Why do some mosquitoes have a taste for human blood?

Even though there are more than 3,000 species of mosquitoes worldwide, there are only a few of these species that bite humans. Researchers set out to determine why certain mosquitoes developed a taste for human blood. They linked the evolution of human biting to a combination of two factors: prolonged dry climate conditions and increased human population density.  

Based on the findings, which have been published by Cell Press, the experts predict that increased urbanization will lead to even more human-biting mosquitoes in the coming decades.

Study co-author Carolyn McBride is a professor in the Department of Ecology and Evolutionary Biology at Princeton University.

“Aedes aegypti mosquitoes are invasive across the global tropics, where a strong preference for human hosts and habitats makes them important disease vectors,” said Professor McBride.

“We found that in their native range of sub-Saharan Africa, they show extremely variable attraction to human hosts, ranging from strong preference for humans to strong preference for non-human animals.”

Study co-author Noah Rose said mosquitoes living near dense human populations in cities such as Kumasi, Ghana, or Ouagadougou, Burkina Faso, showed increased willingness to bite human hosts.

“But they only evolve a strong preference for human hosts in places with intense dry seasons – in particular, in the Sahel region, where rainfall is concentrated in just a couple months out of the year,” said Rose. “We think this is because mosquitoes in these climates are especially dependent on humans and human water storage for their life cycle.”

The researchers noted that while people tend to think of all mosquitoes as major pests, they are actually quite diverse and many of them do not bite humans at all. For the investigation, the team focused on Aedes aegypti – the primary spreader of dengue, Zika, yellow fever, and 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 data,” said Professor McBride.

The researchers used special traps to capture Ae. aegypti eggs from multiple sites in 27 locations across sub-Saharan Africa. In the lab, they tested each mosquito population to examine whether the insects preferred the scent of people or other animals, including guinea pigs and quail.

The study revealed that mosquitoes living in dense urban cities were attracted to people more than those from rural areas. However, the researchers found that this only applied to modern cities, which means it was not the original driver of human biting among certain populations. This led to the discovery that mosquitoes living in places with longer and hotter dry seasons showed a stronger preference for a human scent.

“I was surprised that immediate habitat didn’t have much of an effect – mosquitoes in forests and nearby towns had similar behavior,” said Rose. “We thought that maybe moving into human landscapes would be a key driver of attraction to human hosts. But it seems like mosquitoes fly back and forth too readily 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 to human hosts, but this wasn’t dependent on the precise habitat we collected them from within each region.”

“I was also surprised that climate was more important than urbanization in explaining present day behavioral variation. Many mosquitoes living in fairly dense cities don’t particularly prefer to bite human hosts.”

“I think it will be surprising to people that in many cities in Africa, this species actively discriminates against humans,” said Professor McBride. “It is only when the cities become extremely dense or are located in places with more intense dry seasons that they become more interested in biting humans.”

The researchers noted that climate change in the next few decades is not expected to drive major changes to the dry season dynamics that influence human biting in mosquitoes. But, they said, rapid urbanization could push more mosquitoes to bite humans in many cities across sub-Saharan Africa over the next 30 years.

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer