Pesticides Not Only Linked to Parkinson’s Disease Development, But Accelerating Disease Symptoms

(Beyond Pesticides, January 12, 2023) Exposure to certain pesticides among individuals diagnosed with Parkinson’s disease (PD) can increase the risk of symptom progression. According to a study published in Science of the Total Environment, nearly 20 percent of pesticides associated with the onset of PD also increase the risk of faster decline in motor and non-motor function. Several studies find exposure to chemical toxicants, like pesticides, has neurotoxic effects or exacerbates preexisting chemical damage to the nervous system. Past studies suggest neurological damage from oxidative stress, cell dysfunction, and synapse impairment, among others, can increase the incidence of PD following pesticide exposure. Despite the association between PD onset via pesticide exposure patterns, few epidemiologic studies examine the influence pesticides have on worsening motor and non-motor symptoms in PD.

Parkinson’s disease is the second most common neurodegenerative disease, with at least one million Americans living with PD and about 50,000 new diagnoses annually. The disease affects 50 percent more men than women, and individuals with PD have a variety of symptoms, including loss of muscle control and trembling, anxiety and depression, constipation and urinary difficulties, dementia, and sleep disturbances. Over time, symptoms intensify, but there is no current cure for this fatal disease. While only 10 to 15 percent of PD cases are genetic, PD is quickly becoming the world’s fastest-growing brain disease. Therefore, research like this highlights the need to examine how chemical exposure accelerates disease progression, especially among severe, incurable, and fatal illnesses. The study notes, “Pesticides are not applied in isolation, and people are not singly exposed to one agent over a lifetime. Both scientists and regulators need to consider co- and sequential application hazards and human exposures.”

Using a geographic information system (GIS) tool to gather information on ambient exposure to pesticides in residences and workplaces via California Pesticide Use Report records and land use records. The researchers examine the association between 53 pesticides with links to PD onset to determine PD symptom progression for five years and 2.7 years (respectively) for two patients. Measurements of PD symptom progression include movement disorder specialist-administered Unified Parkinson’s disease Rating Scale part III (UPDRS), Mini-Mental State Examination (MMSE), and Geriatric Depression Scale (GDS). Of the pesticides with links to PD onset, ten or ~18.8 percent (i.e., copper sulfate [pentahydrate], 2-methyl-4-chlorophenoxyacetic acid [MCPA] dimethylamine salt, tribufos, sodium cacodylate, methamidophos, ethephon, propargite, bromoxynil octanoate, monosodium methanearsonate [MSMA], and dicamba) have associations with faster symptom progression. The study identifies a progressive decline among three endpoints: motor skills, cognitive function, and mental health regarding depression. Markedly, individuals living near residential areas or working in occupations with higher exposure to copper sulfate and MCPA experience a rapid decline in all endpoints.

Parkinson’s disease occurs when there is damage to dopaminergic nerve cells (i.e., those activated by or sensitive to dopamine) in the brain responsible for dopamine production, one of the primary neurotransmitters mediating motor function. Although the cause of dopaminergic cell damage remains unknown, evidence suggests that pesticide exposure, especially chronic exposure, may be the culprit. Occupational exposure poses a unique risk, as pesticide exposure is direct via handling and application. A 2017 study finds that occupational use of pesticides (i.e., fungicides, herbicides, or insecticides) increases PD risk by 110 to 211 percent. Even more concerning, some personal protection equipment (PPE) may not adequately protect workers from chemical exposure during application. However, indirect nonoccupational (residential) exposure to pesticides, such as proximity to pesticide-treated areas, can also increase the risk of PD. A Louisiana State University study finds that residents living adjacent to pesticide-treated pasture and forest from the agriculture and timber industry have higher rates of PD incidence. Furthermore, pesticide residues in waterways and on produce present an alternate route for residential pesticide exposure to increase the risk for PD via ingestion. Pesticide contamination in waterways is historically commonplace and widespread in U.S. rivers and streams, with over 90 percent of water samples containing at least five or more pesticides. Pesticide exposure can cause severe health problems even at low residue levels, including endocrine disruption, cancers, reproductive dysfunction, respiratory problems (e.g., asthma, bronchitis), and neurological impacts (e.g., developmental effects and Parkinson’s), among others. Nevertheless, direct occupational and indirect nonoccupational exposure to pesticides can increase the risk of PD. 

