We are not born a killer, we become one. The fault of the society of men, Rousseau would have said. In contact with human society … and a genetic mutation, says a team of thirty-eight researchers, from five continents, in an article published Friday, November 20 in the journal Science. Their study does not, it is true, deal with the causes of human crime. She wonders why an initially harmless mosquito turns into a machine that transmits deadly diseases.
In the matter, Aedes aegypti displays impressive performance. On its own, it transmits dengue fever, yellow fever, chikungunya and Zika. The international team, led by Louis Lambrechts at the Institut Pasteur, focused on this last virus and on the following enigma: why this pathogen, discovered in 1947 on a Ugandan monkey, did it lead to epidemics in Oceania, Asia, America, but hardly ever in Africa?
Louis Lambrechts and his colleagues had a serious clue. We know thatAedes aegypti has split into two subspecies. A few thousand years ago, a drying up of the African continent pushed some mosquitoes out of the humid forests. They adapted to new terrain, through mutation, and changed targets: non-human primates, our vampires opted for humans. And it was thanks to the triangular trade that they left Africa, headed for America and then the rest of the world. Mutants Aedes – since only females feed on blood – stinging more, no wonder they have spread the virus outside the African continent.
A fortuitous development
“It’s true, but that’s only part of the story”, emphasizes Louis Lambrechts. In reality, this “globally invasive” subspecies is also much more susceptible to the virus. When it bites a sick human, it becomes infected more, and thus transmits the pathogen in greater quantities to its next target. To show this, the researchers offered insects from eight provenances (three from Africa, five from outside Africa) with infected blood meals. They then looked for the virus and found that African populations were more protected. “It could be due to the origin of the virus, continues the virologist. We then offered them six distinct strains of the virus, from Africa or elsewhere: whatever the strains, we found the same difference. “
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