To beat mosquitos, researchers must think like mosquitos
Mosquitos are smarter than most people realize. They have four well-developed senses (sight, smell, taste, and hearing), a capacity for making decisions, and an ability to develop flavor preferences. That means, in the battle against malaria it helps to think like a mosquito.
In recent years, researchers have developed new technologies that use a mosquito’s instincts against them. Used alone, one device can’t stop the spread of malaria—a mosquito-borne disease that kills 400,000 people every year, half of which are children under the age of five in Sub-Saharan Africa. But when deployed in a swarm, like the mosquitos themselves, there’s hope they can eliminate malaria at the community level.
To understand how, Dr. Dan Strickman, a senior program officer at the Bill & Melinda Gates Foundation, takes us through the life cycle of a typical Anopheles mosquito (only about 40 species of which transmit malaria) in Mali.
For the sake of this story, let’s call our mosquito Rita (it’s only female mosquitos that bite). She was born on a hot and humid evening in October, emerging from a flooded rice field in Mali. Taking flight, her wings glistened in the setting sun. And she was ravenous.
“When this species of mosquito emerges, they must feed quickly to have the energy to fly and mate,” says Strickman. He has spent the better part of a 40-year career studying mosquito-borne disease prevention. “They need blood to make eggs, but they also need sugar in order to live.”
Flying to the edge of the rice field, Rita saw a footpath used by farm workers traveling from the nearby village of Farabale. Her powerful vision allowed her to see six feet in front of her—remarkable for an insect no bigger than a grain of rice. A mesquite tree grew at the edge of the trail, the blossoms abundant and fragrant. She landed on a flower and took a deep drink of nectar. With her energy restored, Rita flew down the path looking for a mate.
Strickman estimates upwards of 10,000 mosquitos would be found living within a three-mile radius of the village. With half of them males, you would think it easy for Rita to find a mate. But life in the natural world is dangerous for mosquitos who are vulnerable to starvation, changes in weather, and being eaten by birds and bigger insects.
“Every day, 20 percent of the adult mosquito population dies,” Strickman says. “Only about four percent live to be 15 days old, the age when female mosquitos are most likely to transmit malaria.”
By the time Rita reached the age of 15 days, she’d mated and laid eggs several times, and fed on an alternating diet of flower nectar and mammalian blood. One of her first blood meals was from an adult woman who, unbeknownst to Rita, had just recovered from malaria. Adults usually survive the disease, but children aren’t so lucky.
“When a mosquito contracts malaria gametocytes from an infected human,” says Strickman, “it takes 10 days for them to develop as mature parasites in her gut, make their way to her salivary gland and be ready for transmission. In the race to stop malaria from being transmitted, that’s when the clock starts.”
With the malaria parasites fully incubated inside her, Rita was no longer a mere nuisance—she had become one of the deadliest insects on the planet. The next person she fed on would be the unlucky recipient of these malaria parasites.
On a calm and quiet night, Rita flew into the village drawn by the concentrated emissions of carbon dioxide from the sleeping residents. She crossed a field where, in the daytime, kids played soccer. On the edge of town, she noticed a rectangular object hanging on the side of a building. It was a new technology called an attractive targeted sugar bait, or ATSB, developed by a private sector partner through IVCC (the Innovative Vector Control Consortium), with a $50.7 million grant from the Gates Foundation.
“In my career, I have never seen anything work as well as the ATSB trial completed in Mali last December,” says Strickman, a consultant on the study on behalf of the foundation.
An ATSB is made from an impenetrable sheet of plastic the size of a piece of paper. The surface is covered with liquid-filled pillows, each containing a sugary substance mixed with a pesticide. The pillows are made of a special membrane that emits the bait’s odor and allows mosquitos to feed through it, but is impenetrable to other pollinating insects.
During the two-month study, the number of mosquitos in the study area went down by 60 percent. And most of the deaths were adult mosquitos, like Rita.
“The effect on the mosquito population came very close to eliminating all transmission of malaria during the study period,” Strickman says.
