No More Mosquitoes?

One morning this past November, biologist Nsa Dada woke up feeling sluggish. At first, she figured she’d stayed up too late the night before. But she quickly grew weaker and soon spiked a fever. “I couldn’t stand. I couldn’t eat. I couldn’t do anything,” she recalls.

Dada had recently visited relatives in the West African country of Nigeria. During the trip, she’d been bitten by mosquitoes—the very insects she studies. For most people in North America, mosquito bites are an itchy annoyance. But in many other parts of the world, the insects represent a deadly threat. When Dada became ill, “I knew immediately that I might have malaria,” she says. Malaria is a serious flu-like illness caused by Plasmodium parasites, which are carried by certain mosquitoes. When the insects bite, they pass the parasites to people.

One morning last November, Nsa Dada woke up feeling tired. The biologist figured she’d stayed up too late the night before. But she quickly grew weaker. Soon, she spiked a fever. “I couldn’t stand. I couldn’t eat. I couldn’t do anything,” she recalls.

Dada had just visited relatives in Nigeria, a West African country. During the trip, she’d been bitten by mosquitoes. Those are the insects she studies. For most people in North America, mosquito bites are an itchy annoyance. But in many other parts of the world, the insects are a deadly threat. When Dada became ill, “I knew immediately that I might have malaria,” she says. Malaria is a serious flu-like illness. It’s caused by Plasmodium parasites. Certain mosquitoes carry these parasites. When the insects bite, they pass them to people.

Malaria is one of the most dangerous diseases that mosquitoes transmit. In 2020 alone, malaria infected an estimated 241 million people across Africa, Asia, and Central and South America, and killed about 627,000. In Africa, where most cases occur, children under 5 account for three-quarters of malaria deaths. The problem is particularly bad in rural areas, where people don’t have easy access to medical care.

Fortunately, Dada received medication to treat the illness and recovered. But the ordeal was a reminder of why she studies mosquitoes. She and other scientists are working to find ways to stop mosquitoes from making people sick. Some even hope to eliminate populations of the pests entirely.

Malaria is one of the most dangerous diseases that mosquitoes transmit. Malaria infected about 241 million people across Africa, Asia, and Central and South America in 2020 alone. It killed about 627,000. Most cases occur in Africa. There, children under 5 account for three-quarters of malaria deaths. The problem is worse in rural areas. People there don’t have easy access to medical care.

Fortunately, Dada received medication for the illness. She recovered. But the problem reminded her of why she studies mosquitoes. She and other scientists are looking for ways to stop mosquitoes from making people sick. Some even hope to wipe out mosquito populations completely.

Not all mosquitoes are bloodsuckers. Only some adult females bite. Males and females feed mainly on plant nectar. But females of most species need protein from blood to produce eggs. They locate victims by sensing carbon dioxide gas exhaled by humans and other animals. A female mosquito can do this from 10 meters (32 feet) away, says Craig Montell, a neuroscientist who studies mosquitoes at the University of California, Santa Barbara.

Once the mosquito finds a target, the sight, smell, and heat of a person’s skin help guide the bug in for a landing. The insect pierces the skin with its proboscis, a needle-like mouthpart, and extracts its blood meal. As it feeds, it injects saliva. The saliva contains chemicals that keep blood flowing so the mosquito can continue drinking. Our bodies react to these substances by producing itchy welts. If the mosquito carries any pathogens—disease-causing organisms like viruses and parasites—those can enter the victim, too.

Not all mosquitoes are bloodsuckers. Only some adult females bite. Males and females mainly eat plant nectar. But females of most species need protein from blood to produce eggs. Humans and other animals breathe out carbon dioxide gas. Mosquitoes sense this gas to find victims. A female mosquito can do this from 10 meters (32 feet) away, says neuroscientist Craig Montell. He studies mosquitoes at the University of California, Santa Barbara.

When the mosquito finds a target, it heads in to land. The sight, smell, and heat of a person’s skin help guide the bug. The insect pierces the skin with a needle-like mouthpart called a proboscis. Then it draws out its blood meal. As it feeds, it injects saliva. The saliva contains chemicals that keep blood flowing. That way, the mosquito can continue drinking. Our bodies react to these substances by producing itchy welts. The mosquito may also carry disease-causing organisms like viruses and parasites. Then those pathogens can enter the victim too.

Besides malaria, mosquitoes transmit diseases including yellow fever, dengue fever, West Nile virus, and Zika virus. If a mosquito bites someone who has any of these illnesses, it can pick up the pathogens and then infect people it bites in the future. By some estimates, mosquito-borne diseases have killed nearly half of all people who have ever lived, says Montell (see Deadly Animals). “We have a lot of reasons to want to restrict the ability of mosquitoes to bite us,” he says.

Mosquitoes transmit other diseases besides malaria. These include yellow fever, dengue fever, West Nile virus, and Zika virus. If a mosquito bites someone with any of these illnesses, it can pick up the pathogens. Then it can bite other people and infect them. Some experts believe that mosquito-borne diseases have killed nearly half of all people who have ever lived, says Montell (see Deadly Animals). “We have a lot of reasons to want to restrict the ability of mosquitoes to bite us,” he says.

