It is suggested that an exposure to pesticides (either occupational or non-occupational) carries an increased risk of developing Parkinson's Disease.

See e.g. Kab et al, Agricultural activities and the incidence of Parkinson's disease in the general French population, Eur J Epidemiol 2017.

Only about 10 percent of Parkinson's cases have been linked to a genetic cause. Most cases of Parkinson's are called idiopathic or sporadic, meaning the cause is unknown, and researchers believe onset is due to a combination of genetic and environmental factors.

Genetic factors interact with increased occupational exposures among humans to neurotoxic pesticides which in turn act as disease modifying factors that unmask latent Parkinson's disease is one of the more scientifically plausible and likely explanations for an increased risk among farmers. When you think about it, no animals have seen the increase in life expectancy that humans have and they also do not work as pesticide applicators. We found a younger age at onset of PD was associated with occupational exposure to metals and pesticides.

Article Younger age at onset of sporadic Parkinson's disease among s...

Add an additional genetic factor that modifies the metabolism/detoxification of the pesticide to the mix and you have an increased risk of disease among farmers. You can read my review on this topic here

Article A critical review of the interrelationships between genetics...

As for the young onset forms of the disease these have specific recognized genetic risk factors many of which are heritable and as previously mentioned, account for only small subset of people who get the disease; although their disease onset can be expect to also be modified by exposure to neurotoxicants as well as was shown by Jemma Wilk et al 2006. Article Herbicide exposure modifies GSTP1 haplotype association to P...

Parkinson's disease is a neurodegenerative disorder characterized by motor impairment, autonomic dysfunction, and, in many, psychological and cognitive changes. Epidemiologic studies reports increased risk of Parkinson's disease associated with exposure to environmental toxicants such as pesticides. That's the reason why farmers are more exposed to Parkinson's disease. Please take a look at the following references.

Article Rotenone, Paraquat, and Parkinson’s Disease

Article Pesticides and Parkinson's disease--is there a link?

Article Pesticides and Parkinson’s disease: A potential hazard in ag...

Article Pesticide exposure and risk of Parkinson's disease: A family...

Chapter Pesticides and Parkinson�s Disease

Dear Arefeh Shams,

Parkinson's disease is a disease which affects about 50 percent more men than women. Age is the clear risk factor for Parkinson's. Prevalence of Parkinson's disease is continuously increasing. Although most people quite often seen with Parkinson's first develop the disease at about age 60 and PD they live between 10 and 20 years after being diagnosed. The pesticides with the most convincing data connecting them to increased risk of PD and one should consider limiting exposure to these chemicals. Farmers are more prone to Parkinson's than the general population. Over a decade of evidence shows a clear association between pesticide exposure and a higher risk for the second most common neurodegenerative disease, after Alzheimer's.

Most expert scientists agree that this is caused by a combination of genetic and environmental factors. There are two major causes of death with PD are falls and pneumonia. People affected with disease are at higher risk of falling, that require surgery, with medication and anesthesia to protect from heart failure, and blood clots from immobility.

Ashish