Parkinson’s disease is the fastest growingTrusted Source neurological disorder in the world at present, and the reason why is unclear.

Parkinson’s disease is characterized by loss of neurons in the substantia nigraTrusted Source, a part of the brain responsible for motor control.

The neurons in this part of the brain create dopamine, a neurotransmitter that is used for signalling in the brain. Loss of this signalling affects motor control, a common symptom in people with Parkinson’s disease.

There are a number of theories as to how and why Parkinson’s disease develops. One theory is that a peptide called alpha-synuclein accumulates in the substantia nigra neurons of people with Parkinson’s disease.

These accumultations are known as Lewy bodiesTrusted Source, and they appear to damage the neurons and prevent them from creating the dopamine necessary for signalling.

Other theories include genetic inheritance and mitochondrial dysfunction, similar to some theories about Alzheimer’s disease. Another theory is that Parkinson’s disease is caused by exposure to environmental toxins, such as pesticides and herbicides.

The idea that some chemicals could damage the neurons in the substantia nigra, and lead to Parkinson’s disease has been around since the 1980sTrusted Source.

Now, research due to be presented at the American Academy of Neurology’s 76^th Annual Meeting, in Denver, CO, held between April 13–18, 2024, outlines how three more pesticides have been linked to increased risk of Parkinson’s disease.

This research has not yet appeared in a peer-reviewed journal. The study was funded by the Michael J. Fox Foundation.

Researchers from Amherst College, MA, Washington University in St Louis, MI, and Barrow Neurological Institute, AZ, analyzed data on 21,549,400 Medicare beneficiaries in the United States.

They then mapped usage of average annual pesticide application from 1992–2008, down to a county level.

They then used these data to look for an association between exposure to 65 well-documented pesticides and Parkinson’s disease risk across these areas. They also adjusted data for air pollution, rural/ urban residence, and median income.

Results showed an association between 14 pesticides and increased Parkinson’s disease risk in the Rocky Mountain and Great Plains region. Three of these were found to have the strongest relationship with increased risk: simazine, lindane, and atrazine.

People living in counties in the top decile for simazine use were found to have a 36% increased risk of Parkinson’s disease, while people living in the counties with the highest use of atrazine and lindane had a 31% and 25% increased risk of Parkinson’s disease, respectively.

The risk found was dose-dependent. All three of these pesticide are restricted in their use in the European Union and United Kingdom.

These findings support decades of research that has shown pesticide use has a dose-dependent effect on Parkinson’s disease risk, said Prof. Bastiaan R. Bloem, a neurologist and director of the Radboud University Medical Center of Expertise for Parkinson & Movement Disorders in Nijmegen, the Netherlands, who has written extensively on legislation surrounding pesticide and herbicide usage. Prof. Bloem was not involved in this research.

He wondered: “The from the University of Wageningen in the Netherlands found that of the nine European countries where they measured pesticides in the households of farmers and their neighbours, they found no less than 100 different pesticides, and what on Earth is the sum of all those small quantities, of all those pesticides?”

In a recent comment piece published in Nature Reviews Neurology, he points out that the E.U. recently extended marketing authorization for the use of glyphosate, the herbicide found in popular weed killer Roundup, for another 10 years, despite evidence from animal studies showing it affects the substantia nigra.

He argues that rather than the burden of proof of safety being on researchers, it should instead be on the companies, to prove their chemicals are safe.