Webinar: Pesticides and Parkinson’s

Pesticides are chemicals used to kill or control unwanted species (pests), most commonly in agriculture and landscaping. Different types include herbicides for weeds, insecticides for insects, and piscicides for fish. They can be synthesised in a lab or derived from natural sources.

There is a growing body of evidence to suggest that significant exposure to pesticides is associated with an increased risk of Parkinson’s. Researchers believe this may be because pesticides can damage cells and put them under stress, including dopamine neurons – the type of nerve cell affected in Parkinson’s.

This research has involved both laboratory and population studies. Laboratory studies help scientists understand how exactly pesticides interact with cells in the body and why they have their effect. Population studies give a much broader picture, examining how people come into contact with these chemicals and whether rates of Parkinson’s are higher in certain groups.

One of these major population studies was led by Dr Caroline Tanner in 2007. Researchers found that, although increased Parkinson’s prevalence was not associated with overall pesticide use, there were certain pesticides that may put someone more at risk. You can read the full study here. Another helpful paper is one by Dr Ray Dorsey, which summarizes recent evidence on a number of environmental toxicants, including pesticides (read here).

The two pesticides most commonly associated with Parkinson’s are paraquat (herbicide) and rotenone (insecticide, piscicide)[1]. In most countries, these pesticides have either been banned completely or are under heavy restrictions.

[1] Tanner, C. M., Kamel, F., Ross, G. W., Hoppin, J. A., Goldman, S. M., Korell, M., Marras, C., Bhudhikanok, G. S., Kasten, M., Chade, A. R., Comyns, K., Richards, M. B., Meng, C., Priestley, B., Fernandez, H. H., Cambi, F., Umbach, D. M., Blair, A., Sandler, D. P., & Langston, J. W. (2011). Rotenone, paraquat, and Parkinson’s disease. Environmental Health Perspectives, 119(6), 866–872. https://doi.org/10.1289/ehp.1002839

Pesticides are thought to mainly affect two drivers of Parkinson’s progression: mitochondrial dysfunction and oxidative stress. Mitochondria are the parts of our cells that control energy production. Often, in the neurons of people with Parkinson’s, mitochondria do not operate as they should, meaning the cell is not getting enough energy to meet its needs. Pesticides can contribute to this dysfunction by interrupting part of the energy production process; rotenone, for example, blocks a specific step in the electron transport chain[2].

Oxidative stress occurs when there is an imbalance between levels of toxic molecules called reactive oxygen species (ROS) and the ability of cells to detoxify these. Since ROS are a normal by-product of energy production, problems with mitochondria can lead to these molecules building up, causing cell damage.

Pesticides are also thought to interact with several genetic risk factors for Parkinson’s. These are mutations or variations in our genes – segments of our DNA which provide the instructions for building proteins. Changes to genes can cause their associated proteins to not function correctly, creating issues with the pathways the proteins play a role in. Evidence suggests that this is a two-way relationship, where genetic risk factors can worsen the effects of pesticides, and pesticides may also worsen the effects of genetic risk factors. A laboratory study, for instance, found that the presence of a LRRK2 mutation, a known genetic risk factor for Parkinson’s, increased the toxic and inflammatory effects of paraquat[3].

[2] Rudyk, C., Dwyer, Z., Hayley, S., Schurr, E., Brown, E., Gibbings, D., Hayley, S., Park, D., Philpott, D. C., Rioux, J. D., Schlossmacher, M., & CLINT membership. (2019). Leucine-rich repeat kinase-2 (Lrrk2) modulates paraquat-induced inflammatory sickness and stress phenotype. Journal of Neuroinflammation, 16(1), 120. https://doi.org/10.1186/s12974-019-1483-7

[3] Li, N., Ragheb, K., Lawler, G., Sturgis, J., Rajwa, B., Melendez, J. A., & Robinson, J. P. (2003). Mitochondrial complex i inhibitor rotenone induces apoptosis through enhancing mitochondrial reactive oxygen species production. Journal of Biological Chemistry, 278(10), 8516–8525. https://doi.org/10.1074/jbc.M210432200

There is currently no strong evidence to suggest that exposure to pesticides through residue on food increases someone’s risk of developing Parkinson’s. Regardless, it is generally recommended to wash produce before consumption.

Most studies that have looked at this have focused on people who either work in industries where they frequently come into direct contact with these chemicals (such as farmers and manufacturers) or have experienced high, acute exposures (e.g. groundwater contamination). For most people, however, it is difficult to track how much or which pesticides they have come into contact with over their lifetime.

It is important to remember that pesticides are a risk factor, not necessarily a cause. It is generally thought that Parkinson’s is caused by a combination of environmental and genetic factors, which varies widely from person to person.

The main reason most of the evidence we have is on older pesticides is time:

• People can develop Parkinson’s decades after exposure to pesticides. Additionally, as a condition that most often develops later in life, we are only now seeing long-term effects of chemicals that were introduced in the mid-20th century. 
• The types of pesticides we use change over time and vary by country.
• This is a relatively new area of research, and we are only beginning to understand the impacts of these pesticides.

There are many types of pesticides used today in both commercial and domestic farming, and these vary from country to country. One of the most common is the herbicide glyphosate, also known as “Roundup”. Recent research has been trying to understand how glyphosate affects the nervous system and whether it may be associated with certain neurological conditions, including Parkinson’s. Some papers summarising these early findings can be found here and here.

If you work in an industry that commonly uses these chemicals, or are using pesticides at home, it is highly recommended that you follow all personal protective equipment (PPE) guidance.

Being a conscious consumer can be helpful – researching where your food is coming from can allow you to better understand how it is produced and what pesticides may be used. For most people, exposure through food is considered low, but it is always recommended to wash fresh fruits and vegetables before consumption. It may also be helpful to look at local water quality reports and use a water filter in your home.

In general, maintaining a healthy lifestyle is also important, as Parkinson’s risk is influenced by many different factors. Regular exercise is a good way to support general health and is associated with better overall brain health.

Connecting with your local environmental groups is a great way to start getting involved in activism.

Cure Parkinson’s recently supported a campaign by the Soil Association to end use of pre-harvest glyphosate in the UK. Glyphosate is an herbicide known more widely as “Roundup”. A common practice in agriculture is to spray crops with glyphosate right before harvest to help dry out crops faster and more evenly. Having this done so close to harvest, however, increases the residue left on them when they reach consumers. This activity is already banned in the EU, and people in the UK can sign the petition for the UK government to align on this: https://act.soilassociation.org/cut-the-chemicals.