Though we imagine infectious microbes propagating according to their own logic, many are resurging thanks to the unintended consequences of human activity that would seem to have little to do with the biology of microbes, from economics and housing policy to architecture. As these rejuvenated pathogens adapt to the man-made environments that sprawl across the planet, anything from highways to swimming pools can wind up triggering an outbreak.
In a globalized economy, the public health implications of sick people getting on airplanes and spreading germs around the world are obvious. But we’re less used to thinking of things like foreclosed homes, imported tires and decorative bamboo as health risk factors. If we’re going to deal with Zika and the pathogens that will inevitably come after it, we have to start.
Commerce has long provided unexpected opportunities for infectious pathogens to exploit, from the Erie Canal, which slashed the cost of shipping while unwittingly carrying cholera across the country, to the hydropower dams that electrified the South while simultaneously providing succor for scores of malarial mosquitoes. Today, abandoned properties and deteriorating infrastructure, brought on by housing crises and climate change, similarly threaten us with epidemics of mosquito-borne pathogens such as Zika.
Just such an epidemic occurred in 2009, when dengue broke out in Florida. That mosquito-borne disease, which causes joint and bone pain so excruciating that it’s called “breakbone fever” in Latin America and Asia, hadn’t been seen in Florida since 1934. That’s surprising, because the state is surrounded by countries where the disease is endemic, and the mosquitoes that carry the virus, Aedes aegypti, have been established across the Gulf Coast since the 16th century. A few years ago, when I asked Florida Keys mosquito-control expert Lawrence Hribar to explain what had happened, he didn’t point to an invasion of bugs or infected people. He began his explanation with the following: “There were houses in foreclosure.”
The 2008 housing crisis hit Florida hard, Hribar pointed out, and nowhere harder than South Florida, the epicenter of the outbreak. Across the state, more than 380,000 homes had been foreclosed, and many abandoned. That meant a lot of empty swimming pools. Then the rains came. The pools filled with standing water. With nobody home to notice or to let inspectors in, those derelict swimming pools became giant mosquito hatcheries.
Two years earlier, Bakersfield, Calif., experienced a similar outbreak of mosquito-borne West Nile virus. The spring had been dry, and the birds in which the virus lurks were sparse, so public health authorities predicted that there would be few infections that year. Instead, cases soared. It was only after an aerial survey was conducted that experts realized why: Armies of disease-carrying mosquitoes had colonized scores of swimming pools, hot tubs and ornamental ponds. Their neglect was similarly linked to a housing crisis. The previous year, there was a nearly 300 percent spike in delinquency notices.
Climate disruptions have had a similarly unexpected impact on the spread of mosquito-borne disease. When Hurricane Katrina hit New Orleans in 2005, for example, scores of flooded and abandoned swimming pools were quickly transformed into efficient mosquito breeding grounds. And that expansion of mosquito habitat was followed by a rise in severe cases of West Nile virus in the region the next year.
We’re not conditioned to expect this. Rather, the “built environment” — buildings, waterways and other man-made infrastructure that make up our cities and towns — has long been our way of “improving” on the natural environment. It’s supposed to make us more comfortable, productive and secure. Its modern apotheosis, the air-conditioned suburban home replete with endless lawn and sparkling swimming pool, is the bedrock of the American Dream, a dream sold to millions of people who took out subprime mortgages to achieve it.
The built environment functions as intended only when its components are continuously maintained, often at significant expense in time and resources. That might have been feasible in an era of cheap, abundant energy and easy credit. But not anymore. The housing crisis forced 3.1 million American properties into foreclosure in 2008 alone, leaving behind a rash of abandoned homes and neighborhoods that have yet to be fully re-occupied. Even today, there are more than 1 million abandoned properties across the country, and Florida counts more “zombie closures” than almost every other state. And their neglect amplifies and spreads vectors of disease even more effectively than the wild landscapes they replaced.
The backyard swimming pool perfectly encapsulates the problem. Pools have no natural inflow or outflow of water. Without regular maintenance, such as chemical adulteration and continuous filtration, they can rapidly become incubators of insects and microbes. Blooms of algae appear. Leaves and debris collect on the surface. Impregnated female mosquitoes, searching for water in which to lay their eggs, swoop. And the steep, smooth walls of swimming pools preclude the predators that would feed on their eggs in natural bodies of water.