The downside to ATSBs is that a single treatment hanging on the side of a building won’t do much to reduce the number of mosquitos flying around. A mosquito might choose not to feed on sugar that one time. But if there’s a trap hanging around every corner, sooner or later she’s bound to have a snack.
Rita, however, was not in the mood for sugar. She was growing a new batch of eggs and craved the nutrition only a blood meal could provide. Flying to the next house, she noticed the strong smell of carbon dioxide. The house belonged to a family, all of whom were asleep inside.
“Carbon dioxide is a powerful attractant to mosquitos because it travels a long way,” says Strickman. “Mosquitos are able to distinguish between ambient levels of the gas and those coming from a host.”
Arriving at the wall of the home, Rita smelled the gaseous scent wafting down from above. In warm climates, homes are typically built with vents under the eaves. That allows for the exchange of cold and hot air, as well as to ventilate smoke from cooking and heating sources. They also make great passageways for mosquitos to enter a home.
Researchers at In2Care, a Dutch company, realized this behavior pattern led mosquitos into a bottle neck, creating/GFO/Gates Optimists/Research/Week 1/PhotoforErinStuckeypiece an opportunity for treating them with a pesticide. They developed a commercial model of a device known as an “eave tube” which is installed near the roof of a home, along with improvements to windows and closing any open gaps. The tube has a mesh insert electrostatically coated with granules of insecticide. Because they sit well away from human contact, the concentration of insecticide can be much higher than used in proximity to humans. A mosquito trying to enter the home through the tube will bounce off the mesh, come into contact with the insecticide, and die without being able to enter the home.
“By installing eave tubes, you safe guard your home against mosquitos,” Strickman says. “It’s a targeted approach, complimentary to the ATSB’s work at reducing the mosquito population in an entire community level. You and your neighbor need to have ATSBs for them to effectively stop transmission.”
Alas, the eaves Rita was drawn up towards weren’t equipped with eave tubes. In the quiet of night, she slipped through the space and entered a dark room. She smelled the exhalations of three sleeping children. Hovering over a sleeping girl, Rita’s salivary glands were all but bursting with the need to feed. She descended towards the smell of the girl’s quiet breaths.
Suddenly, she ran into something. It was a web-like fabric surrounding the bed. Rita walked across the net, but couldn’t find a hole big enough to crawl through. The net was two-years-old, and the effectiveness of the pesticide was wearing off. Rita’s ancestors had been among the first mosquitos to successfully resist the effects of the pesticide, and she benefited from a genetic resistance.
“The problem with a lot of pesticides is there’s a period at the end of their effectiveness when the intervention allows for survivors,” Strickman says. “You can end up selecting for a mosquito population that is resistant to the insecticide.”
All children in the room had bed nets. Thwarted in her attempt to feed, Rita landed on the wall to rest. But the home had been treated with an indoor residual pesticide, which was absorbed into Rita’s body when her feet made contact with the surface. She started feeling woozy, lost consciousness, and fell to the floor.
In the morning, the children crawled out of their bed nets, asking their mother, again, why they had to go through the fuss.
“Because, dear,” the mother said. “Just, because.”
For more than a decade, people had only bed nets and pesticide sprays to protect them from malaria. It wasn’t enough. Thanks to new devices like ATSBs and eave tubes, people can outsmart mosquitos and protect themselves, their neighbors, and their communities from this deadly disease. Even better, these treatments are affordable.
Yet, mosquitos are resilient and clever foes, capable of developing resistance to pesticides and even behavior modifications that help them to evade treatment. That’s why the Gates Foundation’s $2 billion investment in eradicating malaria includes support for high-tech approaches, such as genetically-modified “friendly” mosquitos that don’t carry malaria, and a malaria vaccine that can protect people from the disease wherever they go.
October is not far away. Rice farmers in Farabale, Mali, will flood their fields. Rita’s great-granddaughters will hatch from these standing bodies of water. And they will be very, very hungry.