Last week, at the bottom of a New York Times story about the Zika virus outbreak, an old and controversial chemical made a brief appearance. DDT, the insecticide made famous for its environmental consequences by Rachel Carson’s 1962 book, Silent Spring, was being “mentioned a bit” in public health circles as a possible means to eradicate the Zika-carrying mosquito, Aedes aegypti, wrote the Times.
DDT works as a neurotoxin, killing mosquitoes and other pests brain-first. Scientists determined decades ago that DDT causes serious environmental damage, leading the U.S. to ban the chemical in 1972; the 150 parties to the 2001 Stockholm Convention agreed to put an end to its use too. More recent studies have drawn connections between DDT and neurotoxic health effects in humans, like Alzheimer's, Parkinson's disease, breast cancer, diabetes and impaired brain development in children.
These concerns, said Lyle Petersen, director of the division of vector-borne diseases at the Centers for Disease Control and Prevention, need to take a backseat in the face of the hazards of Zika. DDT, he argued, could be used in small amounts on the walls inside homes; the environmental damage widely associated with the insecticide was tied with large-scale agricultural use, and scale matters. “That concern about DDT has to be reconsidered in the public health context,” he told the Times.
But according to Joe Conlon, a technical adviser to the American Mosquito Control Association and a former entomologist with the U.S. Navy, using DDT to control Zika is a terrible idea. “DDT seems like a silver bullet, but it isn’t.” First of all, the mosquitoes might be resistant to DDT. Conlon says the Latin American countries where Zika is blooming now used DDT heavily in the 1960s to kill off the Aedes aegypti, which also carries diseases like dengue and yellow fever. It worked, but the mosquitoes in the region developed robust resistance to the pesticide, which may still be lingering in the population. DDT resistance lasts a long time, he says, because the chemical persists in the environment so long. If you spray a wall with DDT today—a method commonly used because mosquitoes are known to rest on walls a moment after having a blood meal—it could still be coated by DDT in 20 years. The mosquito population continues to be bombarded by the chemical, so the resistance shows up in every subsequent mosquito generation.
And even if the mosquitoes aren’t already resistant, they will be. It only takes a “few generations” of mosquito to develop resistance, and when an Aedes mosquito’s life span is about 10 days, that’s not long at all. Conlon speculates mosquitoes could develop resistance within a year. “What’s even worse, resistance to DDT can stir cross-resistance to the other pesticides we use, like the pesticide we use to treat bed nets, to fight malaria.”
In the end, Conlon says, there may be no good chemical solution to the Zika mosquito. The Aedes aegypti are known to prefer breeding in small pools of water, like bottle caps, flower pots or puddles inside discarded tires. Chemical sprays aren’t likely to reach any of those places. “What you need is a change in culture,” Conlon says, a massive push to encourage people to eliminate any standing water they might have around their property. It’s more about getting people to give up on their houseplants than dousing neighborhoods in sprays, he says. “Americans tend to want a chemical solution to everything,” Conlon says, but DDT isn’t it.
Still, the alarming spikes in microcephaly cases among infants born in countries where Zika is spreading is causing tremendous anxiety among world health officials and national health ministries, some of whom have gone so far as to tell women not to get pregnant for the next year or two. The fear of the crippling birth defect understandably makes an “all of the above” approach to eliminating the mosquito attractive, but Jonathan Chevrier, a professor of epidemiology at McGill University, says using DDT in these communities might ensure health problems for decades to come. “We’re talking about exposing generations of people for a one-time emergency.”
Though the CDC official argued that using DDT in homes would have a smaller impact on the environment than the agricultural use of the 1960s, Chevrier says the human toll is likely still very high. “The reality is that the people who live in homes who were spread with DDT have very, very high levels of DDT for their bodies, much higher than what was found back when we used it for agriculture,” Chevrier says. And once in our bodies, it tends to stay there. It takes 10 years to eliminate half the DDT in our system, he says. “It lodges in our fatty tissues. On top of that, DDT crosses the placenta and exposes the fetus. Plus, the only way to get rid of DDT in humans is through breastfeeding.” Breast milk is so fatty that it can flush the DDT out of the mother’s body as she nurses. Of course, “then it’s being transferred to the kid that’s drinking the breast milk.”
Several countries in sub-Saharan Africa have obtained exemptions from the Stockholm Convention to use DDT to control malaria, an illness which killed 438,000 people in 2015. But research has suggested the risks associated with DDT could outweigh the benefits, particularly where children are concerned; A 2003 paper from the National Institute of Environmental Health Sciences found the number of childhood malaria-related deaths that DDT spraying prevented was probably offset by the increase in infant deaths it caused.
In other words, debating whether or not to use DDT is never just a question of choosing the environment over people. Rather, Chevrier says, it’s a matter of weighing one set of human risks, like mosquito-borne illnesses, against another set of risks associated with long term exposure to neurotoxins, a set of risks much more poorly understood but with the potential to be grave.