The controversial pesticide DDT was banned in the US in 1972 but is still used in the agricultural industries of some countries. Now, a study published in the Journal of the American Medical Association Neurology has found a link between having levels of a DDT byproduct in the blood and an increased risk of Alzheimer's disease.
Alzheimer's disease is the most common form of neurodegenerative disease worldwide. There is no cure for the condition, which progressively destroys cognitive functioning - thinking, remembering and reasoning skills - as well as the ability to perform day-to-day tasks.
Most people who get Alzheimer's will have "late-onset" Alzheimer's, the symptoms of which usually only appear after age 60. The reason why some people develop Alzheimer's at this age is not completely understood.
Scientists think having certain genes - particularly a version of the gene apolipoprotein E (APOE) - can have a strong influence on whether somebody develops Alzheimer's or not, although genetics accounts for less than half of all Alzheimer's cases.
Some studies are starting to look at to what extent environmental factors can influence Alzheimer's risk. It has been suggested that some metals, solvents and pesticides may increase the risk of Alzheimer's in people who have jobs where they come regularly into contact with those materials.
Previously, the authors of the new study had found raised levels of a chemical called dichlorodiphenyldichloroethylene (DDE) in a small but significant number of late-onset Alzheimer's patients.
What are DDE and DDT, and what harm do they cause?
DDE is a byproduct of dichlorodiphenyltrichloroethane (DDT). Invented in the 1940s, DDT was one of the first modern insecticides, introduced to prevent the spread of insect-borne human diseases such as malaria and typhus.
Though an effective pesticide, over time, experts became aware of adverse effects caused by DDT to the environment, wildlife and humans, which led to its ban in the US in 1972. DDT is known to cause reproduction-related problems in humans and it is also thought to be the cause of some cancers.
In their new study, the researchers wanted to further explore this link between DDT and Alzheimer's. To do this, they studied existing blood samples from 86 patients with Alzheimer's and 79 members of a control group and measured the levels of DDE present in the blood.
What does the new study say?
The researchers detected DDE in the blood of 70% of the control group and in 80% of the Alzheimer's patients. Although this seems like a high proportion to be exposed to a banned substance, the DDE levels were probably because of DDT's long half-life in contaminated soil and waterways. It is also possible that some people had DDE in their blood because they had eaten food imported from countries where DDT is still used as a pesticide.
In the patients with Alzheimer's, levels of DDE were 3.8 times higher than in the members of the control group. Testing the cognitive function of the Alzheimer's patients, the researchers also found that the disease was more advanced in the people who had the highest levels of DDE and who also carried the version of the APOE gene linked to Alzheimer's.
Because people with the APOE gene linked to Alzheimer's and high levels of DDE had the lowest cognitive score, the researchers speculate that people who carry this gene may be more susceptible to the harmful effects of DDE.
The researchers think that identifying people with high levels of DDE in their blood who have this gene will lead to earlier identification of the disease.
Medical News Today recently reported on a home-based test that may allow symptoms of Alzheimer's to be detected earlier. Earlier detection of the disease means potential treatments could be started much earlier.