For decades, people have relied on insecticides to control mosquitoes. These insect-killing chemicals can be sprayed in areas where mosquitoes breed or rest. Insecticides are also applied to protective netting that people can hang over their beds. Since 2000, governments and nonprofit organizations have distributed more than 1 billion treated bed nets in Africa, Asia, and South America. This has reduced the rate of malaria significantly. But over time, insecticides lose their effectiveness. Dada will soon begin work at Arizona State University, where she’ll continue studying how mosquitoes develop resistance to these chemicals. That could help determine better approaches for managing mosquitoes.

For decades, people have used insecticides to control mosquitoes. They spray these insect-killing chemicals in areas where mosquitoes breed or rest. Insecticides are also applied to protective netting. People can hang the nets over their beds. Governments and nonprofit organizations are working on this. Since 2000, they’ve distributed more than 1 billion treated bed nets in Africa, Asia, and South America. This has greatly reduced the rate of malaria. But over time, insecticides lose their effectiveness. Dada will soon begin work at Arizona State University. She’ll continue studying how mosquitoes develop resistance to these chemicals. That could help reveal better ways to manage mosquitoes.

Some scientists hope to reduce the insects’ ability to transmit disease. In Australia, Brazil, and Indonesia, researchers have infected mosquitoes with Wolbachia bacteria, which reduces the transmission of dengue fever to people. In the wild, those infected mosquitoes pass the bacteria to their offspring, further halting the spread. In some areas, this strategy has cut cases of dengue fever by more than 75 percent. Scientists are studying whether the same bacteria can stop other mosquito-borne diseases too.

Other researchers are altering mosquitoes’ DNA—the molecule that carries hereditary information (see Rewriting the Code). Scientists have modified male mosquitoes with mutations, or genetic changes, that make them unable to produce offspring. Montell’s lab is exploring genetic changes that disrupt mosquitoes’ senses. That could make it harder for the insects to find humans. Omar Akbari, a geneticist at the University of California, San Diego, is experimenting with genes that could make mosquitoes less likely to harbor pathogens. “We’re engineering the mosquitoes to do what we want,” he says.

Some scientists hope to reduce the insects’ ability to transmit disease. In Australia, Brazil, and Indonesia, researchers have infected mosquitoes with Wolbachia bacteria. This reduces the transmission of dengue fever to people. In the wild, those infected mosquitoes pass the bacteria to their offspring. That further reduces spread. In some areas, this method has cut cases of dengue fever by more than 75 percent. Could the same bacteria stop other mosquito-borne diseases? Scientists are trying to find out.

Other researchers are changing mosquitoes’ DNA. This molecule carries hereditary information (see Rewriting the Code). Scientists have altered male mosquitoes so they can’t produce offspring. They’ve done this through genetic changes called mutations. Montell’s lab is exploring genetic changes that interfere with mosquitoes’ senses. That could make it harder for the insects to find humans. Omar Akbari is a geneticist at the University of California, San Diego. He’s experimenting with genes that could make mosquitoes less likely to carry pathogens. “We’re engineering the mosquitoes to do what we want,” he says.

Last year, the British company Oxitec conducted the first field test of genetically modified mosquitoes in the United States. Researchers released 750 million male mosquitoes in the Florida Keys to combat dengue and Zika. The males carried a newly introduced gene, or unit of hereditary material, that prevents their female offspring from hatching. Similar trials in Brazil, the Cayman Islands, Panama, and India have successfully reduced mosquito populations. Opponents argue that this approach could have unintended consequences for ecosystems. But Akbari hopes more research will make the release of modified mosquitoes a safe, accepted strategy.

Last year, the British company Oxitec ran a field test of genetically modified mosquitoes. It was the first test of its kind in the United States. Researchers released 750 million male mosquitoes in the Florida Keys to fight dengue and Zika. The males carried a newly introduced gene, or unit of hereditary material. It prevents their female offspring from hatching. Similar tests have been done in Brazil, the Cayman Islands, Panama, and India. They successfully reduced mosquito populations. Opponents argue that this could affect ecosystems in unexpected ways. But more research is being done. Akbari hopes this will make the release of modified mosquitoes a safe, accepted approach.

Is there a downside to killing off disease-spreading mosquitoes? Akbari doesn’t think so. There are more than 3,200 mosquito species, he points out, and control efforts target only the few that pose a threat to human health. Animals that eat the insects could switch to other bugs for food. But some scientists argue that ecosystems are complex, so it’s difficult to predict the effects of wiping out an entire species.

In Africa, where mosquito-borne diseases are rampant, controlling the insects would make an enormous difference, says Dada. Some people in vulnerable areas restrict their activities to reduce their risk of bites. With fewer mosquitoes, “they wouldn’t have to worry about getting sick when they go out to gather wood in the forest, work on their farms, or chat outside in the evening,” she says. “We would have healthier, happier people.”

Is there a downside to killing off disease-spreading mosquitoes? Akbari doesn’t think so. There are more than 3,200 mosquito species, he points out. Control efforts target only the few that threaten human health. Some animals eat mosquitoes. They could switch to other bugs for food. But some scientists argue that ecosystems are complex. That makes it hard to predict the effects of wiping out an entire species.

In Africa, mosquito-borne diseases are widespread. Controlling the insects there would make a huge difference, says Dada. Some people in dangerous areas try to reduce their risk of bites. So they limit their activities. With fewer mosquitoes, “they wouldn’t have to worry about getting sick when they go out to gather wood in the forest, work on their farms, or chat outside in the evening,” she says. “We would have healthier, happier people.”