Mosquitoes sucked human blood after they could not discover water

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By Michael Marshall

The Aedes aegypti mosquito evolved to bite people because we live near water

BSIP SA / Alamy

According to a study on African mosquitoes, mosquitoes evolved to bite people when they lived in places with an intense dry season. The insects need water to reproduce and may have got stuck on humans because we store it in large quantities.

Many mosquitoes bite a wide variety of animals, but some specialize in biting humans and, until now, no one knew why. Aedes aegypti mosquitoes often specialize in humans and bring with them diseases such as zika, dengue fever, and yellow fever. However, some African populations of this species feed more broadly.

“Nobody has actually gone through and systematically characterized behavioral differences in Africa,” says Noah Rose of Princeton University in New Jersey. For this purpose, he and his colleagues caught A. aegypti eggs from 27 locations in sub-Saharan Africa and raised them in a laboratory.

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Then they put the mosquitoes in a chamber where they could catch a whiff of human or animal – a guinea pig or a quail – to see what ways they would move to try to bite. A variety of preferences were found.

The researchers then built a model to determine what factors influenced the mosquito’s preferences. Those who lived in areas where the dry season was long and intense were much more likely to prefer people. There was also a smaller effect of urbanization: mosquitoes in cities were more likely to favor people.

A long dry season is a problem for A. aegypti, says Rose, because these mosquitoes rely on standing water to raise their young. However, people often create standing water sources, be it by storing rainwater in barrels or by irrigating grain fields. Mosquitoes that lived thousands of years ago may have been attracted to these places and evolved to bite people.

The story could be different with Anopheles mosquitoes, which spread malaria, says Rose. These mosquitoes are only distantly related to A. aegypti and have a different life cycle. “The adults can go into a state called aestheticization, in which they become dehydrated during the dry season,” says Rose.

The model suggests that by 2050 more populations of A. aegypti will prefer humans. Africa is being urbanized and this is expected to have a major impact on mosquito development. Surprisingly, climate change may not make much of a difference in the next three decades as it is not predicted to drastically change Africa’s dry season.

Journal reference: Current Biology, DOI: 10.1016 / j.cub.2020.06.092

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