Jun 30, 2021 8:00 AM

The Experimental African Houses That Outsmart Malaria

A field test of custom-designed homes proves that when carbon dioxide can flow out, mosquitoes stay out too.

Image may contain Animal Invertebrate Insect Arachnid Spider and Mosquito

When Steve Lindsay first traveled to Gambia in 1985, he met a man living in Tally Ya village whom he remembers as “the professor.” The professor knew how to keep the mosquitoes away.

That’s a big deal for people who live in this small West African country, which serves as the namesake for one of the most deadly bugs on the planet: Anopheles gambiae. “It’s probably the best vector of malaria in the world,” says Lindsay, a public health entomologist at Durham University in the United Kingdom. Malaria kills 384,000 people a year in Africa, 93 percent of whom are under 5 years old. The mosquito exploits human behavior by feeding at night when people are sleeping, transmitting the Plasmodium parasite that causes flu-like symptoms, organ failure, and death. “It's adapted for getting inside houses and biting people,” says Lindsay.

But many houses in Gambia aren’t very well adapted for keeping the mosquitoes out. Sleeping people are an unguarded buffet for the insects, which are attracted to carbon dioxide. A home full of stagnant, exhaled air, and the complex cocktails of body odor, lure them in like flesh-seeking missiles. Mosquitoes are able to get inside because many of the houses have thatched roofs made of mud and dry vegetation, which often leaves gaps under the eaves. These houses often don’t have windows, and when they do, they don’t always have screens. And while there is an existing solution for this problem—insecticide-treated bed nets—nets exacerbate the uncomfortable heat. That’s a big reason why people don’t always use them.

The professor had figured out that the way to avoid getting bitten wasn’t just nets; it was architecture. He had filled in the holes in his home’s eaves. “We asked him: ‘Why do you do that?’” recalls Lindsay.

“So I get fewer mosquitoes coming in,” he replied.

Outside of their homes, some people build “banta bas,” knee-high stick platforms where they rest on warm evenings. “But his was 2 meters high, under a tree. We said, ‘Why do you build your banta ba up there?’” Lindsay says. Again, the professor replied, the height was a ploy to evade mosquito bites.

So starting in 2017, Lindsay’s team began building small experimental huts to test which designs would keep mosquitoes out and let people remain cool and comfortable. Their tweaks, which ranged from adding small screened windows to raising the homes on stilts, made a huge difference. Some configurations dropped mosquito visits by up to 95 percent. Lindsay’s team published the results in two reports of the Journal of the Royal Society Interface in May.

The results are encouraging to experts who say that improved housing can save children from malaria. “Creating a mosquito-free house does not necessarily mean building an opaque house,” says Fredros Okumu, a biologist with the Ifakara Health Institute in Tanzania not involved in the work. This evidence shows that comfort and design are not at odds with preventing malaria sustainably, he adds. “It simply means putting together these beautiful design features so that, even if you're a low-income person in a small house, you can still have a livable house that is also mosquito-proof.”

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The population of Sub-Saharan Africa is expected to double by 2050—adding 1.05 billion people—according to a 2019 report from the United Nations. This growth has catalyzed a boom in new housing. Urbanization and luxury developments are increasingly common, but so are “informal” residences that often lack basic infrastructure. These rural homes remain susceptible to mosquitoes carrying the malaria parasite. Warm, muggy places with pools of water are prime real estate for mosquitoes like An. gambiae, which lay their eggs in shallow sunlit puddles. Humans have a knack for leaving behind such surface waters, whether in the form of an irrigated field or a large flooded tire print in the mud.

So for that reason, the tests the team ran in 2017 took place near a rice field. The mud houses with metal roofs they built to test what design choices best thwart mosquitoes were intentionally designed to let some in: They all had badly fitting doors, since these gaps reflect a common mosquito entry point in rural homes. Volunteers from the neighboring Welingara village signed on to sleep two per house, and the researchers set up light traps—which lure female mosquitoes with light and suck them into a net with a fan. (Male mosquitoes don’t feed on people or transmit disease.)