This study is one of the few, possibly the first, to identify that pesticides can contribute to the progression of Parkinson’s disease. The study identifies 53 pesticides associated with PD onset, with ten directly accelerating declines in motor and non-motor function and mental health from amplified disease progression. However, pesticides’ worsening disease risk following exposure is not an unfamiliar phenomenon for either physical or psychiatric health. For instance, pesticide exposure can cause injury to cells responsible for safeguarding against viral infections, inducing more severe disease progression. Since the start of the pandemic, studies evaluating disease outcomes acknowledge excessive and improper use of pesticides, like disinfectants, as a culprit of immunocompromising the respiratory system of COVID-19 patients. COVID-19 is a systemic (general) disease that overwhelmingly impacts the respiratory system of many patients. The respiratory system is essential to human survival, regulating gas exchange (oxygen-carbon dioxide) in the body to balance acid and base tissue cells for normal function. Damage to the respiratory system can cause many issues—from asthma and bronchitis to oxidative stress that triggers the development of extra-respiratory, systemic manifestations like rheumatoid arthritis and cardiovascular disease. However, just as the respiratory system is far from the only bodily system affected by the virus, pesticides’ adverse effects can span multiple bodily systems, even co-concurrently. Furthermore, underlying medical conditions (i.e., heart/kidney disease, diabetes, cancer, high blood pressure, obesity, etc.) heighten risks associated with severe illness from disease. Additionally, this study is not the first to identify an association between multiple disease risks and proximity to areas with regular pesticide applications. Studies can match disease risk to zip code, with individuals in low-income, indigenous, and people of color communities at the greatest risk of developing pesticide and other environmentally induced diseases.

Over 300 environmental contaminants and their byproducts, including pesticides, are chemicals commonly present in human blood and urine samples and can increase neurotoxicity risk. For instance, 90 percent of Americans have at least one pesticide compound in their body, primarily from dietary exposure, like food and drinking water. These compounds have a global distribution, with evaporation and precipitation facilitating long-range atmospheric transport, deposition, and bioaccumulation of hazardous chemicals in the environment. Thus, exposure to these toxicants can cause several adverse environmental and biological health effects. With the increasing ubiquity of pesticides, current measures safeguarding against pesticide use must adequately detect and assess total chemical contaminants. 

The study concludes, “Identifying modifiable risk factors for disease progression may help identify new targets for research, perhaps leading to mechanistic insights important for medication development, and importantly help revise public health policy, aiming to reduce exposure to disease-modifying agents. Our study has implicated individual pesticides in Parkinson’s disease progression in several domains. For some, previous epidemiologic or experimental data are supportive of our findings. Further investigation should target both these individual pesticides and the cumulative risk of their mixtures to tease out potential synergistic effects.”

The scientific literature demonstrates pesticides’ long history of severe adverse effects on human health (i.e., endocrine disruption, cancer, reproductive/birth problems, neurotoxicity, loss of biodiversity, etc.) and wildlife and biodiversity. In addition to this research, several studies demonstrate autism, mood disorders (e.g., depression), and degenerative neurological conditions (e.g., ALS, Alzheimer’s, Parkinson’s) among aquatic and terrestrial animals, including humans, exposed to pesticides. However, there are several limitations in defining real-world poisoning as captured by epidemiologic studies in Beyond Pesticides’ Pesticide-Induced Diseases Database. The adverse health effects of pesticides, exposure, and the aggregate risk of pesticides showcase a need for more extensive research on occupational and nonoccupational pesticide exposure, especially in agriculture. For more information on the effects of pesticide exposure on neurological health, see PIDD pages on Parkinson’s disease, dementia-like diseases, such as Alzheimer’s, and other impacts on cognitive function. 

Parkinson’s disease has no cure, but preventive practices like organics can eliminate exposure to toxic PD-inducing pesticides. Organic agriculture represents a safer, healthier approach to crop production that does not necessitate toxic pesticide use. Beyond Pesticides encourages farmers to embrace regenerative, organic practices and consumers to purchase organically grown food. A complement to buying organic is contacting various organic farming organizations to learn more about what you can do. Those affected by pesticide drift can refer to Beyond Pesticides’ webpage on What to Do in a Pesticide Emergency and contact the organization for additional information. Furthermore, see Beyond Pesticides’ Parkinson’s Disease article from the Spring 2008 issue of Pesticides and You.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Parkinson’s News Today, Science of the Total Environment