The Mosquito Menace

Along hundreds of miles of Lake Victoria’s shoreline in Kenya, a squadron of young scientists and an army of volunteers are waging an all-out war on a creature that threatens the health of more people than any other on Earth: the mosquito.

They’re testing new insecticides and ingenious new ways to deliver them. They’re peering into windows at night, watching for mosquitoes on sleeping people. They’re collecting blood—from babies, from moto-taxi drivers, from goatherds and their goats—to track the parasites the mosquitoes carry.

But Eric Ochomo, the entomologist leading this effort on the front lines of global public health, stood in the swampy grass, laptop in hand, and acknowledged a grim reality.

“It seems,” he says, “as though the mosquitoes are winning.”

Less than a decade ago, we humans appeared to have gained the clear edge in the long fight against the mosquito. But over the past few years, that progress has reversed.

A group of young scientists and volunteers are working along the coast of Lake Victoria in Kenya. They are fighting a creature that threatens the health of more people than any other on Earth. They are attacking the mosquito.

They’re testing new insecticides and new ways to deliver them. They’re looking into windows at night. They are watching for mosquitoes on sleeping people. They’re collecting blood from everyone including babies, moto-taxi drivers, and goatherds. They are even testing the goats. They are tracking the parasites the mosquitoes carry.

Eric Ochomo is the entomologist leading this effort on the front lines of the global public health crisis. He acknowledged a grim reality while standing in the swampy grass.

“It seems,” he said, “as though the mosquitoes are winning.”

Less than 10 years ago, we appeared to have gained the clear advantage in the long fight against the mosquito. But over the past few years, that progress has reversed.

The insecticides, used since the 1970s to spray in houses and on bed nets to protect sleeping children, have become far less effective. After declining to a historic low in 2015, cases of malaria—a mosquito-borne disease—are rising. So are malaria deaths.

Climate change has brought mosquitoes carrying viruses that cause dengue and chikungunya, excruciating and sometimes deadly fevers, to places where they’ve never been before. Once a purely tropical disease, dengue is now being transmitted in Florida and France. This past summer, the U.S. saw its first locally transmitted cases of malaria in 20 years, with nine cases reported, in Texas, Florida, and Maryland.

“The situation has become challenging in new ways in places that have historically had these mosquitoes,” says Ochomo, “and also, at the same time, other places are going to face new threats because of climate and environmental factors.”

The insecticides sprayed in houses and on bed nets have become less effective. They have been used since the 1970s to protect sleeping children. Malaria, a mosquito-borne disease, had declined to a historic low in 2015. But now cases of malaria are rising and so are related deaths.

Climate change has brought mosquitoes carrying viruses to places where they’ve never been before. Now cases of dengue and chikungunya, excruciating and sometimes deadly fevers, are being seen in these places. Once a purely tropical disease, dengue is now being transmitted in Florida and France. This past summer, the U.S. saw its first locally transmitted cases of malaria in 20 years. Nine cases were reported, in Texas, Florida, and Maryland.

“The situation has become challenging in new ways in places that have historically had these mosquitoes,” says Ochomo, “and also, at the same time, other places are going to face new threats because of climate and environmental factors.”

Only female mosquitoes bite; they need the protein in human and animal blood to produce eggs. When mosquitoes bite infected humans, the insects can become infected with some diseases. The mosquitoes can then inject the disease into the next humans they bite through their saliva. (There’s no evidence that they can transmit Covid-19.)

Malaria has killed more people than any other disease over the course of human history. But then between 2000 and 2015, malaria cases dropped by a third worldwide. Widespread use of insecticide inside homes, insecticide-coated bed nets, and better treatments caused malaria mortalities to decrease by nearly half.

Malaria deaths fell to a historical low of about 575,000 in 2019. But then they climbed over the next two years and stood at 620,000 in 2021.

Cases of and deaths from dengue, chikungunya, and other mosquito-borne infections have also started rising in many regions of the world.

Mosquitoes are highly adaptable. With more and more people protected by nets or sprays at home, the insects have begun to bite more outdoors and in the daytime, instead of indoors and at night. Because the genetic makeup of mosquitoes evolves quickly in response to changing environmental conditions, they’ve also developed resistance to the class of insecticides in wide use.

Also, a new mosquito that thrives in urban areas has come from Asia to Africa, where the spread of malaria had always been confined largely to the countryside. That change has made more than 100 million additional people vulnerable to mosquito-borne infections, researchers at the University of Oxford recently estimated.

Only female mosquitoes bite. They need the protein in human and animal blood to produce eggs. When mosquitoes bite infected humans, the insects can become infected with some diseases.

The mosquitoes can then pass the disease to the next humans they bite through their saliva. There’s no evidence that they can transmit Covid-19.

Malaria has killed more people than any other disease in human history. But then between 2000 and 2015, malaria cases dropped by a third worldwide. This is attributed to the widespread use of insecticides inside homes and insecticide-coated bed nets. Better treatments caused malaria mortalities to decrease by nearly half.

Malaria deaths fell to a historical low of about 575,000 in 2019. Then deaths began to rise over the next two years, and in 2021, 620,000 people died of malaria.

Cases of and deaths from dengue, chikungunya, and other mosquito-borne infections have also started rising in many regions of the world.

