Dense human population and dry climate drove mosquitoes to develop an appetite for human blood: Study
Mosquitoes have varied tastes. While most are not fussy, only a few specifically target humans and spread infectious diseases such as zika, dengue and chikungunya. To develop ways to control the disease spread, scientists have to answer a question: What factor drove these insects to develop a taste for people?
According to a new study, long hot, dry seasons and recent increases in human population density are responsible. By 2050, scientists expect mosquitoes to evolve to bite humans in many big cities. “Mosquito evolution is related to human history,” Noah Rose, a postdoctoral research fellow at Princeton University, the first author of the study, said in a statement. The findings suggest that human behavior has had a bearing on mosquito evolution. “They evolved in response to changes in how humans live, they spread around the world in response to historical events, and they’re spreading disease in a way that reflects that," Rose explained. For instance, storing water for drinking, cooking and washing has helped the eggs hatch.
One species that specifically sucks human blood is Aedes aegypti, which spread dengue, zika, chikungunya and yellow fever. They flourish in urban regions across the American and Asian tropics. The species makes over 100 million people sick each year. The World Health Organization estimates that more than half of the world's population lives in areas where the species exist.
“What sets this species apart is that it specifically targets humans,” said Carolyn “Lindy” McBride, an assistant professor of ecology and evolutionary biology and neuroscience at Princeton University and the senior author of the paper. McBride also points put that not all individuals from the same species target humans the same way.
To learn more about these mosquitoes, scientists collected eggs from 27 locations in Sub-Saharian Africa. They covered seven countries, including both urban and rural regions, along with dry and cold places. These eggs were made to form 50 colonies. The team then tested how the lab-bred mosquitoes responded to smell — from humans and another animal. They were placed in two chambers. Mosquitoes from the first chamber were exposed to the lead author's hand while those in the other had a guinea pig. The results showed that mosquitoes from urban and dry regions showed an affinity for humans. “It wasn’t living with people per se that made mosquitoes specialize in biting humans,” Rose said. “It was actually them adapting to these really hot and dry places where they lived intimately with humans.”
Rose and her team also scanned the genes of mosquitoes that only target humans and compared them with those that had a broader taste. They found that the former group had some hotspots in their genome, supporting their appetite for people's blood. “When we see those genes flowing into new populations, mosquitoes in those populations start biting humans,” explained Rose. Climate change and increased urbanization could help human-loving mosquitoes to thrive.“[Climate] is changing, but it’s not changing quickly enough and in the right ways to cause major changes in preference [by 2050],” Rose said.
The study is published in Current Biology.