Do pesticides cause Parkinson’s?
Research on the effects of pesticides on human brain cells in the lab has recently appeared in the news. We go behind the headlines and find out what is known about the link between these chemicals and Parkinson’s.
The causes of Parkinson’s are complex and not fully understood. For the vast majority of people with the condition, multiple factors will have played a role and, as such, pinpointing a single cause is not possible.
However, understanding the factors that both increase and decrease risk of Parkinson’s is of interest, particularly to researchers who may be able to use the insight to develop new and better treatments, or help to reduce the risk for future generations. You can read more about this in a previous post:
Can you catch Parkinson’s?
Is the role of infection in Parkinson’s being overlooked, and what would this mean for prevention?
When it comes to pesticides, we have known about the association between certain types of pesticide and an increase in risk of Parkinson’s for a while. In fact it was back in the 1980s when scientists first became suspicious of one pesticide called paraquat.
Paraquat has a structure similar to a chemical called MPTP. But it wasn’t until the 1980s that it was discovered just how damaging the MPTP is for the dopamine producing brain cells of the substantia nigra.
The discovery came about by accident when a botched batch of synthetic heroin containing the chemical caused Parkinson’s-like symptoms almost overnight in users in Northern California. With the similarities in the chemical structure, researchers then turned their attention to what exposure to pesticides may be doing to the brain.
Which pesticides increase risk?
Pesticides are a broad group of chemicals that are structurally and functionally diverse, they have different actions and potential toxicities. As such, the broad brush approach of previous research which looked for the association between Parkinson’s and a variety of chemicals used in agriculture often fails to identify individual chemicals which increase risk.
The chemical with the strongest association is the herbicide paraquat, which has been reported to cause a 2.5 fold increase in the odds of getting Parkinson’s. And in the lab, when combined with a chemical fungicide, called maneb, researchers have discovered that exposure may active a toxic inflammatory response, lead to the formation of damaging types alpha-synuclein protein, and cause the loss of dopamine-producing brain cells.
But caution should be taken before suggesting that paraquat can cause Parkinson’s directly as a more recent meta-analysis states that the evidence so far cannot fully support a causal link between this chemical and Parkinson’s.
Despite this, many countries and the whole of the EU have already banned paraquat, but not for its connection to Parkinson’s. The chemical is acutely toxic when inhaled or comes into contact with the skin. In other countries, where it has not been banned, restrictive use measures have been established — such as limiting its concentration in formulated products and only allowing licensed mixers and ground applicators to manipulate it.
There is also strong evidence that continued exposure to rotenone is linked to a slightly increased risk of Parkinson’s, particularly in the absence of glove use. While this is another common pesticide, in certain countries its use is also being phased out.
While the association between Parkinson’s and other chemicals used in agriculture is less well established, other chemicals of note include organochlorines, which have also been linked to Parkinson’s in both epidemiology and laboratory studies. And organophosphates and pyrethroids, which have limited but suggestive human and animal data linking them to Parkinson’s.
Are people that use pesticides more likely to get Parkinson’s?
In occupational terms, there is more limited research. The most relevant evidence comes in the form of a recent systematic review and meta-analysis, which has identified at least a 50% increase in risk with occupational exposure to pesticides.
We know that Parkinson’s affects more men than women, and occupational exposures to pesticides and chemicals is often listed as just one of the reasons for this imbalance. However, not everyone who is exposed to risk factors for Parkinson’s go on to develop the condition.
But research in the news may help to explain why.
This study used stem cells, some of which carried a mutation in the alpha-synuclein gene, and used them to make dopamine producing brain cells in the lab. They then exposed these new brain cells to paraquat and maneb, or rotenone to assess what effect these chemicals had.
The team, led by Professor Ryan, discovered that the pesticides altered the way mitochondria (the cellular power stations) move around, and believe this could affect the energy levels of the cell. And we already know that the dopamine producing brain cells that are lost in Parkinson’s are particularly sensitive to problems with mitochondria.
But more importantly, the cells that contained genetic changes were more affected by the pesticides. This highlights that our genetic makeup likely plays a huge role in how susceptible we are to toxin exposure in our environment, and may help to explain why most people exposed to these chemicals do not go on to develop Parkinson’s.
So how worried should we be about pesticides?
Long term exposure to pesticides may increase risk of Parkinson’s, but you might still be wondering by how much. Often factors that increase risk are presented as a percentage or a fold increase, for instance in this post we quote research that suggests the risk of Parkinson’s with environmental exposure to pesticides is increase by at least 50%. But what does that mean in real terms?
Working these statistics out is complex and requires a large amount of data and specialist skills, but for the purpose of understanding how much a 50% increase in risk really matters, I am going to do something a lot simpler….
Well if we use the latest stats that lifetime risk of Parkinson’s is around 1 in 37, a 50% increase would put this number at 1.5 in 37, or around 1 in 25. So still, with exposure to pesticides, risk of Parkinson's is relatively low.
We are still learning about how environmental factors affect all aspects of our health, but research like this plays a powerful role in making our environment safer and also improving our understanding of what is happening in conditions like Parkinson’s so we can develop new and better treatments.