To predict and help control the spread of infections transmitted by mosquitoes It is important to know where and why, evolutionarily speaking, certain of these insects have a taste for biting humans in the first place. Now, researchers have identified two main factors: dry weather and city life.
Based on these findings, they predict that further urbanization in the coming decades will mean even more human biting mosquitoes in the future, according to the study published in the magazine “Current Biology“
Of some 3,500 species of mosquitoes worldwide, only a few have specialized in biting people, becoming major spreaders of infectious diseases.
“The mosquitos Aedes aegypti They are invasive in the global tropics, where a strong preference for hosts and human habitats makes them important disease vectors, says Carolyn McBride of Princeton University. We found that in their native range in sub-Saharan Africa, they show extremely variable attraction to human hosts, ranging from a strong preference for humans to a strong preference for non-human animals. “
“The mosquitoes that live near dense human populations in cities like Kumasi, Ghana or Ouagadougou in Burkina Faso, they showed a greater willingness to bite human hosts, “adds Noah Rose, also of Princeton.
“But they only develop a strong preference for human hosts in places with intense dry seasons, particularly in the Sahel region, where rainfall is concentrated in just a couple of months a year,” he added. It is because mosquitoes in these climates are especially dependent on humans and human water storage for their life cycle. “
Photo: AP / Felipe Dana, File
People tend to think of all mosquitoes as important pests for people. But in fact, the researchers explain, mosquitoes are quite diverse. Some of them will not bite humans at all. Only a few species specialize in biting people. In the new study, the researchers focused on Aedes aegypti, the main spread of dengue, zika, yellow fever and the Chikungunya virus.
“Many people have speculated about why this species evolved to selectively bite humans, but our study is the first to address this question directly with systematic empirical dataMcBride says.
The researchers used special traps to collect eggs from Ae. aegypti from multiple outdoor venues at each of the 27 locations in sub-Saharan Africa. Back in the lab, they tested the preferences of each of those mosquito populations for the scent of people versus other animals, including guinea pigs and quail.
Their studies led to two main findings. First, they show that mosquitoes living in dense urban cities were more attracted to people than those in more rural or wild places.
However, the researchers note that this only applies to particularly dense modern cities and is therefore unlikely to be the original reason that a certain population of Ae. The mosquitos aegypti They evolved to specialize in human biting.
Their second discovery was that mosquitoes living in places with longer, warmer dry seasons showed a strong preference for a human scent versus an animal.
Photo: AP / USDA, file
“I was surprised that the immediate habitat didn’t have much of an effect: mosquitoes in forests and nearby cities behaved similarly,” Rose says. “We thought perhaps moving to human landscapes would be a key pull factor for human hosts. But mosquitoes seem to fly too fast between these habitats for their behavior to diverge in many cases. “
“When we took a more regional view of things, we saw that regions with dense human populations had mosquitoes with a greater attraction for human hosts, but this did not depend on the precise habitat in which we collected them within each region,” Rose continues. .
“I was also surprised that climate was more important than urbanization to explain current behavioral variation,” he adds. “Many mosquitoes living in fairly dense cities do not particularly prefer to bite human hosts.”
“I think it will be surprising to people who in many cities in Africa, this species actively discriminates against humans,” says McBride. “Only when cities become extremely dense or are they located in places with more intense dry seasons, are they interested more on biting humans. “
The researchers show that many genes concentrated in some key parts of the genome drove this evolutionary change in mosquito bite preferences.
Based on their findings, the researchers wondered how the short-term climate change and urban growth shape the behavior of mosquitoes in the near future. And it is not good news.
The researchers say that climate change in the coming decades is not expected to lead to major changes in dry season dynamics that they found important to mosquitoes. But, they say, rapid urbanization could push more mosquitoes to bite humans in many cities in sub-Saharan Africa in the next 30 years.
The researchers will continue to study the interaction between mosquito bite preferences, climate, and urban life. They would also like to understand why mosquitoes specialize in certain hosts to begin with and which specific genes and genetic changes are most important.