BTT!!!!!!!
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 the United States banned the use of DDT.
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 began, 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.
In fact, pockets of resistance to DDT in some mosquito species in Africa are already well-documented. There are strains of mosquitoes that can metabolize DDT into harmless byproducts and mosquitoes whose nervous systems are immune to DDT. There are even mosquitoes who avoid the toxic effects of DDT by resting between meals not on the interior walls of houses, where chemicals are sprayed, but on the exterior walls, where they don't encounter the chemical at all.
The truth is that DDT is neither superhero nor supervillain - it's just a tool. And if entomologists have learned anything in the last half-century of dealing with the million-plus species of insects in the world, it's that there is no such thing as an all-purpose weapon when it comes to pest management. DDT may be useful in controlling malaria in some places in Africa, but it's essential to determine whether target populations are resistant; if they are, then no amount of DDT will be effective.
We have new means of determining whether populations are genetically prone to developing resistance. DDT advocates are right to suggest that DDT may be useful as a precision instrument under some circumstances, particularly considering that environmental contamination in Africa may be less of a problem than it has been in temperate ecosystems because the chemical can degrade faster due to higher temperatures, moisture levels and microbial activity. Moreover, resistance evolves because of random mutation, so there are, by chance, malaria-carrying mosquito species in Africa that remain susceptible to DDT despite more than two decades of exposure to the chemical.
But environmentalists are right to worry that the unwise use of DDT may cause environmental harm without any benefit. In 2000, I chaired a National Research Council committee that published a study titled "The Future Role of Pesticides in U.S. Agriculture." Our principal recommendation is germane to discussions of malaria management: "There is no justification for completely abandoning chemicals per se as components in the defensive toolbox used for managing pests. The committee recommends maintaining a diversity of tools for maximizing flexibility, precision and stability of pest management."
Overselling a chemical's capacity to solve a problem can do irretrievable harm not only by raising false hopes but by delaying the use of more effective long-term methods. So let's drop the hyperbole and overblown rhetoric - it's not what Africa needs. What's needed is a recognition of the problem's complexity and a willingness to use every available weapon to fight disease in an informed and rational way.
The truth is that DDT is neither superhero nor supervillain - it's just a tool.