Study co-author Dr. Dwight German, professor of psychiatry at the University of Texas Southwestern Medical Center in Dallas, said previous studies linked chronic diseases such as cancer and diabetes to DDT, but this is the first clinical study to link the U.S.-banned pesticide to Alzheimer's disease.
The study, published online in JAMA Neurology, found elevated levels of the DDT metabolite, DDE, in patients diagnosed with Alzheimer's disease -- 3.8 times higher than in control subjects.
German's team, in partnership with researchers at Emory University School of Medicine and Rutgers Robert Wood Johnson Medical School, found elevated levels of DDE in blood samples of 86 patients with Alzheimer's disease as compared to 79 control patients from the UT Southwestern Alzheimer's Disease Center and the Emory University Alzheimer's Disease Center.
Study participants, average age 74, underwent preliminary testing to ensure they didn't have symptoms of other dementia-related diseases. Control subjects were, on average, age 70.
The researchers linked DDE and Alzheimer's by measuring three components -- blood serum levels, severity of the patient's Alzheimer's disease as measured by the Mini-Mental State Examination and its relation to serum DDE levels, and the reaction of isolated nerve cells to DDE.
Treatment of human nerve cells with DDE caused them to increase the production of the amyloid precursor protein that is directly linked to Alzheimer's disease, German said.
DDT was used extensively as an insecticide in the 1940s, but banned in some places beginning in 1968. It is still used in some countries to combat the spread of malaria.
It has been estimated that a total of 1.8 million tons were produced globally since the 1940s and more than 600,000 tons were applied in the United States before the 1972 ban.
DDT is a persistent organic pollutant that is adsorbed in soils and sediments, which can act as long-term sources of exposure. Depending on conditions, its soil half life can range from 22 days to 30 years, the Agency for Toxic Substances and Disease Registry said.