I wrote this article for Nature Medicine in July 2006. This was a big scoop. The news flew mostly under the radar, with the big announcement due that September. But I had heard rumblings of it at conferences, and was able to get fantastic access to (and quotes from) Arata Kochi, then head of the WHO’s malaria program. You can download a pdf of this article.)

The most infamous pesticide in history is also the most effective weapon against malaria. Ready or not, DDT is on its way back to Africa. Apoorva Mandavilli reports.

It’s possibly the most reviled chemical on the planet. Every adult anywhere in the world could list its supposed evils. And it’s been the subject of a long and bitter battle that defies traditional divisions of liberal and conservative.

But here it is, poised for a comeback.

After decades of being marginalized as a dangerous pesticide, DDT—short for dichloro-diphenyl-trichloro ethane—is set to be reintroduced into countries that have tried, and failed, to win the fight against malaria.

On 2 May, the United States Agency for International Development (USAID), arguably the most powerful donor agency in the world, endorsed the indoor spraying of DDT for malaria control. The World Health Organization (WHO) is set to follow. In its new guidelines, a final version of which is expected to be released later this summer, the WHO is unequivocal in its recommendation of DDT for indoor residual spraying.

For the many African countries riddled with malaria, this is welcome news. “We in southern Africa feel extremely happy,” says John Govere, integrated vector control officer for the WHO’s Southern Africa Malaria Control Programme.

Malaria kills as many as 1 million people each year, about 90% of them in Africa. Someone dies of malaria every 30 seconds—and most of those are pregnant women and children under the age of five. Even the millions who survive can be reinfected, leaving them bedridden and decimating economies.

Little surprise then, that just days after USAID’s announcement, Tanzania said that it would lift its DDT ban. Others soon followed.

For these impoverished countries, the choice may seem clear: DDT is cheap and lasts longer than other pesticides, so it has to be sprayed less often. Most pesticides work by killing mosquitoes on contact, but DDT also repels them.

“DDT is the most effective chemical, the most effective insecticide in terms of malaria,” says Arata Kochi, director of the WHO’s Global Malaria Programme.

Silent spring
If DDT is that effective, why has it been so vilified? “You’ve heard of Rachel Carson? Well that’s your answer,” says Maureen Coetzee, chief of vector control research at South Africa’s National Institute for Communicable Diseases. “DDT was so abused in the ’50s and ’60s that it is still suffering from that abuse.”

In the 1940s, DDT was considered a miracle chemical. Airplanes sprayed thousands of tons of the pesticide, coating acres of crops, villages and cities with abandon.

By 1949, the US was malaria free. Between 1955 and 1969, the Global Malaria Eradication Campaign also relied heavily on DDT. In Europe, India, South America, Africa, wherever it was used widely, DDT cut malaria rates dramatically and saved millions of lives.

Then came Carson’s Silent Spring— credited with launching the environmental movement—which famously described DDT’s horrific effects on the food chain, and the stark silence left behind by dying birds. The book was so effective that to this day, any mention of DDT instantly evokes images of bald eagles and thinning eggshells.

DDT soon became a symbol of Western governments’ rash embrace of science. In the US and in Europe, environmental groups waged a successful campaign against the pesticide. Based largely on its effect on the environment—and on public opinion—the US Environmental Protection Agency in 1972 banned DDT. Norway and Sweden had begun the trend in 1970, and the UK signed on in 1986.

Environmental groups that still oppose DDT see its use in developing countries as a double standard. On the other hand, note champions of DDT, most of those groups are based in countries where malaria is only a distant memory.

“I think the whole push of the environmentalists like Rachel Carson and many others to eliminate all uses of DDT are, quite honestly, responsible for millions and millions and millions of human deaths,” says Don Roberts, professor of tropical public health at the Uniformed Services University of the Health Sciences in Maryland.

Money talks
In theory, any country is free to use DDT. The Stockholm Convention of 2001 sought a global ban on DDT, but many countries and scientists argued against the ban, citing its value in malaria control. The final treaty made an exemption for DDT’s use in public health, but called for countries to gradually phase out the pesticide.

Still, in places where malaria was still endemic, the treaty spelled disaster.

Most African nations are heavily dependent on foreign aid and can ill afford to cross a line drawn by donor agencies.

USAID never banned DDT outright, for instance, but nor did it fund DDT’s purchase—which amounts to the same thing. For that reason, the May announcement is widely seen as a change in policy even though the agency doesn’t position it as such.

The World Bank went one step further, making the ban of DDT a condition for loans. The WHO supported the use of bednets dipped in insecticide over indoor spraying, even though malaria rates continued to increase. DDT was “further ignored and intentionally or unintentionally suppressed,” by these agencies, says Kochi.

“People are very emotional about DDT, even within the WHO,” Kochi says, adding that much of the reaction to DDT was a response to political pressure. Since his arrival at the agency in late 2005, he has pushed for the return of indoor spraying not just to Africa, but wherever malaria continues to be a problem.

