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  February 7, 2020

How mosquitoes find humans to bite

  by Lawrence Goodman, [35]Brandeis University
  How mosquitoes find humans to bite Anopheles gambiae mosquito. Credit:
  Willem Laursen, Garrity lab

  In a paper appearing online February 6 in Science, professor of biology
  Paul Garrity, Ph.D. student Chloe Greppi, post-doctoral fellow Willem
  Laursen and several colleagues report that they've figured out an
  important part of how mosquitoes hone in on human warmth to find and
  bite people.

  Mosquitoes are one of the planet's deadliest animals. Hundreds of
  thousands of people die each year from such mosquito-borne illnesses as
  malaria, dengue, West Nile virus and yellow fever, most of them
  children. Another 200 million are infected and suffer the symptoms.

  The discovery holds out the possibility of one day being able to fool
  or knock-out the insects' [36]temperature sensors so they don't spread
  disease.

  "Sensory systems like these are excellent targets for developing new
  ways to repel or confuse mosquitoes to keep them from biting us or to
  create new ways to help trap and kill these disease-spreading
  creatures," Garrity said.

  A quick history lesson

  At the beginning of the 20th century, Frank Milburn Howlett, a British
  scientist serving in India, noticed mosquitoes were always hovering
  around his teapot at tea-time. As an experiment, he filled a loose
  gauze bag with the insects and placed it near a test tube filled with
  [37]hot water.

  When warmth from the tube reached the animals, "the effect was most
  interesting," he wrote in a 1910 research paper. The mosquitoes were
  drawn to the side of the bag closest to the rising hot air.

  Howlett also observed that mosquitoes didn't seem to attack
  cold-blooded animals, suggesting that it was [38]body heat that drew
  them to humans.

  Other research has since shown that over distances of many feet,
  mosquitoes rely on the carbon dioxide we exhale, the odors we give off,
  and visual cues to find us. But when they get within a few inches, it's
  our bodies' [39]temperature that plays a major role in guiding them.

  Only the females of the species behave this way. As was later learned,
  they use the protein in our blood to nourish their eggs. Males sup only
  on fruit and plant nectar.

  The heat-seeking behavior of Anopheles gambiae mosquitoes activated by
  a puff of carbon dioxide. Credit: Greppi et al., Science (2019)

  Heat-seeking or cool-avoiding?

  Last year, Garrity and several colleagues published a paper in the
  journal [40]Neuron that upended the conventional thinking about the
  temperature-sensing receptors at the tip of flies' antennas.

  [INS: :INS]

  Traditionally, these receptors were thought to act like thermometers,
  taking the temperature of the surroundings to let the fly know if the
  environment is hot or cold. Instead, Garrity and his colleagues found
  that the receptors only detected whether the temperature was changing,
  letting the fly know if things were getting hotter or colder.

  For this reason, Garrity renamed these temperature sensors the Cooling
  Cells and Heating Cells. They're so sensitive they can detect a few
  hundredths of a degree change in temperature per second.

  Mosquitoes, who are close evolutionary relatives of flies, also have
  Cooling Cells and Heating Cells.

  While it would seem to make sense to look at the insects' heating cells
  to understand what draws them to human warmth, Garrity's group
  considered an alternative —- and counterintuitive —- hypothesis. Maybe
  it wasn't that the insects were flying toward the heat; maybe they were
  flying away from the cold. This would mean the Cooling Cells would be
  the ones to focus on.

  The specific Cooling Cells Garrity and his fellow scientists studied
  for their paper in Science rely on a molecular receptor called IR21a.
  IR stands for ionotropic receptor, a group of proteins that help
  neurons to transmit signals. IR21a facilitates the transmission of a
  signal that the temperature around the insect is falling.

  How they did it

  In their experiment, the researchers knocked out the mosquito gene
  responsible for producing the IR21a receptor. They then placed about 60
  of the mutant insects into a shoebox-sized container with a plate on
  its back wall heated to near core body temperature, 98.6 degrees, and
  gave the mosquitoes a puff of carbon dioxide to mimic human breath.

  While non-mutant mosquitoes rapidly congregated on the body temperature
  plate, trying to feed, the mutant mosquitoes largely ignored the plate.
  Without the IR21a receptor, they could no longer direct themselves to
  the hottest spot in their vicinity.

  In a second experiment, the mosquitoes were placed in a small mesh
  cage. Above the cage, the researchers placed two vials full of
  [41]human blood, with one heated to 73 degrees (room temperature) and
  the other to 88 degrees (the surface temperature of a human hand).
  Compared to non-mutant mosquitoes run through the same setup, the
  mutants showed a reduced preference for the 88-degree blood.
  Anopheles gambiae mosquitoes lacking the IR21a receptor show greatly
  reduced heat-seeking behavior. Credit: Greppi et al., Science (2019)

  "Is the world getting better or are things getting worse?"

  According to Garrity, the IR21a receptor is activated whenever
  mosquitoes move toward a cooler temperature. Since humans are usually
  warmer than their surroundings, this means that as a mosquito is
  approaching a human, IR21a is silent. But if the animal should deviate
  from its course and start to move away from its warm-blooded prey,
  IR21a becomes activated, only shutting off once the insect
  course-corrects.

  Ultimately tracking temperature change is extremely useful in helping
  these animals determine precisely where to bite us because blood
  vessels are the warmest spot on our skin.

  Garrity said IR21a seems to act like "an annoying alarm. It goes off
  whenever the female mosquito heads towards cooler climes. When they are
  seeking humans, they seem to be driven to do whatever it takes to turn
  down the sound."

  How it all began

  The gene for IR21 originated in a marine creature that lived over 400
  million years ago and eventually gave rise to both modern crustaceans
  (like lobsters and crabs) and insects.

  Once the ancestors of the modern insects finally ventured onto land,
  the gene was passed on to the common ancestor of both flies and
  mosquitoes. When the evolutionary trajectories of these insects
  diverged some 200 million years ago, each species developed different
  uses for the IR21a receptor. Flies use it to avoid warmth,
  [42]mosquitoes to find warmth and feed on human blood.

  The other authors of the study are: Gonzalo Budelli, Elaine C. Chang,
  Abigail M. Daniels '19 and Lena van Giesen of Brandeis; and Andrea L.
  Smidler and Flaminia Catteruccia of the Harvard T.H. Chan School of
  Public Health.
    __________________________________________________________________

  Explore further
  [43]How do insects feel the heat?
    __________________________________________________________________

  More information: C. Greppi el al., "Mosquito heat seeking is driven by
  an ancestral cooling receptor," Science (2020).
  [44]science.sciencemag.org/cgi/doi … 1126/science.aay9847

  C.R. Lazzari at Institut de Recherche sur la Biologie de l'Insecte in
  Tours, France el al., "In the heat of the night," Science (2020).
  [45]science.sciencemag.org/cgi/doi … 1126/science.aba4484
  Journal information: [46]Science , [47]Neuron

  Provided by [48]Brandeis University

  Citation: How mosquitoes find humans to bite (2020, February 7)
  retrieved 7 February 2020 from
  https://phys.org/news/2020-02-mosquitoes-humans.html
  This document is subject to copyright. Apart from any fair dealing for
  the purpose of private study or research, no part may be reproduced
  without the written permission. The content is provided for information
  purposes only.

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