Why Bringing Back DDT to Fight Zika Mosquitoes Could Backfire

Less than a century since DDT was first sprayed, the spread of the Zika virus through mosquitoes has left some wondering whether regulators were too quick to move away from the chemical. But public health officials warn returning the controversial pesticide to use could do more harm than good in some places.

DDT, which first came into use as an insecticide in the 1930s, played a crucial role helping Allied forces protect themselves from mosquitoes carrying malaria in World World II and eventually helped eliminate the deadly disease from the U.S. But despite its successes, public opinion turned quickly in the 1960s as outrage grew over DDT’s potential risks to human health and animals and many countries joined the U.S. in ultimately banning its use.

Today, health officials caution that while the benefits of the pesticide can outweigh health concerns in some contexts, spraying DDT to combat Zika would be premature and potentially very misguided.

“It’s a difficult question, and it’s a very controversial question,” says Jonathan Chevrier, an assistant professor at McGill University, of how policymakers weigh the use of DDT to protect public health. “What the Zika virus is potentially doing is terrible. But using any pesticide needs to be considered very, very carefully.”

Read More: All the Ways Humans Try to Kill Mosquitoes—and Why We’re Still Losing

The first concern for public health officials when considering whether to use DDT is if the method of applying the pesticide will actually kill the targeted mosquitoes. Most applications of the pesticide today occur on the walls of indoor dwellings—targeting mosquitoes that prey on humans while they sleep at night. This tactic, frequently used in Africa, works in areas where mosquitoes carrying disease bite at night while people are asleep, according to Lynn Goldman, dean of the George Washington University School of Public Health. More research is needed to understand other mosquito populations and learn whether DDT use will be effective.

“You have to be using the right weapons for the right mosquito,” says Goldman. “Firing at any mosquito we see is not going to be useful.”

Read More: See All the Places Where the Zika Virus Has Spread

Policymakers also have to weigh the potential effects of DDT on human health. The impact of the chemical is not necessarily immediate, but can be devastating to those who ingest it. Research has suggested that DDT has the potential to disrupt the human nervous system in the same way it does to insects. That may mean cancer, infertility and other long-term health effects including developmental problems in young children. DDT remains in the environment where it’s sprayed for years, potentially affecting multiple generations.

Taking the risk of exposing people to DDT can make sense in areas at high risk for malaria—though the question remains a topic of debate for health officials. After all, that disease kills nearly 500,000 people each year. But it less makes sense, at least for now, when combatting a disease like Zika that doesn’t kill those it infects and has yet to spread as widely. Unnecessary use of DDT will also make it less effective going forward as mosquitoes grow resistant. The first round of spraying DDT will effectively kill a large share of the local mosquito population, but fewer and fewer of the insects will die with each consecutive spray, limiting its utility in a future crisis.

Instead of focusing on pesticides to address Zika, public health officials are now advocating a comprehensive approach to addressing the virus that includes studying the species of mosquito that spreads the virus, developing a Zika vaccine, eliminating places where mosquitoes can breed and potentially introducing genetically modified mosquitoes that will help reduce the population.

“You want to marshal efforts that are actually saving people’s lives,” says Goldman. “A lot of mosquito control is going house to house to house and getting rid of small amounts of water where these vectors can be breeding.”