Courtesy of Steve Lindsay

Over the next four months, the researchers kept making changes to the homes, then counting the mosquitoes to isolate the effect of each architectural decision. That way they could compare the benefit of, say, having one screened window, or three, or none. “As the area of screening on the windows goes up, the number of mosquitoes declines quite precipitously,” Lindsay says. They caught 95 percent fewer mosquitoes in homes with three large screened windows compared to homes with solid metal “windows.”

What Lindsay’s team found is that mosquito-proofing doesn’t have to mean creating an impenetrable fortress, but can rather be about letting exhaled breath out. Ventilation foils the hungry mosquitoes, because it prevents CO₂ from building up overnight. Physicists with the team modeled the fluid dynamics of carbon dioxide in these homes, confirming how airflow breaks up the stagnant clouds that attract mosquitoes. Adding more screened space dropped CO₂ levels by up to 36 percent.

“It almost becomes what would be a stealth house,” Lindsay says. “You're hiding the house and the occupants from mosquitoes."

The windows also played another role: They made the houses about 1 degree cooler. “This is a real win-win,” Lindsay says. “Because it's not just about reducing malaria, it’s about getting a good night's sleep.”

In 2019, the team built another set of homes—this time, ones that could be elevated, harking back to the professor’s extra-tall platform. Mosquitoes tend to hunt only a few feet off the ground, so a raised house, they hypothesized, would get humans out of their way. The wood and tin homes, which sat on stilts, could be raised up to 3 meters above ground with a pulley system, like the kind mechanics use to pull engines out of cars.

It worked. They found that homes raised 1 meter high attracted 40 percent fewer mosquitoes. At 2 meters, it was 68 percent fewer, and at 3 meters, 84 percent fewer.

“I was surprised by how large an impact they saw,” says Kelly Searle, a University of Minnesota epidemiologist who did not take part in the study. Searle, who has explored how construction materials, such as brick, mud, and metal, affect malaria transmission herself, says this level of reduction is convincing. “We do see really strong evidence that the housing construction can be protective against malaria infection,” she says.

“It's really important,” she continues, because bed nets and insecticide spraying aren’t enough. “If we could have additional tools that we can use to prevent malaria, that's fantastic.”

Adopting this design for new homes or retrofits in real communities will be a challenge, though. “The number of people who will be influenced by [the academic studies] to actually change their home will be quite small,” says Patrick Kelley, vice president of the Terwilliger Center for Innovation in Shelter at Habitat for Humanity International. It's a hurdle—but it’s not insurmountable.

One path to making widespread change for the growing population would be through building codes that could be enforced by local governments. But another would be changes in consumer behavior: people’s tastes in houses updating as they learn what designs make sense—counterintuitively large windows, for example, but with screens. “I'm more optimistic about the consumer behavior route, putting knowledge into people's hands,” Kelley says. “There are ways to bring some of that messaging into the home improvement markets where people go to buy wood—to buy screening.”

Lindsay agrees. “The way that architects think about making change,” he says, “is to build something new then get people to look at it and say, ‘Hey, that's cool!’ and copy it.” If local people see the appeal of these science-based designs, they’ll be more likely to build that way too.

Okumu believes that design is a more sustainable way to control malaria than by using commercial products like bed nets, insecticide, and drugs. The goal is simple: keep mosquitoes from finding humans. “I have learned over the years that we have to go back to the basic biology of the disease,” Okumu says. “And malaria is primarily a problem of poor housing and surface water.”

Lindsay has a large clinical trial ongoing in Tanzania called the Star Homes Project, designed by team member Jakob Knudsen, a Danish architect, testing the resilience of two-story homes with walls made of breathable shade-cloth, inspired by designs from Southeast Asia. The study will run for three years and will track malaria transmission among the kids living in 110 Star Homes across 60 villages, compared to rates for others living in 440 traditional homes.

“They're really very beautiful,” says Lindsay.

Each home has beds upstairs from an airy screened living space. Wind flows in, exhaled breaths flow out, and mosquitoes, presumably, stay away. In the evening, lights glow faintly through the translucent walls—yet the house stays hidden in plain sight.