Neglected swimming pools often go undetected, since pools are frequently surrounded by privacy hedges and high vegetation, obscuring problems from passersby and public health authorities. Even when pools turn green and buggy, satellite images can fail to detect them. And those that are noticed can be difficult to access. In many jurisdictions, by law, when owners aren’t home, swimming pools must be enclosed behind locked gates. That helps prevent accidental drownings, but it also blocks public health workers from treating neglected pools with mosquito-killing chemicals or stocking them with mosquito-eating fish.
That’s not the only way human activity has heightened the disease risk posed by mosquitoes. Thanks to the logic of rapid growth and resource consumption, the United States is home not just to the Culex species that carries West Nile virus, but also to two invasive species of mosquitoes from the genus Aedes, which are carriers of much more deadly diseases such as yellow fever, dengue, chikungunya and Zika.
Neither Aedes aegypti, which hails from Africa, nor Aedes albopictus, from Asia, are native to the United States. But two peculiarly American industries brought them to our shores. The slave trade ferried Aedes aegypti to the New World in the 16th century. These mosquitoes caused repeated epidemics of yellow fever in American cities in the 18th and 19th centuries. Then, in the mid-20th century, another engine of the economy, the automobile, brought Aedes albopictus. More specifically, the trade in used tires, required to service our fleet of vehicles, gave them a lift. Many of these tires were imported from Asia, where albopictus lived in dark, wet tree holes. The tires proved pleasingly similar, and as the tires traveled across the continents, so did albopictus.
In 2001, more albopictus pioneers caught a ride in shipments of a popular decorative plant called lucky bamboo, packaged in standing water for the long voyage from south China to California. The used-tire and lucky bamboo trades have since broadcast albopictus across Europe, Africa and Latin America. They join a smattering of other trades known to have disseminated pathogens throughout the globe. Trade in frogs has been implicated in the spread of a pathogen called chytrid fungus that’s wiping out amphibians; an air shipment of rodents from Ghana introduced a deadly virus called monkeypox into the United States in 2003.
Albopictus pushed aegypti to the edges of its habitat. Unlike aegypti, which thrives in tropical weather and bites humans exclusively, albopictus tolerates temperate areas as well and can bite people or animals, spending its days hiding in green vegetation. Aegypti retreated to the Gulf Coast, concealed in neglected neighborhoods where pockets of stagnant water collect in garbage-strewn lots, and where human blood is easily accessible through the broken window screens of dilapidated homes. Albopictus colonized the leafy suburbs that sprawl across temperate parts of the country, hidden in the high bushes and hedges that surround its gleaming necklace of pools and hot tubs.
Today, 60 percent of the U.S. population is vulnerable to albopictus mosquitoes. A few emerge from my garden every time I walk out my front door in the Baltimore suburbs, awakened by the scent of my exhales, hungry for blood (yes, that’s all it takes). After they bite, they’ll fly off in search of still water to deposit their eggs. In my neighborhood of privacy hedges and lush vegetation, there’s no telling where they’ll land.
For now, there’s little federal funding on tap to finance research and development of new tools to fight Zika, despite months of pleading from the CDC and other agencies. What we do have: an uptick in reported cases of Zika; high-profile athletes pulling out of the 2016 Olympics — which might protect them, but is in no way a meaningful public health response for the rest of us; and a Congress unable to reach consensus on emergency funding to fight the virus. The rest of us are left to slather on mosquito repellent and wait uneasily for the inevitable news of infants born with microcephaly.
There’s more we could do. We could assess the public health implications of our built environment the way we assess its environmental impact — before construction begins. We could empower public health officials to do more than combat outbreaks after the fact, but also help regulate the social and economic conditions that contribute to outbreaks in the first place.
As the strange tales of swimming-pool outbreaks show, housing crises, economic shocks, climate disruptions, and other economic and social factors can affect public health in unexpected ways. Rather than wait for swimming pools to turn into mosquito hatcheries, we could start to address the underlying economic and social drivers that create these and other public health hazards, before epidemics occur.
That would first mean critically acknowledging and assessing our role in bringing about the conditions driving epidemics, whether abandoned homes or neglected neighborhoods, aging infrastructure or trade patterns. And then we would need to enlist a response not just from the biomedical establishment but from all sectors of society, from consumers to banks to builders.