The study, published in JAMA Neurology, found higher levels of DDE, the chemical compound left when DDT breaks down, in the blood of late-onset Alzheimer's disease patients compared to those without the disease - suggesting a possible link between exposure to the pesticide and risk of developing the condition.
Led by Jason Richardson of Rutgers University, the team noted that researchers have known for many years that the synthetic pesticide DDT is harmful to bird habitats and a threat to the environment. Indeed, DDT is now banned in the majority of countries - however historical exposures and the fact that some countries still use the pesticide mean a possible link to Alzheimer's should be taken seriously, said the authors.
"I think these results demonstrate that more attention should be focused on potential environmental contributors and their interaction with genetic susceptibility," said Richardson.
"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," he said.
While it is true that levels of DDT and DDE have decreased significantly globally since its ban, data from blood samples collected by the US Centers for Disease Control and Prevention as part of a national health and nutrition survey shos that the toxic pesticide is still found in 75 to 80% of the people's blood.
This occurs because the chemical can take decades to breakdown in the environment, said the team.
In addition, people may be exposed to the pesticide by consuming imported fruits, vegetables and grains where DDT is still being used and eating fish from contaminated waterways, they added.
The Rutgers research is the first to link environmental exposure to DDT with Alzheimer's disease after the team found that 74 out of the 86 Alzheimer's patients involved in the study – with an average age was 74 – had DDE blood levels almost four times higher than the 79 people in the control group who did not have Alzheimer's disease.
Patients with a version of ApoE gene (ApoE4), which greatly increases the risk of developing Alzheimer's, and high blood levels of DDE exhibited even more severe cognitive impairment than the patients without the risk gene, they added.
Brain cell studies also found that DDT and DDE increased the amount of a protein associated with amyloid plaques, which are believed to be a hallmark of Alzheimer's disease.
Richardson said that these findings are very important because it suggests that DDT and DDE may directly contribute to the process of plaque development.
"We need to conduct further research to determine whether this occurs and how the chemical compound interacts with the ApoE4 gene" he said.