Around 3,500 mosquito species live on our planet. Most of them feed on the blood of various animals including mammals, birds and even reptiles. Only a few though have a strong preference for humans and they contribute to the transmission of vector-borne pathogens. Amongst these species, the yellow fever mosquito (Aedes aegypti) is probably the queen. Not only it feeds primarily on people (up to 95% of the times), but it also breeds in close association with us and thrives in cities. Over the years though several studies reported that different populations of Ae. aegypti show a variety of host preferences, so the trait certainly shows some variability.
The question of why some mosquitoes evolved a preference towards humans has always puzzled scientists. Some authors suggested that when people started to live as sedentary populations approximately 10,000 years ago, they may have provided a easy-to-access and reliable source of blood. Others speculated that this adaptation was triggered by the dependence of mosquitoes on ‘artificial’ sources of water provided by humans in arid regions, where natural sources are scarce.
A team of researchers decided to investigate the factors driving human preference in the yellow fever mosquito. The results, recently published in the journal Current Biology, certainly provide some interesting insights.
The team went on collecting Ae. aegypti eggs from 27 different locations in sub-Saharan Africa, and they used them to establish mosquito colonies in the laboratory. Collection sites varied greatly in terms of human population density and climate (from being arid to having rain year-round). Females from the different colonies were then tested for their preference towards human or animal odor using a two-port olfactometer. In this system mosquitoes are offered the choice of moving towards two chambers, one with the arm of a human volunteer and the other with either a guinea pig or a quail. The researchers then counted how many mosquitoes had moved in one or the other chamber. The team then tested for relationships between the host preference exhibited by the mosquito colonies and the characteristics of the location (in particular human density and climate) from which they originated.
What did the researchers find? First, that the host preference varied a lot between colonies (from extreme animal preference to clear predilection for humans). Second, that the number of people living around the locality of collection was a predictor of human preference – with population density explaining around 18% of the behavioral variation in mosquitoes. Third, two climate variables (monthly rainfall variability and precipitations level in the warmest quarter of the year) were even more significant and they explained up to 65% of host preference variation. Basically, mosquitoes showed a stronger attraction to people in areas where the rain was scarce in the hottest part of the year – basically, in places with a more intense dry season.
These findings suggest that environmental conditions during the dry season play a role in driving the host preference of Ae. aegypti, possibly due to the necessity of surviving by exploiting the water sources provided by people. The study though is not without limitations, as pointed by the authors. Other factors such as the reduced availability of animal hosts or vegetation during the dry season may force mosquitoes to stay close to humans. Also, collecting and testing insects from more locations could help refine the model predictions.
It would be very interesting to test these hypotheses in other important vectors with strong attraction to humans but different breeding ecologies, including malaria vectors. A better understanding on which factors affect host preference may help in predicting how climate change or human activities impact mosquito behavior and, ultimately, the epidemiology of vector-borne pathogens.
Rose NH, Sylla M, Badolo A, Lutomiah J, Ayala D, Aribodor OB, Ibe N, Akorli J, Otoo S, Mutebi JP, Kriete AL, Ewing EG, Sang R, Gloria-Soria A, Powell JR, Baker RE, White BJ, Crawford JE, McBride CS. Climate and Urbanization Drive Mosquito Preference for Humans. Curr Biol. 2020 Sep 21;30(18):3570-3579.e6. doi: 10.1016/j.cub.2020.06.092.