If Malaria's the Problem, DDT's Not the Only Answer
In the pantheon of poisons, DDT occupies a special place. It's the only pesticide celebrated with a Nobel Prize: Swiss chemist Paul Mueller won in 1948 for having discovered its insecticidal properties. But it's also the only pesticide condemned in pop song lyrics -- Joni Mitchell's famous "Hey, farmer, farmer put away your DDT now" -- for damaging the environment. Banned in the United States more than 30 years ago, it remains America's best known toxic substance. Like some sort of rap star, it's known just by its initials; it's the Notorious B.I.G. of pesticides.
Now DDT is making headlines again. Many African governments are calling for access to the pesticide, believing that it's their best hope against malaria, a disease that infects more than 300 million people worldwide a year and kills at least 3 million, a large proportion of them children. And this has raised a controversy of Solomonic dimensions, pitting environmentalists against advocates of DDT use.
The dispute between them centers on whether the potential benefits of reducing malaria transmission outweigh the potential risks to the environment. But the problem isn't that simple. This is a dispute in which science should play a significant role, but what science tells us is that DDT is neither the ultimate pesticide nor the ultimate poison, and that the lessons of the past are being ignored in today's discussion.
The United Nations Environment Program has identified DDT as a persistent organic pollutant that can cause environmental harm and lists it as one of a "dirty dozen" whose use is scheduled for worldwide reduction or elimination. But some DDT advocates have resorted to anti-environmentalist drama to make their case for its use in Africa.
They have accused environmental activists of having "blood on their hands" and causing more than 50 million "needless deaths" by enforcing DDT bans in developing nations. In his best-selling anti-environmentalist novel "State of Fear," Michael Crichton writes that a ban on using DDT to control malaria "has killed more people than Hitler."
Such statements make good copy, but in reality, chemicals do not wear white hats or black hats, and scientists know that there really are no miracles.
Malaria is caused by a protozoan parasite that is transmitted by mosquitoes. For decades, there have been two major strategies for curbing the disease: killing the infectious agent or killing the carrier. Reliably killing the protozoan has proved difficult; many older drugs are no longer effective, new ones are prohibitively expensive, and delivering and administering drugs to the susceptible populace presents daunting challenges. Killing the carrier has long been an attractive alternative.
And DDT has been an astonishingly effective killer of mosquitoes. DDT (which stands for the far less catchy dichloro-diphenyl-trichloroethane) is a synthetic chemical that didn't exist anywhere on the planet until it was cooked up for no particular purpose in a German laboratory in 1874. Decades later, in 1939, Mueller pulled it off a shelf and tested it, along with many other synthetic substances, for its ability to kill insects. DDT distinguished itself both by its amazing efficacy and its breadth of action -- by interfering with nervous system function, it proved deadly to almost anything with six, or even eight, legs. And it was dirt-cheap compared to other chemicals in use -- it could be quickly and easily synthesized in chemical laboratories from inexpensive ingredients.
Soon after its insecticidal properties were discovered, DDT was put to use combating wartime insect-borne diseases that have bedeviled troops mobilized around the world for centuries. It stemmed a louse-borne typhus outbreak in Italy and prevented mosquito-borne diseases in the Pacific theater, including malaria and yellow fever, to almost miraculous effect. This military success emboldened governments around the world to use DDT after World War II to try to eradicate the longtime scourge of malaria. And in many parts of the world, malaria deaths dropped precipitously. This spectacular success is why many people are calling for the use of DDT specifically for malaria control.
At the same time that malaria deaths were dropping in some places, however, the environmental persistence of DDT was creating major problems for wildlife, as famously documented in Rachel Carson's classic 1962 book, "Silent Spring." By 1972, the pesticide had become the "poster poison" for fat-soluble chemicals that accumulate in food chains and cause extensive collateral damage to wildlife (including charismatic predators such as songbirds and raptors), and a total ban on the use of DDT went into effect in the United States.
What people aren't remembering about the history of DDT is that, in many places, it failed to eradicate malaria not because of environmentalist restrictions on its use but because it simply stopped working. Insects have a phenomenal capacity to adapt to new poisons; anything that kills a large proportion of a population ends up changing the insects' genetic composition so as to favor those few individuals that manage to survive due to random mutation. In the continued presence of the insecticide, susceptible populations can be rapidly replaced by resistant ones. Though widespread use of DDT didn't begin until WWII, there were resistant houseflies in Europe by 1947, and by 1949, DDT-resistant mosquitoes were documented on two continents.
By 1972, when the U.S. DDT ban went into effect, 19 species of mosquitoes capable of transmitting malaria, including some in Africa, were resistant to DDT. Genes for DDT resistance can persist in populations for decades. Spraying DDT on the interior walls of houses -- the form of chemical use advocated as the solution to Africa's malaria problem -- led to the evolution of resistance 40 years ago and will almost certainly lead to it again in many places unless resistance monitoring and management strategies are put into place.