Parkinson's disease: Common pesticides may damage brain cells

Parkinson's disease is a neurodegenerative condition characterized by motor symptoms, including tremor and rigidity, and nonmotor symptoms such as cognitive impairment and sleep disorders.

What causes Parkinson's disease is currently unknown. However, the condition is thought to arise from a mix of factors, including genetics, environmental factors, and aging. Increasing evidence suggests that exposure to certain pesticides may increase Parkinson's disease risk.

While pesticides are important for modern commercial agriculture and maximizing food production, how they may contribute to Parkinson's disease remains unknown.

Understanding more about how pesticides affect Parkinson's disease risk could inform agricultural policies and prevention strategies for the condition.

Dr. Daniel Truong, neurologist, and medical director of The Parkinson's and Movement Disorder Institute at MemorialCare Orange Coast Medical Center, not involved in the study, told Medical News Today that the findings have many implications should they be confirmed by future studies.

"Pesticide regulation as well as agricultural practices and worker safety may need to be changed. Farm workers may need to be monitored to see if indeed a higher incidence may occur. Foremost however the study needs to be confirmed by other means such as animal studies," he told us.

For the study, the researchers included data from 829 patients with Parkinson's disease and 824 controls alongside their residential areas.

Officials have recorded commercial pesticide use in California since 1972, meaning the researchers were able to estimate individual pesticide exposure over time. The researchers included 722 pesticides in their analysis.

In the end, they found that patients with Parkinson's disease were more likely to live and work near agricultural faculties with higher pesticide use than controls.

On average, Parkinson's disease patients lived in nearby areas where 50 different kinds of pesticides were used, whereas controls lived near areas where an average of 45 were used.

Next, the researchers assessed how a subgroup of 288 pesticides affected Parkinson's disease risk. Each of these pesticides affected at least 25 participants in the first analysis.

In doing so, they identified 53 pesticides that appeared to be linked to Parkinson's disease. The researchers noted that these Parkinson's disease-associated pesticides were 2–3 times more likely to contaminate groundwater than those more prone to diffuse in the air soon after application.

Afterward, the researchers tested 39 of these pesticides for toxicity in dopaminergic neurons derived from patients with Parkinson's disease.

Ten of these pesticides resulted in substantial neuronal death. They included:

The researchers wrote that eight of these 10 toxic pesticides are still registered for use by the US Environmental Protection Agency (EPA).

Finally, the researchers analyzed pesticides used in cotton farming. They found that commonly used herbicide, trifluralin, leads to toxicity in dopaminergic neurons and mitochondrial dysfunction.

They now plan to study the epigenetic and metabolomic effects of the identified pesticides to better understand their link to Parkinson's disease.

Dr. David Houghton, vice chair/ chief of Movement & Memory Disorders at Ochsner Health, who was not involved in the study, commented that: "This study […] may give us a clue to how toxin exposure interacts with a genetic predisposition to [Parkinson's disease]. Ultimately, the complicated interplay between environment and genetics likely holds the best clues for why Parkinson's disease occurs."

MNT spoke with Dr. Truong about the study's limitations. He noted that the study is observational and thus does not establish causation. He added that the study has a limited sample size and geographic scope.

Dr. Kelly Johnson-Arbor, medical toxicologist, co-medical director, and interim executive director at the National Capital Poison Center, who was not involved in the study, also not involved in the study, told MNT that the researchers applied pesticides directly to cells, meaning they were likely exposed to higher quantities than consumers may be exposed to naturally.

"The study investigators applied pesticides directly to cells, which did not take into account the presence of a blood-brain barrier that limits transfer of toxins into the central nervous system in humans. Because of these factors, the results of this study may not be fully applicable to humans," she added.

Dr. Elsa Rodarte, a movement disorders fellow at McGovern Medical School at UTHealth Houston, not involved in the study, also told MNT that the findings may be limited as they used dopaminergic neurons derived from only one patient.

"Using cells from a single patient that survive the whole preparation process results in very specific vulnerabilities. In other words, Jane Doe's cells will be susceptible to damage from pesticide X and not from pesticide Y. This information can help her avoid working with pesticide X, but John Deer might tolerate pesticide X and not pesticide Y. Banning one or the other would benefit different people. This method may be useful for the most toxic ones that affect a larger number of people," she explained.

Nevertheless, reducing pesticides in foods may be an important measure in minimizing the potential risks to health that may come with some of these chemicals.

Dr. Howard Pratt, board-certified medical director at Community Health of South Florida (CHI), not involved in the study, emphasized to MNT that: "While these studies suggest that we should be thoughtful about our fruit and vegetable consumption, and work to make sure they are appropriately washed before consumption, fruits, and vegetables remain an essential part of our diet. By identifying pesticides which may contribute to the development of Parkinson's we can remove or reduce them from our consumption."

Dr. Johnson-Arbor advised: "To reduce the amount of pesticides in foods, agricultural scientists can evaluate whether alternative pesticide formulations — for example, solid instead of liquid formulations, or microencapsulated suspensions instead of concentrated solutions- are a viable alternative to traditional pesticides. Scientists may also be able to develop alternative farming systems that require less pesticide use."

"For consumers, washing fruits and vegetables under room-temperature running water can help remove chemicals from produce surfaces. The US EPA recommends that people wash produce instead of soaking it, as washing more effectively removes chemicals and germs from the skin of fruits and vegetables. Also, peeling fruits and vegetables prior to eating them can also help reduce the chemical content in these foods, as most of the pesticides are found on the outer surface of the produce."

– Dr. Kelly Johnson-Arbor

Dr. Truong added that buying organic, local, and seasonal produce can also reduce pesticide consumption. He further noted that supporting integrated pest management (IPM) practices may also be beneficial.

"IPM focuses on using a combination of techniques such as biological control, crop rotation, and pest-resistant varieties to reduce the need for pesticides," he noted.

Dr. Johnson-Arbor noted that those who work with pesticides should take special caution to reduce their exposure.

"Always keep pesticides in their original packaging, and avoid transferring them into other containers — like bottles or cups — where people could accidentally drink them," she advised.

"Wear protective clothing, including gloves, when handling pesticides. Since wind can cause pesticides to spread over larger regions or unintended areas after application, don't apply pesticides on windy days," Dr. Johnson-Arbor added.

Elana Clar, movement disorders specialist and neurologist at New Jersey Brain and Spine in Oradell, NJ, not involved in the study, also told MNT that the study may help reinforce legislative efforts to ban these pesticides from widespread use.