Mosquitoes are highly adaptable. In the past, mosquitoes used to bite indoors and at night. However, with more and more people protected by nets or sprays at home, the insects have begun to bite more outdoors and in the daytime. They have also developed resistance to commonly used insecticides because the genetic makeup of mosquitoes evolves quickly in response to changing environmental conditions.

In Africa, malaria used to be confined mainly to the countryside.  But a new mosquito that thrives in urban areas has come to these regions. Now more than 100 million additional people are exposed to mosquito-borne infections, according to researchers at the University of Oxford.

The multiplying risks, experts say, mean there’s an urgent need for a method to protect people from all mosquitoes—one that will help defend against malaria and also dengue, yellow fever, and whatever pathogen lurks around the corner.

But it takes a decade or more to design, develop, test, and produce a new technology or intervention. Contrast that with the six-week life span of mosquitoes, which are constantly evolving to elude the ways we try to kill them.

Most of the current insecticides in use are pyrethroids, which were developed in the 1970s and derived from the chemical compounds in an ancient mosquito deterrent made by crushing aster flowers. These insecticides have been used for everything, including coating bed nets and spraying on walls. But with mosquitoes around the world now highly resistant to them, there’s an urgent search for something new.

The world’s multiplying mosquito problems increasingly require solutions tailored to specific situations, experts say: What works to protect children in Africa’s Sahel region may not safeguard loggers in forests in Cambodia.

The multiplying risks, experts say, mean there’s an urgent need for a method to protect people from all mosquitoes. It will need to help defend against malaria and also dengue, yellow fever, and whatever pathogen lurks around the corner.

But it takes a decade or more to design, develop, test, and produce a new technology or intervention. Contrast that with the six-week life span of mosquitoes, which are constantly evolving to resist the ways we try to kill them.

Most of the current insecticides in use are pyrethroids. They were developed in the 1970s and derived from the chemical compounds that are made by crushing aster flowers, an ancient mosquito repellent. These insecticides have been used for everything, including coating bed nets and spraying on walls. But now mosquitoes around the world are highly resistant to these compounds. There’s an urgent search for something new.

The world’s multiplying mosquito problems increasingly require solutions tailored to specific situations. Experts say what works to protect children in Africa’s Sahel region may not keep loggers safe in the forests of Cambodia.

In the towns and villages of Busia County in Kenya, the roads begin to fill while the sky is still the streaky purple of dawn, with farmers on the way to their fields, children in freshly pressed uniforms walking to school, and moto-taxi drivers reporting to the market. Four in 10 people on these red-dirt roads are carrying the malaria parasite, even if they don’t have symptoms.

Ochomo’s research has found that the mosquito Anopheles funestus is feasting on them: The species, once thought to bite mostly sleepers in bed at night, now bites in the daytime.

Mary Oketeti, a farmer who lives about an hour’s drive outside of Busia, gets malaria three times a year. Her 12-year-old daughter gets it twice as often. Repeated bouts of malaria keep children out of school and adults from working. Oketeti says she must stay home from the fields she farms to care for a sick family member for a few days every month.

“When there is a malaria case in the house, it suspends life,” she says.

In the towns and villages of Busia County in Kenya, the roads begin to fill while the sky is still the streaky purple of dawn. Farmers are on the way to their fields, children are walking to school, and moto-taxi drivers are reporting to the market. Four in 10 people on these red-dirt roads are carrying the malaria parasite. They might not have any symptoms.

Ochomo’s research has found that the mosquito Anopheles funestus is feasting on them. The species used to bite mostly sleepers in bed at night, but it now bites in the daytime.

Mary Oketeti, a farmer who lives about an hour’s drive outside Busia, gets malaria three times a year. Her 12-year-old daughter gets it twice as often. Repeated cases of malaria keep children out of school and adults from working. Oketeti says she must stay home from the fields where she works to care for a sick family member for a few days every month.

“When there is a malaria case in the house, it suspends life,” she says.

Ochomo is the lead investigator on two large randomized clinical trials of mosquito interventions. In one $33 million project, researchers are testing the effectiveness of spatial repellents—squares of plastic film that can be hung on the walls inside homes. They dispense low doses of a chemical that confuses mosquitoes and prevents them from biting in both dengue and malaria risk areas.

But the protection they offer is temporary, as is the funding for them. The spatial repellents must be replaced about every six months to a year. Even so, Ochomo says it won’t be hard to convince people in Busia to use the special repellents.

“People already know that nets are not enough, they need something more, and they’re happy to see us,” he says. “They say, ‘Finally, someone is coming to try to help with this.’”

Ochomo is the lead investigator on two large randomized clinical trials of mosquito interventions. In one $33 million project, researchers are testing the effectiveness of spatial repellents. These are plastic film squares that can be hung on the walls inside homes. They dispense low doses of a chemical that confuses mosquitoes. It prevents them from biting in both dengue and malaria risk areas.

But the protection they offer is temporary. The funding for them is also temporary. The spatial repellents must be replaced about every six months to a year. Even so, Ochomo says it won’t be hard to convince people in Busia to use the special repellents.

“People already know that nets are not enough, they need something more, and they’re happy to see us,” he says. “They say, ‘Finally, someone is coming to try to help with this.’”

Stephanie Nolen covers global health for The New York Times.

Stephanie Nolen covers global health for The New York Times.