From readers.

DDT and Malaria control

In her article "Malaria, Mosquitoes, and DDT" (May/June), Anne Platt McGinn suggests that DDT, when used indoors against malaria vectors, is a "poison to our soils, our waters, and ourselves." However, though this is a widespread belief, there is no convincing supporting evidence. Rachel Carson's accounts of the harmful effects of DDT, via the food chain, on eggs of birds of prey, referred to the massive outdoor use of DDT in agriculture in the 1950s and 60s. The situation with indoor residual spraying against vector insects is incomparably different. This underlies the carefully worded South African amendment to the Convention on Persistent Organic Pollutants (POPs treaty), which doe not mention use only in "the last resort," but simply states that those governments planning to use DDT against vectors (or already using it) should inform UN Environment Programme. This amendment was accepted without a single opposing voice among the 150 national delegations at the POPs treaty negotiations in Johannesburg in De cember 2000.

DDT-based anti-malaria programs have not proved to be "toxic futility." In the 1950s and 60s, with no evidence for toxicity, they reduced:

* India's annual malaria incidence from about 75 million to about 105,000 (99.86 percent reduction);

* Zanzibar's holoendemic level of malaria almost to eradication, before a misguided political decision to terminate the program.

Recent results in those areas have not been so good, apparently because of vector resistance and/or failure to achieve such high household coverage as in the past. However, DDT spraying can still work well in appropriate circumstances, as shown by recent data showing that reversion to DDT spraying has:

* terminated an epidemic in the Madagascar highlands which had killed about 40,000 people in the late 1980s;

* turned around a four-fold rise in malaria incidence which occurred in the four years after South Africa switched from DDT to pyrethroid spraying; and

* achieved a 59-percent reduction in the year 2001 after the switch back to DDT.

Pyrethroid-treated bednets have done very well in the case of the national program in Vietnam and moderately well in the many small programs in Africa. However my review with A.E.P. Mnzava of the relevant data in Bulletin of the World Health Organization (78: 138 9-1400) does not support the claim that treated bednets have proved to be a "superior technology." Recent results with treated nets have been distinctly worse than in several house-spraying programs in Africa in the 1950s, 60s and 70s. We commented that the move from spraying to bednets may be partly motivated by the fact that it allows governments and other agencies to shift responsibility for the funding of malaria prevention from influential, affluent taxpayers to poor tropical villagers.

CHRIS CURTIS

The author responds:

Professor of Medical Entomology London School of Hygiene & Tropical Medicine, UK

While it provides for continued use of DDT, the Stockholm Convention also encourages countries to develop and implement alternatives that pose less risk to human health and the environment.

It's significant that the successes of spray programs of 30 to 50 years ago were achieved before there was widespread drug and insecticide resistance. As you and Mnzava conclude in your above-mentioned review, more recent studies show that pyrethroid-treated nets are "at least as efficacious" as house spraying.

Landscape management on a global scale

Howard Youth's article, "The Plight of Birds" (May/June) is a beautiful summary. The advocacy of landscape management is especially noteworthy. However, I'm disappointed that the author did not advocate landscape management on a global scale. The examples cited are too piecemeal and small-scale. Until world population growth is brought under control and Third World countries are convinced not to emulate the current U.S. standard of living (and until the U.S. sets a much better example), I see little hope that...