Mosquitoes can get their fill of blood from many animals other than humans and, for the vast majority of the more than 3,000 mosquito species in the world, most do, says Noah Rose, a mosquito researcher at Princeton University. But a few just seem to love the taste of human blood.
“They are the ones that spread the most disease,” Rose says. “Yellow fever, Zika, chikungunya, dengue. No one knew quite why those mosquitoes evolved to have a preference for human hosts. If we understood [that], then we could understand more about why they spread disease so effectively and ways to stop them.”
That would be a welcome advantage for humans in many parts of the world, including Massachusetts, where in addition to causing itchy red welts, mosquitoes can also spread diseases like West Nile virus and Eastern equine encephalitis, both of which have already been detected in mosquito samples this summer.
A few years ago, Rose started studying why some mosquitoes evolved to have a preference for human blood. For three years, he and his colleagues traveled around sub-Saharan Africa, the ancestral home of the human-biting mosquito Aedes aegypti. These mosquitoes live all around the world now, where they almost exclusively feed on people. But in their native range in Africa, Rose says Aedes aegypti have a wide degree of preferences.
“They’ll bite lots of stuff. Humans are one of them,” Rose says.
To capture mosquitoes for the study, the scientists put out cups filled with water to lure the mosquitoes in. A strip of paper collected their eggs. Rose wanted to gather samples from a range of environments to get a sense for how mosquitoes differ in their genetics and food preferences.
“Things like living in the middle of a dense rainforest where they rarely encounter humans, to a busy city where they only encounter humans. Places where it’s wet and rainy most days, to places where there’s a long dry season that’s quite hostile to mosquitoes,” he says. “We wanted to better understand what drives them to become adapted to humans.”
Back at the lab, Rose hatched the mosquito eggs he collected and then put them in a chamber to see if they would rather suck human blood or some other animal’s blood. On one end of the chamber, there was a human arm behind a mosquito screen. On the other end, there would be some other animal like a guinea pig, a rat or a dog, also behind a screen.
Rose found that urban mosquitoes also had more of a taste for people,
but that wasn’t always the case. In some populated areas that get regular rainfall, Rose says mosquitoes preferred the non-human option. It was the mosquitoes from places with a long dry season, he explains, that showed the greatest desire to consume human blood.
Rose thinks that means climate is the main reason why some mosquitoes evolved to prefer humans. In order to survive in dry places, where the only available water and blood came from humans, mosquitoes needed to learn to love the taste of people, Rose says.
“In these places, the mosquitoes are dependent on human water storage to make it through that long, hot, dry season,” Rose says.
When the researchers ran ecological on this hypothesis, they found that the models again predicted that the intensity of a dry season and human population density were mostly responsible for mosquitoes evolving a taste for human blood. The researchers published their work in the journal Current Biology on Thursday.
“The only good news is that this is a very scientifically rigorous paper,” says Leslie Vosshall, a biologist at Rockefeller University who did not work on this study. “The key conclusion, which is very bad news, is that a dry climate will drive mosquitoes toward humans. And humans, we’re making the planet hotter and more drought prone.”
Vosshall says the study suggests as climate change continues, mosquitoes may desire human blood even more than they already do, particularly in sub-Saharan Africa, where most would currently rather feed on other animals. That could make mosquito-borne illnesses a greater threat in the coming years, Vosshall says.
The paper also provides an explanation for why hosts of mosquitoes descend on humans everywhere, Rose says. He believes that once mosquitoes made the evolutionary jump to finding people delicious, they followed their food out of Africa and throughout the world, where they became legion.
This article was originally published on July 24, 2020.