitoes.
Microbes that change the smell
This is not the only case where a microorganism manipulates the physiology of mosquitoes and human hosts to promote its transmission.
For example, people infected with the parasite responsible for malaria, Plasmodium falciparum, are more attractive than healthy individuals to Anopheles gambiae mosquitoes, the vector of the disease.
The reason remains unknown, but could be related to the fact that Plasmodium falciparum produces an isoprenoid precursor, called (E)-4-hydroxy-3-methyl-but2-enyl pyrophosphate (HMBPP), which affects foraging behaviors and mosquito blood supply. as well as susceptibility to infections.
Specifically, HMBPP activates human red blood cells to increase the release of CO₂, aldehydes and monoterpenes, which together attract the mosquito more strongly and invite it to “suck our blood”.
Additionally, by adding HMBPP to blood samples, greatly increases attraction which arouses in other species of mosquitoes, such as Anopheles coluzzii, Anopheles arabiensis, Aedes aegypti and species of the Culex pipiens/Culex torrentium complex.
understand what are the factors that influence the preference shown by mosquitoes to bite one person or another will determine and reduce the risk of spreading infectious diseases transmitted by vectors.
By Raúl Rivas González
*Raúl Rivas González is professor of microbiology at the University of Salamanca. His original article was published on The Conversation which you can read here.