Mosquitoes can fill themselves with blood from many animals other than humans, and for the vast majority of the more than 3,000 species of mosquitoes in the world most do, says Noah Rose, a mosquito researcher at Princeton University. But some just seem to love the taste of human blood.
“They are the ones who spread most diseases,” says Rose. “Yellow fever, zika, chikungunya, dengue fever. Nobody knew exactly why these mosquitoes preferred human hosts. When we understand [that]Then we could better understand why they are so effective at spreading diseases and how they can be stopped. “
This would be a welcome benefit to humans in many parts of the world, including Massachusetts, where mosquitoes not only cause itchy red welts, but can also spread diseases such as West Nile virus and eastern equine encephalitis, both of which have already been detected in mosquitos Rehearsals this summer.
A few years ago Rose began to investigate why some mosquitoes evolved to prefer human blood. For three years he and his colleagues traveled through Africa south of the Sahara, the home of the human-biting mosquito Aedes aegypti. These mosquitoes now live all over the world, where they feed almost exclusively on humans. But in their native Africa, says Rose, Aedes aegypti have a wide range of preferences.
“You will bite a lot of things. Man is one of them, ”says Rose.
To catch mosquitoes for the study, the scientists put out cups filled with water to attract the mosquitoes. A strip of paper collected their eggs. Rose wanted to collect samples from different environments to get a feel for how mosquitoes differ in their genetics and food preferences.
“Things like living in the middle of a dense rainforest where you rarely meet people, in a busy city where you only meet people. Places that are wet and rainy most days, places that have a long dry season that is very mosquito-repellent, ”he says. “We wanted to understand better what makes them adapt to people.”
Back in the lab, Rose hatched the mosquito eggs he’d collected and then put them in a chamber to see if they’d rather suckle human blood or the blood of another animal. At one end of the chamber was a human arm behind a mosquito repellent. At the other end there would be another animal like a guinea pig, rat or dog, also behind a screen.
Rose noted that urban mosquitos also tasted more tasteful to people, but that wasn’t always the case. In some populated areas where it rains regularly, according to Rose, mosquitoes prefer the non-human option. It was the mosquitoes from places with a long dry season that showed the greatest desire to consume human blood.
Rose believes that climate is the main reason some mosquitoes evolved to favor humans. To survive in arid places where only humans would provide water and blood, mosquitoes had to learn to love the taste of humans, says Rose.
“In these places the mosquitoes depend on the storage of human water in order to survive this long, hot and dry season,” says Rose.
When the researchers examined this hypothesis ecologically, they found that the models again predicted that the intensity of a dry season and human population density are primarily responsible for mosquitoes that develop a taste for human blood. The researchers published their work Thursday in the journal Current Biology.
“The only good news is that this is a very scientifically rigorous paper,” says Leslie Vosshall, a Rockefeller University biologist who did not work on this study. “The main conclusion, which is very bad, is that a dry climate drives mosquitoes towards people. And humans, we’re making the planet hotter and more prone to drought. “
According to Vosshall, the study found, mosquitoes may want more human blood than before as a result of climate change, especially in sub-Saharan Africa, where most would currently prefer to feed on other animals. That could make mosquito-borne diseases a bigger threat in the years to come, says Vosshall.
The paper also provides an explanation for why hosts of mosquitoes come down on people everywhere, says Rose. He believes that after mosquitoes made the evolutionary leap to find people delicious, they followed their food from Africa and around the world, where they became legions.