Mosquitoes contribute to the transmission of deadly diseases such as Zika, dengue, Chikungunya fever, Rif Valley fever, and malaria. Of all of them, malaria is the one that most threatens people’s lives: In 2019 alone, 229 million cases and more than 400,000 deaths were recorded. Africa accounted for 67% (274,000) of all deaths in the world as a result of malaria.
The pandemic disrupts the fight against HIV, malaria and tuberculosis in poor countries: “The risk is going back years”
Malaria is caused by parasites that are transmitted to humans by the bite of infected female Anopheles mosquitoes. Different vector control strategies, such as indoor residual spraying and deployment of long-lasting insecticide nets, have played a key role in reducing malaria cases.
Yet these interventions also pose a number of problems.
On the one hand, in Africa, the insecticide resistance of most malaria vectors is increasing and continuing to increase.
On the other hand, and closely related to the above, is the fact that insecticides and mosquito nets, alone or in combination, will never be able to eradicate malaria in areas of high transmission. In particular, they are not considered at all effective in controlling anopheles vectors. They feed and rest indoors, and usually eat at night.
Consequently, new strategies are needed to complement the current controls. The key to designing them is understanding what attracts and repels these mosquitoes in relation to certain people. Precisely this opens the door to new tools and strategies, such as baits and chemical traps, to control and monitor malaria vectors.
In this sense, our research group at the University of Pretoria is working on a project that seeks to answer the following question: why do mosquitoes prefer some people over others?
We investigate if there really was any chemical difference between the skin surface of people who considered themselves attractive to mosquitoes and those who did not. In this regard, we were able to detect chemical differences between both groups.
In this way, our findings bring to the table two possible avenues of investigation. First, the chemical compounds most associated with individuals that attract mosquitoes could be used in chemical baits to trap mosquitoes outdoors. Second, it is possible to work with those other compounds that are more present in individuals that arouse less interest for mosquitoes and develop new repellants with them.
How mosquitoes find food
Female mosquitoes need to ingest blood for their eggs to develop. First, the female mosquito must find her ‘victim’. It can be very selective. For example, the mosquito Culex quinquefasciatus it feeds exclusively from birds.
Mosquitoes are in complex environments full of different signals or stimuli. Locating your favorite ‘victim’ involves a series of behaviors. It all starts when the mosquito realizes the presence of the ‘victim’. They often do this with the use of a wide variety of signals, such as carbon dioxide or visual cues.
From there, the female mosquito is guided by the signs of heat and humidity that surround the ‘victim’ and, finally, it is the body odor that influences the decision who and where to bite. These chemical signals present on the surface of the skin and used for communication within a species are called semiochemicals.
The reason why mosquitoes prefer some individuals over others lies precisely in the different semiochemicals found on the surface of human skin. The complexity of our skin poses a real challenge for chemical analysis. According to studies conducted to date from human skin secretions, there are more than 500 skin compounds. And there are many more chemicals that are not yet known.
Nowadays, thanks to sophisticated analytical techniques, we are beginning to know the semiochemicals of mosquitoes and we are even investigating possible semiochemical mixtures. It is suspected that some chemicals could be working together to attract or repel mosquitoes.
The volunteers participating in the study were compared with each other to try to discover how attractive each one was to the mosquitoes, and if they preferred to bite some areas or others of their skin.
Volatile and semi-volatile compounds (those that mosquitoes use to find and navigate their human host) of a very wide variety of chemicals (69 in total) were detected and it was concluded that these are the ones that give rise to the differences that exist in the chemical profiles of the skin’s surface.
To our knowledge, there was no prior knowledge of 31 of the compounds detected on the skin surface.
Also we set out to investigate the last step mosquitoes take in their search for a victim: how to land on the right host and then treat them to a bite. For this we use a high performance liquid chromatography system with high resolution mass spectrometry and ion mobility.
In this way, we identified 20 compounds involved in the final mosquito bite.
A way forward
The attractant or repellent compounds that we have identified could prove useful for future malaria vector control programs. Therefore, more biological studies are needed to test these compounds in female mosquitoes.
The non-invasive techniques for taking skin samples used by our research group have laid the foundations for the detailed examination of the surface of the human skin, not only for applications related to vector control, but also for its use in the usual ones. population health reviews.