Why do mosquitoes bite some people more than others? | science and technology

Disease-carrying mosquitoes have killed more people than all the wars in recorded history combined. In fact, statistics indicate that the mosquito is by far the deadliest creature in the world to humans.

Mosquitoes caused an estimated 725,000 deaths in 2018 alone. The second leading cause of human death that year was other people, responsible for 437,000 deaths. Far behind were the combined deaths caused by snakes, dogs, poisonous snails, crocodiles, hippos, elephants, lions, wolves and sharks.

This worrying situation prompted the World Health Assembly in 2017 to endorse the Global Vector Control Response (GVCR) 2017-2030, an agenda that provides strategic direction to countries that urgently need to combat vector control. vectors, especially mosquitoes.

Vector control is essential for the prevention of mosquito-borne diseases and the response to epidemics. These insects 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 fever, Semliki forest virus. infection, Sindbis fever, Japanese encephalitis, Ross River virus disease, Barmah Forest virus disease and malaria, the last of which caused 627,000 deaths in 2020 alone. It is understandable that people want to know what causes mosquitoes to bite one person and not another.

Carbon dioxide and body odor

Both male and female mosquitoes can survive without biting other animals, but females need blood to complete the breeding cycle. Carbon dioxide (CO₂) was identified as an attractant for mosquitoes almost a century ago. This gas has also been used to trap female mosquitoes seeking to consume blood with the nutrients needed for oogenesis or egg generation.

However, there is no evidence that CO₂ is responsible for the differential attraction. In other words, carbon dioxide emission levels do not explain why mosquitoes consistently prefer one person over another. So what is the explanation? There are other physical and chemical cues that attract mosquitoes, such as heat, water vapor, humidity, visual cues, and most importantly, body odor.

Although the scents most attractive to mosquitoes are not well known, several studies indicate that molecules such as indole, nonanol, octenol, and lactic acid are prime 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 allows the Temples of the Egyptians mosquito to detect lactic acid. This type of mosquito is a known transmitter of dengue fever, chikungunya and Zika. When scientists mutated the IR8a receptor on the insect’s antennae, they discovered that mosquitoes were unable to detect lactic acid and other acidic odors from humans.

Acetophenone, the “perfume” that attracts mosquitoes

Recent research indicates that dengue and Zika viruses alter the smell of mice and humans they infect to make them more attractive to mosquitoes. This is an interesting contagion strategy, as it encourages mosquitoes to bite the host, extract the infected blood, and then carry the virus to another individual. Viruses achieve this by modifying the emission of an aromatic ketone – acetophenone – which is particularly attractive to mosquitoes.

Human and rodent skin normally produces an antimicrobial peptide that controls bacterial populations. However, in mice infected with dengue fever or Zika, the concentration of this peptide drops, and certain bacteria (genus Bacillus) proliferate, triggering the production of acetophenone. Something similar happens in humans: odors collected from the armpits of dengue fever patients contain more acetophenone than those of healthy people.

Interestingly, this increased emission of acetophenone can be corrected. Some of the dengue-infected mice were treated with isotretinoin, which resulted in decreased acetophenone emissions, making the host less attractive to mosquitoes.

Microbes that change smell

This is not the only example of a microorganism manipulating the physiology of mosquitoes and human hosts to enhance transmissibility. For example, people infected with Plasmodium falciparumthe parasite that causes malaria, are more attractive than healthy individuals to Anopheles gambiae mosquitoes, vectors of the disease.

The reason is not known, but may be related to Plasmodium falciparum producing an isoprenoid precursor called (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), which affects the mosquito’s bloodthirsty and bloodthirsty behaviors, as well as its susceptibility to infection. Specifically, HMBPP activates human red blood cells to release more CO₂, aldehydes and monoterpenes, which combine to attract mosquitoes and invite them to “suck our blood”.

Moreover, the addition of HMBPP to blood samples greatly intensifies the attraction of other mosquito species, such as Anopheles coluzzii, Anopheles arabiensis, Aedes aegyptiand some species of Culex pipiens/Culex torrentium complex.

Understanding the factors that make some people more attractive to mosquitoes than others will help identify and reduce the risk of transmission of vector-borne infectious diseases.

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