The study, which appears in the current issue of the international medical journal Lancet, was carried out by scientists at the National Institute of Environmental Health Sciences, the National Institute of Child Health and Human Development, the University of North Carolina at Chapel Hill, and the Centers for Disease Control and Prevention.
The scientists said they found elevated levels of DDT's breakdown product, DDE, in the stored blood of mothers recorded as giving birth to premature or low birth weight infants. Pre-term births are a major contributor to infant mortality.
"DDT levels in the U.S. are now low and likely not causing any harm," said Matthew Longnecker, M.D., Sc.D., NIEHS, lead author on the study. "But we have to be concerned about what might be happening in those 25 countries where DDT is still used. Also, looking back on earlier decades in the U.S., we may have had an epidemic of pre-term births that we are just now discovering."
The U.S. Collaborative Perinatal Project, a program of the National Institutes of Health and 12 universities, still has stored blood serum from the mothers of thousands of children born between 1959 and 1966. A sample group of 2,380 was studied. Of these women's births, 361 were born pre-term, and 221 were small for gestational age; that is, they weighed less than most infants their age. Mothers of the affected infants had higher levels of DDE in their blood, indicating higher exposure to DDT in the environment. Average levels were about five times higher than at present.
DDT has long been suspected of reproductive toxicity. It was identified by Rachel Carson as being a potent reproductive toxin in birds in her pioneering environmental book Silent Spring published in 1962. The book forecast a time when DDT and other persistent pesticides used at that time could produce a spring where there were no birds left to sing. In fact, bald eagle and the brown pelican were nearly driven to extinction before the banning of DDT in the U.S. in 1972 brought their numbers back.
Studies since then on human reproductive effects have been suggestive of the human reproductive toxicity of DDT, a pesticide still widely used and highly effective in areas where mosquito-borne malaria is a major public health problem. Previous studies have drawn data from smaller samples and were not statistically powerful.
"The findings of our study strongly suggest that DDT use increases pre-term births, which is a major contributor to infant mortality," Dr. Longnecker said. "If this association is causal, it should be included in any assessment of the costs and benefits of insect control using DDT."
Dr. Longnecker points out that other agents that are less toxic and less persistent, but more expensive, can be used to control malaria. He is now working with epidemiologists in Mexico to see if women from malaria areas, highly exposed to DDT, are affected like the U.S. women were.
NIEHS, which is in Research Triangle Park, N.C., and NICHD, in Bethesda, Md., are institutes of the federal National Institutes of Health. UNC-CH provided biostatistical support for the study. The National Center for Environmental Health is the agency of the Centers for Disease Control and Prevention in Atlanta participating in the study.
The authors are Matthew P. Longnecker, M.D., Sc.D., NIEHS; Mark A. Klebanoff, M.D., NICHD; Haibo Zhou, Ph.D., UNC-CH; and John W. Brock, Ph.D., NCEH.