With any pesticide, over-use can have harmful effects on the environment. 

DDT has not been used in America for over four decades but Rutgers scholars say that exposure to DDT may also increase the risk and severity of Alzheimer's disease in some people, particularly those over the age of 60. 

The cause of Alzheimer's disease, which about five million Americans have now, is not known, but scientists hypothesize that late-onset Alzheimer's may be linked to a combination of genetic, environmental and lifestyle factors. Much of the research into Alzheimer's and other neurodegenerative diseases has mostly been centered on finding genetic connections but a paper in JAMA Neurology says that levels of DDE, the chemical compound left when DDT breaks down, were higher in the blood of late-onset Alzheimer's disease patients compared to those without the disease.  Many of the authors contributed to a similar article from 2009, linking DDT to Parkinson's Disease.

DDT was introduced as a pesticide during World War II. It has been unmatched in its ability to control malaria and so countries where that is a concern have benefited greatly from it, but due to public outcry after an advocacy book against it, "Silent Spring" by Rachel Carson, was published, it was banned in America. The new study is correlational, two matching curves. The consensus is that the problem with DDT was not the product itself, but overuse, or everyone in America would have cancer and Alzheimer's and everything else once blamed on DDT.

Traces of DDT can still found in 75 to 80 percent of the blood samples collected from the Centers for Disease Control and Prevention for a national health and nutrition survey, along with 148 other chemicals, including metals, cotinine, polycyclic aromatic hydrocarbons (PAHs), dioxins, furans and polychlorinated biphenyls (PCBs), phthalates, phytoestrogens, organochlorine pesticides, organophosphate pesticides, herbicides, pyrethroid insecticides, other pesticides, and carbamate insecticides.  

The authors contend DDT may be acquired by consuming imported fruits, vegetables and grains where DDT is still being used, or eating fish from contaminated waterways. 

"I think these results demonstrate that more attention should be focused on potential environmental contributors and their interaction with genetic susceptibility," says Jason R. Richardson, associate professor in the Department of Environmental and Occupational Medicine at Rutgers. "Our data may help identify those that are at risk for Alzheimer's disease and could potentially lead to earlier diagnosis and an improved outcome."

With Emory University Alzheimer's Disease Research Center and the University of Texas Southwestern Medical School's Alzheimer's Disease Center, 74 of 86 Alzheimer's patients whose average age was 74 had DDE blood levels 3.8X higher than the 79 people in the control group who did not have Alzheimer's disease. Yet patients also had a version of ApoE gene (ApoE4), which greatly increases the risk of developing Alzheimer's. Their brain cell studies also found that DDT and DDE increased the amount of a protein associated with plaques believed to be a hallmark of Alzheimer's disease. These sticky amyloid proteins – which may form in regions of the brain involved in memory, learning and thinking – break off and clump together in the brain and increase as the disease progresses.

DDT and DDE may directly contribute to the process of plaque development, they conclude. 

"We need to conduct further research to determine whether this occurs and how the chemical compound interacts with the ApoE4 gene" Richardson says. "This study demonstrates that there are additional contributors to Alzheimer's disease that must be examined and that may help identify those at risk of developing Alzheimer's. It is important because when it comes to diagnosing and treating this and other neurodegenerative diseases, the earlier someone is diagnosed, the more options there may be available."

Citation: Jason R. Richardson, PhD; Ananya Roy, ScD; Stuart L. Shalat, ScD; Richard T. von Stein, PhD; Muhammad M. Hossain, PhD; Brian Buckley, PhD; Marla Gearing, PhD; Allan I. Levey, MD, PhD; Dwight C. German, PhD, 'Elevated Serum Pesticide Levels and Risk for Alzheimer Disease', JAMA Neurology January 27 2014 doi:10.1001/jamaneurol.2013.6030