Wednesday, December 8th, 2021

Mosquitoes’ taste for human blood may grow as African cities expand

Where water is scarce, Aedes mosquitoes mingle more with humans.

Nigel Cattlin/Science Source

In most of the world, the Aedes aegypti mosquito is notorious for biting humans and spreading dengue, Zika, and other viruses. But in Africa, where the mosquito is native, most Aedes prefer to suck blood from other animals, such as monkeys and rodents. A new study suggests, though, that their taste for humans may rapidly expand—and with it their ability to spread disease.

By surveying the range of Aedes biting preferences across Africa, the study shows that dryness and dense populations favor strains that target people. Those conditions are likely to intensify in Africa with climate change and increasing urbanization, though not everywhere.

“The work is significant because the better we can understand where and why mosquitoes like humans, the better equipped we will be to predict and mitigate disease spread,” says Mara Lawniczak, an evolutionary geneticist at the Wellcome Sanger Institute, who was not involved in the study.

Noah Rose, a postdoctoral fellow working with Carolyn McBride at Princeton University, and African colleagues collected Aedes eggs from 27 places in sub-Saharan Africa, from the dry savanna to moist forests, and from places with varying numbers of human inhabitants. Rose used those eggs to start lab colonies and tested the biting preferences of the offspring. Placed in a plastic box with two tubes sticking out, groups of 100 mosquitoes had the option of heading down one tube with Rose’s forearm at the end or the other toward a guinea pig. (Screens prevented the mosquitoes from biting either target.)

He also sequenced the genomes of 389 mosquitoes to see how those with different preferences were related. “It [was] a huge amount of work,” says Anna Cohuet, a medical entomologist at the French National Research Institute for Sustainable Development in Montpellier, who was not involved with the work.

The mosquitoes had different, consistent preferences for human or guinea pig depending on where they were collected, Rose and his colleagues reported at the Biology of Genomes meeting, held online last week, and in a February preprint. Insects from African forests, where Aedes aegypti originated, preferred the guinea pig. Only insects from the Sahel region, the semiarid belt south of the Sahara, consistently preferred humans, the team found. For the rest, the closer their kinship to those from the Sahel region, the more likely they were to bite humans. (The Sahel mosquitoes likely spread to the Americas hundreds of years ago with the slave trade.)

One factor that tips the scales toward humans, Rose found, is population density. With more people around, Cohuet explains, “being specialized [on humans] becomes more efficient.” But that couldn’t be the full explanation, because mosquitoes collected from several towns were not human-centric. Instead, a hot and dry climate for most of the year, with just a short rainy season—the conditions found in the Sahel—seems to foster a taste for humans. In those places, Rose says, the mosquitoes seem to become more dependent for breeding on water stored by people or trapped in humanmade items such as tires.

“The implication is that if we have rapid urbanization in areas with these dry seasons, you are going to have a proliferation of these strains that bite humans,” says Elaine Ostrander, a geneticist at the National Human Genome Research Institute. The potential for the spread of dengue and other Aedes-transmitted diseases worries Ostrander. “That could be absolutely devastating,” she says.

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