Aedes aegypti—the mosquito species that carries the Zika virus, yellow fever, dengue fever, and chikungunya—is about as domesticated as insects get. It prefers to live among people, feed on people, and deposit its eggs in trash produced by people. This isn’t a coincidence: Over the past 40 years, haphazard and ever-accelerating urbanization and globalization have created ideal conditions for mosquitoes like Aedes to thrive. And now, the world’s most vulnerable are bearing the brunt of the burden.
After a spate of effective mosquito eradication programs in the 1950s and ‘60s, lax mosquito abatement efforts coincided with an explosion of urban growth in the Americas and southeast Asia. Land use and lifestyles shifted dramatically. Deforestation, for example, helped bring mosquitoes into closer proximity to humans, while a more global consumer economy gave the insects new places to reproduce.
“Forty years ago, you could say the mosquitoes were living in the forests,” Albert Ko, a professor of epidemiology at Yale University and collaborating researcher at the Oswaldo Cruz Foundation at the Brazilian Ministry of Health, tells CityLab. “But now the complexity of the urban landscape has changed enormously. Mosquitoes are breeding in places where we traditionally wouldn’t think they would breed, or in places that we can’t find.”
For example: old tires and plastic containers. In a 2011 paper tracing the spread of dengue fever since the 1970s, Duane J. Gubler, a professor and founding director of the Signature Research Program in Emerging Infectious Diseases at the Duke-NUS Medical School in Singapore, writes:
The global automobile population has exploded during this time, and used automobile and truck tires provide ideal oviposition sites and larval habitats for the mosquito vectors. As a result, they also served as the principal mechanism for the geographic spread of mosquitoes. In addition, most consumer goods are packaged in non-biodegradable plastic containers, most of which are discarded into the environment and provide another ideal larval habitat for the mosquito vectors of dengue.
Rapid urbanization has hardly been equitable. Approximately one billion people live in the world’s urban slums, which often lack adequate water supplies. Residents must store their water in cisterns or jars, which make ideal breeding grounds for Aedes mosquitos in tropical environments. The sheer density, lack of proper drainage, and poor garbage management in slum communities pose additional challenges to mosquito eradication.
“In a favela, it’s complicated,” Gilberto de Souza, a Brazilian public health worker, recently told The Guardian as he discovered a pallet of upturned beer bottles—another ideal mosquito breeding ground—on a street corner in a Rio de Janeiro slum.
The current Zika epidemic, which may be linked to high number of microcephaly in infants born to infected mothers, began in Brazil in May 2015. Since then, an estimated 1.5 million Brazilians have caught it. Many of these cases have been concentrated in northeastern Brazil, the country’s poorest and least developed region.
“Most mothers with children with microcephaly have come from poor communities,” Claudio Maierovitch, director of the Communicable Disease Surveillance Department at the health ministry, told The Guardian. “We don’t know the reason yet, but it has been an observation noted by the people working on the ground.”
The reasons are actually pretty clear. Global lifestyle shifts, rapid changes in land use, and poor urban planning and public health policy have left the planet’s most vulnerable most susceptible to epidemics like Zika. As Mario Moscatelli, a Brazilian biologist and conservationist, told The Guardian, “We are reaping what we sowed.”