Acquired taste: mosquitoes may evolve to favour humans over animals
More species of mosquito may evolve to bite humans instead of other animals and spread disease because of urbanisation, according to a scientific study.
While the vast majority of the 3,500 species of mosquito do not bite humans, scientists studied Aedes aegypti, an invasive species which has evolved a taste for humans, and become the primary spreader of infectious diseases including dengue and yellow fever.
By collecting eggs of the species at 27 sites across its ancestral range in subSaharan Africa, the researchers discovered a hitherto unrecognised variation in its preference for human odour verses other animals. Writing in Current Biology, they identified the two major factors in its taste for humans: a dry climate and city life.
According to the scientists, mosquitoes living in densely-populated urban areas were more attracted to biting people than those insects from more rural environments. Mosquitoes living in places with longer and hotter dry seasons also showed a strong preference for human over other animal scents, which researchers suggest is because human sources of water have enabled them to survive dry seasons.
“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 of Princeton University, the lead author of 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.”
The researchers tested in a lab the preferences of each of the 27 mosquito populations for the scent of people versus other animals, including guinea pigs and quail.
The mosquitoes’ immediate habitat – whether they were living in a forest or town – did not have much of an effect on their biting preferences because it appears that mosquitoes fly between nearby habitats too routinely to cause a sharp divergence in their behaviour. But when the researchers looked at the insects’ preferences at a regional level, they identified that in regions with dense human populations the mosquitoes had a greater attraction to humans.
The researchers undertook modelling to determine how global heating and rapid urbanisation currently under way in Africa might shape mosquito behaviour in the future. While climate change is not expected to cause major changes to dry season dynamics that are important to mosquitoes, the scientists predict that rapid urbanisation could cause more mosquitoes to bite humans in cities across sub-Saharan Africa over the next 30 years.
Co-author Carolyn McBride, also of Princeton University, said: “I think it will be surprising to people that in many cities in Africa, this species actively discriminates against humans. 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.”
Aedes aegypti is one of the most globally widespread mosquito species, although currently only sporadically found in Europe after accidental arrivals on ships. It is a vector for diseases including the chikungunya virus and Zika virus.