DDT is in the dock again, this time for making babies arrive smaller and sooner.
Exposure to DDT may have caused premature births, increasing infant mortality in the United States during the 1960s. Banned in the developed world, DDT - a cheap and highly effective insecticide - is still widely used to control malaria-carrying mosquitoes in the developing world.
High levels of DDT may have tripled mothers' chances of having a pre-term birth, suggest researchers at the National Institute of Environmental Health Sciences in Bethesda, Maryland1.
Having also found an independent correlation between low birth weight and DDT exposure, the team is now calling for the benefits of DDT to be re-evaluated. "I hope the publication of these findings brings the issue to the fore," says epidemiologist Matthew Longnecker, who led study.
DDT is slow to break down, so has long been recognized as an environmental concern. Animal tests had suggested that it might cause developmental problems in humans, but no strong evidence for any effect due to low environmental levels had been found before.
Instead of looking at the effects of DDT on modern-day mothers, Longnecker's team took advantage of archived material from when DDT use was widespread.
Between 1959 and 1966, the US Collaborative Perinatal Project (CPP) took blood samples from 42,000 pregnant women in around 12 US cities, and followed them and their 55,000 children for 7 years.
Longnecker and his colleagues based their analysis on 2,380 mothers, eliminating those for which there was insufficient data. They looked for DDE, the stable breakdown product of DDT, in the archived blood samples. They compared this data with the babies' birth dates, health and birth weights.
After controlling for factors known to contribute to premature birth, such as smoking, income level and size of the mother, the researchers found that the likelihood of premature birth in their sample increased with increasing DDE concentrations.
If indeed - and it's a big 'if' - the premature births found in the analysis were a direct result of DDT exposure, and if those premature babies had the same likelihood of survival as babies born prematurely for other reasons, as much as 15 per cent of infant mortality between 1959 and 1966 may have been due to DDT, says Longnecker.
"This is an important study," says Gertrud Berkowitz, another epidemiologist at Mount Sinai School of Medicine in New York, who studies the effects of DDT on pregnant women. Although the findings will require confirmation by others, Longnecker's discovery that risk of premature birth increased with increasing DDE levels "strengthens his results," says Berkowitz
In countries that still use DDT, mothers' DDE levels are as high, if not higher, than those of the CCP women, says Longnecker, who is now studying mothers in agricultural Mexico, where DDT is still deployed. Given that mosquito control alternatives are getting cheaper, findings like his should "play a greater role in decisions of pesticide choice," he argues.
Though DDT is unpopular with environmentalists, disease-control experts have long argued that its benefits for beating malaria far outweigh the costs, making it a necessary evil in some poor countries.
"The greatest burden of malaria is on pregnant women and young children," points out malaria control expert Donald Roberts, a DDT advocate at the Uniformed Services University in Bethesda, Maryland. "What is [DDT] as a public health risk factor compared to malaria?" he asks.
Longnecker, M. P., Klebanoff, M. A., Zhou, H. & Brock, J. W. Association between maternal serum concentration of the DDT metabolite DDE and preterm and small-for-gestational-age babies at birth. Lancet 358, 110 - 114 (2001).
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Association between maternal serum concentration of the DDT metabolite DDE and preterm and small-for-gestational-age babies at birth. Nature (2001). https://doi.org/10.1038/news010719-3