By Donald Roberts, an emeritus professor of tropical medicine at the Uniformed Services University of the Health Sciences and a board member of the nonprofit health advocacy group Africa Fighting Malaria (THE NEW YORK TIMES, 20/08/07):
DDT, the miracle insecticide turned environmental bogeyman, is once again playing an important role in public health. In the malaria-plagued regions of Africa, where mosquitoes are becoming resistant to other chemicals, DDT is now being used as an indoor repellent. Research that I and my colleagues recently conducted shows that DDT is the most effective pesticide for spraying on walls, because it can keep mosquitoes from even entering the room.
The news may seem surprising, as some mosquitoes worldwide are already resistant to DDT. But we’ve learned that even mosquitoes that have developed an immunity to being directly poisoned by DDT are still repelled by it.
Malaria accounts for nearly 90 percent of all deaths from vector-borne disease globally. And it is surging in Africa, surpassing AIDS as the biggest killer of African children under age 5.
From the 1940s onward, DDT was used to kill agricultural pests and disease-carrying insects because it was cheap and lasted longer than other insecticides. DDT helped much of the developed world, including the United States and Europe, eradicate malaria. Then in the 1970s, after the publication of Rachel Carson’s “Silent Spring,” which raised concern over DDT’s effects on wildlife and people, the chemical was banned in many countries. Birds, especially, were said to be vulnerable, and the chemical was blamed for reduced populations of bald eagles, falcons and pelicans. Scientific scrutiny has failed to find conclusive evidence that DDT causes cancer or other health problems in humans.
Today, indoor DDT spraying to control malaria in Africa is supported by the World Health Organization; the Global Fund to Fight AIDS, Tuberculosis and Malaria; and the United States Agency for International Development.
The remaining concern has been that the greater use of DDT in Africa would only lead mosquitoes to develop resistance to it. Decades ago, such resistance developed wherever DDT crop spraying was common. After the DDT bans went into effect in the United States and elsewhere, it continued to be used extensively for agriculture in Africa, and this exerted a powerful pressure on mosquitoes there to develop resistance. Although DDT is now prohibited for crop spraying in Africa, a few mosquito species there are still resistant to it. But DDT has other mechanisms of acting against mosquitoes beyond killing them. It also functions as a “spatial repellent,” keeping mosquitoes from entering areas where it has been sprayed, and as a “contact irritant,” making insects that come in contact with it so irritated they leave.
In our studies, in which we sprayed DDT on the walls of huts in Thailand, three out of every five test mosquitoes sensed the presence of DDT molecules and would not enter the huts. Many of those that did enter and made contact with DDT became irritated and quickly flew out.
The mosquitoes we used were the kind that carry dengue and yellow fever, not malaria. But there is ample evidence that malaria-carrying mosquitoes respond similarly to DDT. Several malaria-carrying species are even more sensitive to DDT’s repellent effects.
When we sprayed the huts with either dieldrin or alphacypermethrin, in contrast, all the test mosquitoes entered. Alphacypermethrin acted as a contact irritant, and it killed others that lingered on a treated surface. Dieldrin worked only as a poison — a powerful one, killing 92 percent of mosquitoes that made contact with it, far more than alphacypermethrin or DDT.
But dieldrin’s strong toxicity means that mosquitoes quickly develop resistance to it. Its use against malaria was short-lived, ending in the 1950s, because it so quickly became powerless.
Alphacypermethrin and others like it in the family of so-called pyrethroid insecticides are viewed as environmentally friendly, so they are used heavily in agriculture, in Africa and elsewhere. They are also used for treating bed nets and in indoor spraying programs to control malaria. But these multiple uses, combined with fact that the insecticide must make contact with the insect in order to work, have made pyrethroid resistance a large and growing problem for pest control programs in Africa.
DDT’s spatial repellency, by keeping mosquitoes from making physical contact, reduces the likelihood that the insects will develop resistance. Even those mosquitoes already resistant to poisoning by DDT are repelled by it.
It would be a mistake to think we could rely on DDT alone to fight mosquitoes in Africa. Fortunately, research aimed at developing new and better insecticides continues — thanks especially to the work of the international Innovative Vector Control Consortium. Until a suitable alternative is found, however, DDT remains the cheapest and most effective long-term malaria fighter we have.