How Mosquitoes Evolved to Bite Humans (and What That Means for the Future)

Not all mosquitoes are the same, even if they look the same when they bite you.

There are about 3,500 species of mosquitoes worldwide, but only a few are responsible for spreading infectious diseases to humans.
Among the mosquitoes that thirst for human blood is the Aedes aegypti, the main spreader of yellow fever, Zika virus, dengue fever and chikungunya virus.

But how did Aedes aegypti mosquitoes evolve to specialize in biting people? New research, published Thursday in a journal, revealed that two factors played a role: dry climate and city life.

Scientists studied these mosquitoes in sub-Saharan Africa and discovered that the insects actually have powerful palates.

"There is great diversity in mosquito preferences. Some like to bite humans and some don't," explained paper co-author Noah Rose, a postdoctoral researcher at Princeton University in New Jersey.

"At first we thought that mosquitoes that lived near people would just like to bite people and that those that lived in the woods wouldn't like it," Rose said. "We were very surprised that this was not the case," he added.

While mosquitoes living near high-density cities, such as Kumasi in Ghana or Ouagadougou in Burkina Faso, showed a greater willingness to bite humans, the researchers found that city life alone does not explain the evolution of these insects. In fact, mosquitoes living in large cities prefer to bite animals rather than humans.

That's where the second factor, dry climate, comes in. In areas with severe dry seasons, such as the Sahel region of Africa -- which stretches from Senegal to Sudan and Eritrea -- mosquitoes evolved to have a strong preference for humans.

"Mosquitoes depend on water containers for their larvae," Rose commented. "So, in places with an intense and long dry season mosquitoes become very dependent on humans storing water," he added.

The evolution of mosquitoes to bite humans is a consequence of their dependence on breeding in areas close to human city life. That means that urbanization in the coming decades could lead to more mosquitoes biting humans in the future.

Urbanization may lead to greater change in mosquitoes
Sub-Saharan Africa is expected to have an increase in the total number of births over the next few decades, as well as longer life expectancy, according to United Nations population projection data. That implies an estimated population increase of 1.005 million in sub-Saharan African countries over the next three decades, for a projected total population of more than 2.1 billion people.

The new research predicts that this rapid urbanization will drive mosquito evolution and generate a shift toward human bites in many large cities by 2050.

"We should be watching these mosquitoes closely," Rose said, noting that the future of these insects' adaptations is still uncertain.

"We really don't know what will happen when the urbanization of sub-Saharan Africa goes beyond what we see today. But we do know that something will happen and we think it will be a shift to biting more humans," he added.

And that also means that the way mosquitoes transmit disease could change.

Rose said that if scientists and public health officials take into account how urbanization and climate change will affect mosquitoes, people can be better prepared to deal with changes in the way diseases spread.

But in the short term, the researchers said, climate change is not expected to create major alterations in the dry-season dynamics that impact mosquito behavior.

Mosquitoes can be in different habitats
It took the international team of scientists more than three years to conduct the research and collect samples of mosquito eggs in a wide range of habitats at 27 sites in Africa.

"I was surprised that the immediate habitat didn't have that much impact: mosquitoes in nearby forests and villages exhibited similar behavior," Rose said.

"We thought that perhaps moving into human landscapes would be a key factor in attracting them to people. But it seems that mosquitoes fly too easily between these two habitats for their behavior to diverge in many cases," he explained.

Mosquitoes can live in a "mosaic of habitats" within the same region, making preventing mosquito-borne diseases an even greater challenge, Rose added.

"Even if you get rid of mosquitoes that bite humans in a given location, there's a great diversity of mosquitoes in a habitat. They are really good at problem solving and surviving in different habitats," Rose explained.

The World Health Organization (WHO) has said that mosquito control can effectively reduce the spread of vector-borne diseases such as Zika and yellow fever, but "mosquito control is complex, costly and tempered by the spread of insecticide resistance."

More than half of the world's population lives in areas where Aedes aegypti mosquitoes are present, according to WHO, and mosquitoes cause millions of deaths each year, making them one of the world's deadliest animals.

How to protect yourself from mosquitoes?

For people living in or traveling to places where mosquito-borne diseases are prevalent, there are a number of recommendations you can put into practice to prevent mosquito bites, according to the National Institutes of Health and WHO:

• Use insect repellent that has one of the following active ingredients: DEET, picaridin (known internationally as KBR 3023 or icaridin), IR3535, oil of lemon eucalyptus, para-methanediol or 2-undecanone. In the United States, be sure to use an insect repellent approved by an Environmental Agency. It is important to reapply the product every three to four hours.
• Wear clothing that covers your arms, legs and other exposed skin. You can also wear hats with mosquito netting to protect your neck and face. And, although it's not very fashionable, tucking your pants into your socks also helps.
• Put permethrin on the clothing and gear you wear, as it helps repel mosquitoes and will protect you even after multiple washings. But don't use permethrin products directly on the skin.
• Keep mosquitoes outside with screens on your windows and doors, or by leaving windows closed and using air conditioning, if possible.
• Sleep under a mosquito net if it is difficult to keep mosquitoes out of your home or hotel, or if you are traveling to an area with disease-carrying mosquitoes.
• Workplaces in areas with human-biting mosquitoes can reduce the risk to employees by eliminating standing water; removing tires, buckets, and other items that collect water; and covering and storing equipment outdoors.