How mosquitoes got their taste for human blood and what it means for the future

Date:
July 23, 2020
Source:
Cell Press
Summary:
To predict and help control the spread of mosquito-borne illnesses,
it's important to know where and why certain mosquitoes got their
taste for biting humans in the first place. Now, researchers have
identified two major factors: a dry climate and city life. Based
on these findings, they predict that increased urbanization in
the coming decades will mean even more human-biting mosquitoes in
the future.



FULL STORY ==========================================================================
Of about 3,500 mosquito species around the world, only a few have taken
to specifically targeting people for biting, making them important
spreaders of infectious diseases. To predict and help control the spread
of those mosquito- borne illnesses, it's important to know where and
why, evolutionarily speaking, certain mosquitoes got their taste for
biting humans in the first place. Now, researchers reporting in the
journal Current Biology on July 23 have identified two major factors:
a dry climate and city life. Based on these findings, they predict
that increased urbanization in the coming decades will mean even more human-biting mosquitoes in the future.


========================================================================== "Aedes aegypti mosquitoes are invasive across the global tropics, where
a strong preference for human hosts and habitats makes them important
disease vectors," says Carolyn McBride of Princeton University. "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."
"Mosquitoes living near dense human populations in cities such as Kumasi, Ghana, or Ouagadougou, Burkina Faso, showed increased willingness to bite
human hosts," adds Noah Rose, also of Princeton. "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." People tend to think of all mosquitoes as major
pests to people. But, in fact, the researchers explain, mosquitoes are
quite diverse. Some of them won't bite humans at all. Only a few species specialize in biting people. In the new study, the researchers focused
their attention 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," McBride says.

To do this, the researchers took advantage of the fact that this species
came from Africa and that many populations in Africa still don't like
to bite humans. They asked a simple question: where specifically do
the mosquitoes like humans? And, where do they prefer to bite other
animals instead?


==========================================================================
The researchers used special traps to collect Ae. aegypti eggs from
multiple outdoor sites in each of 27 locations across 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 attracted to people more than those from
more rural or wild places. However, the researchers note that this only
applied to especially dense modern cities and therefore isn't likely to
be the original reason that certain population of Ae. aegypti mosquitoes evolved to specialize in biting humans.

Their second discovery was that mosquitoes living in places with longer
and hotter dry seasons showed a strong preference for a human versus
animal scent.

"I was surprised that immediate habitat didn't have much of an effect -
- mosquitoes in forests and nearby towns had similar behavior," Rose
says. "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," Rose continues. "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," McBride says. "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 show that many genes concentrated in a few
key parts of the genome drove this evolutionary shift in mosquitoes'
biting preferences. Based on their findings, the researchers asked how near-term climate change and urban growth are expected to shape mosquito behavior in the near future. And, it's not good news.

The researchers say that climate change in the next few decades isn't
expected to drive major changes to the dry season dynamics they found
were important to mosquitoes. But, they say, rapid urbanization could
push more mosquitoes to bite humans in many cities across sub-Saharan
Africa over the next 30 years.

The researchers will continue to study the interplay between mosquitoes'
biting preferences, climate, and urban life. They'd also like to
understand why mosquitoes specialize on certain hosts to begin with and
which specific genes and genetic changes are most important.


========================================================================== Story Source: Materials provided by Cell_Press. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. 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. Climate
and Urbanization Drive Mosquito Preference for Humans. Current
Biology, 2020; DOI: 10.1016/ j.cub.2020.06.092 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200723115921.htm

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