The exact cause of Alzheimer's disease (AD), a type of dementia which currently affects five million Americans, is unknown though scientists believe the late-onset version of the disease may be linked to a blend of genetic, environmental, and lifestyle factors. In a new study, a team of researchers discovered that DDE, the chemical compound that remains after the pesticide DDT breaks down, showed higher levels in the blood of patients with late-onset Alzheimer’s than in people without the disease.

“Our data demonstrate that having DDE levels in the top tertile increased risk of Alzheimer’s in all [people],” Jason R. Richardson, associate professor in the Department of Environmental and Occupational Medicine at Robert Wood Johnson Medical School, Rutgers University, told Medical Daily in an email. In fact, exposure to the toxic pesticide, he and his co-researchers found, may increase not only the risk but also the severity of Alzheimer’s in some people. Their research appears online in JAMA Neurology.

International Contamination

First synthesized in 1874, dichlorodiphenyltrichloroethane, commonly known as DDT, was discovered to perform effectively as a contact insecticide in 1939. Shortly thereafter, DDT was used to control malaria and typhus during World War II, and after the conclusion of the war, the chemical became available for use as an agricultural pesticide in the U.S. In 1962, biologist Rachel Carson published her book Silent Spring, which suggested that DDT and other pesticides may cause cancer while also exerting a detrimental environmental impact, harming wildlife, particularly birds. General public condemnation led to a ban on the agricultural use of DDT throughout the U.S. in 1972. Despite a subsequent worldwide ban on its agricultural use under the Stockholm Convention, an international environmental treaty signed in 2001, DDT remains in use today in malaria-prone countries. Although China stopped manufacturing the chemical in 2007, India continues as the world’s sole producer of DDT and also as its largest consumer.

In the U.S. today, the toxic pesticide is still found in 75 to 80 percent of the blood samples collected from citizens by the Centers for Disease Control and Prevention. The reason it remains in the blood of so many Americans, scientists say, is because the chemical can take decades to breakdown in the environment. People may also be exposed to the pesticide when they eat imported fruits, vegetables, and grains from countries where DDT is still being used as well as by eating fish from contaminated waterways.

“There is still contamination of some foods from the U.S. (i.e., fish from contaminated waters etc.),” Richardson told Medical Daily. “The foods most likely to contain organochlorine residues, including DDE, include dairy, meat, and fish.” Because of the possible prevalence of contaminated foods here in the U.S., he and his team believe their research into how DDT and DDE may trigger Alzheimer’s and other neurodegenerative diseases is crucial.

Results of the Study

For their new study, Richardson and his scientific team began by noting occupational pesticide exposures have been shown to be associated with AD, while serum levels of DDE have been shown to be elevated in a small number of patients. To further evaluate DDE and the link to Alzheimer’s, the researchers designed a new study, whereby they measured serum levels of DDE in 86 people with Alzheimer’s (average age of 74) and 79 people without.

Surprisingly, the researchers discovered 74 of the 86 Alzheimer's patients had DDE blood levels that were nearly four times higher than the people in the control group. Next, the researchers compared the Mini-Mental State Examination (MMSE) test results of both the patients with a version of ApoE gene (ApoE4), which greatly increases the risk of developing Alzheimer's, and patients without the risk gene. Here, the researchers discovered “the cognitive measure (MMSE) was worse in those that had both” the genetic risk and high levels of DDE, Richardson told Medical Daily. Finally, the researchers conducted brain cell studies and found high DDT and DDE levels also seemed to increase the amount of a beta-amyloid protein that causes the plaques, a familiar hallmark of the disease. These sticky plaques break off and clump together, often in regions of the brain responsible for memory, learning, and thinking; as the disease progresses, so do these plaques. The study’s evidence, then, suggests that DDT and DDE may directly contribute to the process of plaque development.

"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," Richardson stated in a press release. "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." In the meantime, those who wish to minimize their chances of developing the progressive, neurological disorder might focus on eating only locally grown fruits and vegetables while also selecting their fish and seafood with great care.

 

Source: Richardson JR, Roy A, Shalat SL, von Stein RT, Hossain MM, et al.  Elevated Serum Pesticide Levels and Risk for Alzheimer Disease. JAMA Neurology. 2014.