Why Are Mosquitoes So Thirsty For Our Blood, and What Does It Mean for the Way They Spread Disease?

The Context: Several infectious diseases such as Zika, dengue, yellow fever, and Chikungunya virus are transmissible through mosquito bites. Not all types of mosquitoes bite humans, however, and the reasons why some show a preference for biting us is not well understood.

The Study: Mosquitoes in dry and urbanized environments tend to show affinity for biting humans rather than animals, finds a new study in Current Biology by NYSCF – Robertson Neuroscience Investigator Lindy McBride, PhD, of Princeton University.

The Importance:  Understanding why certain mosquito species have developed a preference for biting humans will help scientists track and contain infectious diseases spread through mosquito bites.

If you’ve ever gone camping, or just taken a summer walk, it likely wasn’t long before the mosquito bites started showing up. At their most benign, they are a nuisance, but in certain areas of the world, mosquito bites can transfer infectious diseases like Zika, dengue, or yellow fever.

Surprisingly, of the over 3,000 species of mosquitoes buzzing around the Earth, only a few have developed a taste for human blood. If we want to understand and contain the infectious diseases they spread, we have to understand what drives them to bite humans. New research in Current Biology by NYSCF – Robertson Neuroscience Investigator Lindy McBride, PhD, of Princeton University finds that two main factors drive mosquitos to target us: dry environments and urbanization. 

The Perfect Mosquito Candidate 

The researchers focused their studies on Aedes aegypti, a species of mosquito notorious for spreading infectious disease. Aedes aegypti are found in Africa, where many mosquitos do not bite humans, and studying what has caused this particular species to start seeking human blood could shed light on why this happens across all human-biting mosquito species.

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

Why Humans?

The team took a closer look at where exactly Aedes aegypti mosquitoes were biting humans more than animals. The researchers set up traps to collect mosquitoes from 27 regions in sub-saharan Africa. They then ran laboratory tests to see whether the mosquitoes preferred the scents of humans or animals. The results showed that mosquitoes from very dense, modern cities developed an affinity for human blood, as did mosquitoes from areas with longer, hotter seasons. The team discovered several genes that may play a role in driving this evolutionary shift in biting preference.

“Mosquitoes living near dense human populations in cities such as Kumasi, Ghana, or Ouagadougou, Burkina Faso, showed increased willingness to bite human hosts,” said Noah Rose, PhD, a postdoctoral fellow in Dr. McBride’s lab and lead author on the study. “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. We think this is because mosquitoes in these climates are especially dependent on humans and human water storage for their life cycle.”

“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,” Dr. Rose continued. “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,” agreed Dr. 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.”

Climate Change, Urbanization, and the Future

While climate change is not expected to greatly impact the dry season dynamics that influence mosquito behavior, increased urbanization will likely play a role. As cities in sub-saharan Africa become more crowded over the next 30 years, that could mean more mosquito bites, and more spread of disease.

Looking forward, the team plans to further study the interplay between climates, urbanization, and mosquito biting, as well as the genetic drivers of biting preference, hopefully identifying methods for containing the spread of disease.

Journal Article:

Climate and Urbanization Drive Mosquito Preference for Humans
Noah H. Rose, Massamba Sylla, Athanase Badolo, Joel Lutomiah, Diego Ayala, Ogechukwu B. Aribodor, Nnenna Ibe, Jewelna Akorli, Sampson Otoo, John-Paul Mutebi, Alexis L. Kriete, Eliza G. Ewing, Rosemary Sang, Andrea Gloria-Soria, Jeffrey R. Powell, Rachel E. Baker, Bradley J. White, Jacob E. Crawford, Carolyn S. McBride. Current Biology. 2020. DOI: https://doi.org/10.1016/j.cub.2020.06.092

Photo credit: Sameer A. Khan

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