The WHO plans to field test its new guidelines for indoor residual spraying in Yemen, Sudan and two countries in south eastern Asia. “Quite often in this field, politics comes first and science second,” Kochi says. “We must take a position based on the science and the data.”

Disputed dangers

The WHO recommends 12 insecticides including DDT—which is an organochlorine—six pyrethroids, three organophosphates and two carbamates. In terms of toxicity to humans, DDT is considered safer than many of the others.

Because only small quantities are sprayed on the walls for malaria control, “unless you go and lick the wall, you’re not likely to ingest the insecticide,” notes Coetzee.

Over the years, however, based primarily on data from agricultural use, DDT has been linked to various illnesses—but those studies are rarely convincing and never consistent.

DDT was thought to cause liver and breast cancer and disrupt hormonal balance, for example, but the claims have since been debunked (Lancet356, 267–268; 2000; Nat. Med. 6, 729–731; 2000). Studies have found that in areas where DDT is sprayed for malaria control, mothers show traces of DDT in their blood and breast milk (Bull. World Health Organ. 68, 761–768; 1990). Some scientists say those levels might trigger premature births and a shorter time for breast-feeding (Lancet 366, 763–773; 2005), but the results are much disputed.

Most recently, researchers in California reported in July that babies of mothers exposed to DDT fare worse on mental tests and motor skills tests (Pediatrics 118, 233–241). But critics note that the study followed only 360 women and that because the mothers were born in Mexico—where DDT was used for agriculture till 1995—the researchers cannot link the exposure to malaria control.

But even critics of DDT agree that in countries ravaged by malaria, the benefits far outweigh the risks. “I’d rather have a child with three IQ points less than have a dead child,” says Brenda Eskenazi, lead researcher of the Mexico study.

Although DDT might be a short-term solution for malaria control, however, more research is needed before it can be regarded as entirely safe, Eskenazi and others caution.

For instance, when assessing DDT’s effect on malaria, global agencies should measure rates of total infant mortality, and not just malaria-related deaths, says Walter Rogan, an epidemiologist at the US National Institute on Environmental Health Sciences. That would help catch any adverse affects DDT may have on infant health, he says.

“I think that DDT is not entirely a benign compound,” Rogan says. “If you’re going to use it, you should consider the idea that it might do harm.”

Within walls
DDT’s use in malaria is far different from its application in agriculture. In contrast to indiscriminate aerial spraying several times a year, individuals spray a small amount—about two grams per square meter, a fraction of that used in agriculture—on the inside walls and eaves of houses, where mosquitoes rest.

Because DDT is not sprayed outdoors, there is little chance of it getting into the environment, notes Simon Kunene, chair of the malaria subcommittee for the Southern African Development Community (SADC).

But with thousands of tons of the cheap pesticide available, some of it will inevitably be diverted for agricultural use, experts warn.

“What concerns me a lot is that it’s going to be misused and end up in the environment,” says Mark Rowland, a malaria control expert at the London School of Hygiene and Tropical Medicine.

The WHO is considering a method that South Africa has developed over the years. Precise amounts of DDT are shipped from a central location to the area where they are needed, minimizing middlemen en route, and once spraying is complete, the containers are returned for checking.

But South Africa, with its relatively well-developed infrastructure, may be an exception, argues Rowland. From 1991 to 1998, Rowland led a malaria control program for Afghan refugees in Pakistan. That program employed a similar method, using packets of DDT that were closely supervised. Still, some packets were misused, Rowland says. “It’s very difficult to monitor every kilogram that gets distributed.”

Fine print
The best long-term prospect may be to develop safe and effective alternatives, several of which are being developed by public-private partnerships.

Meanwhile, the use of DDT, and of indoor spraying generally, is expected to rise sharply.

“People are now beginning to appreciate how effective indoor residual spraying can be,” says Richard Greene, director of the Office of Health, Infectious Disease and Nutrition in USAID’s Bureau for Global Health.

Unless done well, however, using DDT may be dangerous—and powerless against malaria.

For spraying to be effective, at least 80% of the affected area must be covered. Global agencies must first map the distribution and behavior of different kinds of mosquitoes and, most important, patterns of resistance to insecticides. “The last thing you want to do is select for resistance,” says Coetzee.

Convincing the local community to let people into their homes and spray their walls is another formidable challenge. At the height of the epidemic, people are generally terrified enough to do whatever it takes. But once the mosquitoes—and, as a bonus, the cockroaches—disappear, so does people’s tolerance.

Because DDT lasts longer than other pesticides—up to a year compared to six months with pyrethroids—it needs to be sprayed less often, making its use less challenging for governments.

“What can I say? It has worked for us,” says Kunene. “They’ll see the results. Give me one year or two years. There’ll be a big difference.”

Published online: 27 July 2006.