Pesticides, Parkinson's and the Precautionary Principle

In May, two Italian scientists, Gianni Pezzoli and Emanuele Cereda, published a study in Neurology linking exposure to pesticides and solvents to Parkinson’s disease. I was immediately very curious. In my article “Kerala’s endosulfan tragedy---Did it really happen?”, I spoke about a very wide range of diseases being attributed to endosulfan exposure in Kasargod, Kerala. Among them was Parkinson’s Disease.

So what does this new study mean? Can we now say that all the people in Kerala receiving compensation for Parkinson’s are being classified correctly?

I emailed Cereda, one of the authors of the study, who was kind enough to respond.  This is what I understood. The paper, ‘Exposure to pesticides or solvents and risk of Parkinson’s disease’ is basically a meta-analysis of 104 past studies on pesticides. A meta-analysis is when you compare a number of different studies and hope to come across a trend you wouldn't have seen in an individual study.

What this study did was check if there was any correlation between occurrence of Parkinson’s disease and living in rural areas, consuming well water (likely contaminated by pesticides) and farming. And it found a significant link, although it isn't ‘conclusive’.

So does this mean endosulfan is likely to cause Parkinson’s? We still don’t know, because according to Cereda, 51 of the 104 studies were about exposure to a mix of pesticides. So the conclusion applies to people exposed to a mix of pesticides, and not any single one. We do not know which pesticides in this mix cause Parkinson’s, and it is highly likely that there is a group of 4-5 pesticides common to these 51 studies that are the culprits.

In Cereda’s study, there were only about 20 studies that linked specific pesticides to Parkinson’s.  Among these, exposure to Paraquat, a herbicide, caused a 2-fold increase in risk of Parkinson’s. On the other hand, no association was found with DDT, Maneb or Mancozeb.

This study, however, does not say what dose of exposure causes the disease. Cereda said this was the main limitation of his study, apart from the usual limitations of a meta-analysis.

In general though, this study goes much farther than previous studies in establishing a clear link between pesticide exposure and Parkinson’s.

Going back to the question I started with. Does this mean a chemical like endosulfan can cause Parkinson’s?  I guess we continue to remain in the same boat. There is a likelihood, but we can’t say for sure. The precautionary principle would come into play here, and farmers and people living in rural areas would have to reduce exposure to such pesticides.

On a related note, when the Royal College of Obstetricians and Gynaecologists (RCOG) issued guidelines this month to pregnant women, asking them to avoid chemicals in common household products such as cosmetics, sunscreen, air-freshener and plastic food containers, several scientists criticized the College for scaremongering, because, again, evidence of the endocrine disrupting properties of several of these chemicals is ‘inconclusive’.  There it is again. That dirty word.

One of the authors of the report, Richard Sharpe, however, defended the report, saying that if his wife was pregnant, they would be taking all those precautions.

I tried to contact some of the critics of RCOG’s report, but received no response.

Any opinions from readers on whether RCOG’s report amounts to scaremongering?