ABC Landline in Australia recently broadcast a segment about paraquat, a product used to control weeds, which contained serious allegations that paraquat causes Parkinson’s disease.
Syngenta categorically denies these allegations, based both on its own scientific analysis and the independent scientific literature. While it is heartbreaking to see people suffer from Parkinson’s disease, no peer-reviewed scientific analysis has ever established a causal relationship between a farmer’s use of paraquat and Parkinson’s disease.
The ABC report was not a balanced or fair discussion of the issue. Without being able to establish causation, the report focused on emotional stories of patients with Parkinson’s, and used innuendos to portray Syngenta negatively. Regrettably, the report omitted important facts, some of which are elaborated below.
We have also added links to our full response to ABC Landline.
We appreciate your interest in this complex issue and invite you to read our responses to some of the points made in the program, and reach your own conclusions.
Parkinson’s disease is a naturally occurring neurodegenerative disease that was first identified in 1817, more than 100 years before paraquat was first commercialized and sold. Parkinson’s affects millions of people around the world who have never used paraquat. Gene mutations are the only known cause of Parkinson’s.
Syngenta rejects the claims of a causal link between paraquat and Parkinson’s disease because it is not supported by scientific evidence. Despite more than six decades of investigation and more than 1,200 epidemiological and laboratory studies, no scientist or doctor has ever concluded in a peer-reviewed scientific analysis that paraquat causes Parkinson’s. Nor has any medical textbook or treatise concluded that paraquat causes Parkinson’s. In short, the hypothesis that paraquat causes Parkinson’s is not accepted in the medical community or peer-reviewed science, nor has it been accepted at any time in the past. In fact, according to the peer reviewed literature: there is a “consensus in the scientific community that the available evidence does not warrant a claim that paraquat causes Parkinson’s disease.” (Weed 2021).
In particular,
The Largest, Most Recent publication from the Independent Agricultural Health Study Finds No Casual Link
The Agricultural Health Study (“AHS”), which is sponsored by the US National Institutes of Health and several independent public health institutions, including the National Institute of Environmental Health Sciences and National Institute for Occupational Safety and Health, has followed more than 66,000 chemical applicators and their spouses since 1993. In 2020, using data from the AHS study, Drs. Srishti Shrestha and Dale P. Sandler, both with the National Institute of Environmental Health Science’s Epidemiology Branch, published their 25-year update, focusing specifically on a potential link between the use of pesticides and herbicides (including paraquat) and Parkinson’s. (Shrestha 2020). That study found no statistically significant link between paraquat and Parkinson’s, and in fact, did not find any increased risk of Parkinson’s with the increased use of paraquat.1
The Tanner 2011 publication referenced was an interim study -- a snapshot in time of a small subset of people who were part of a much bigger study. All of the study participants included in the Tanner publication were also included in the Shrestha 2020 Ag Health study, which was ten years longer, one hundred times larger, and superseded Tanner. In other words, Shrestha 2020 was a continuation of the Tanner 2011 study, published by many of the same authors, but now using the complete set of data rather than a subset. Dr. Tanner herself was involved in reviewing the data for Shrestha 2020 and was specifically acknowledged for her support in the publication.
Animal Studies Do Not Replicate Real World Paraquat Use Conditions
Beginning in the 1990s, researchers injected rodents with near-fatal doses of paraquat – typically into the abdomen. Syngenta did not believe that injecting mice with massive doses of paraquat necessarily raised concerns about the health of farmers applying paraquat in their fields. As the US Environmental Protection Agency recently put it in analysing the mouse-injection studies, “injection is not a relevant route of exposure for pesticidal uses of paraquat and cannot be used to evaluate toxicity and risk for anticipated exposure scenarios.” (EPA 2020). Nevertheless, Syngenta took the hypothesis and the method being used to study it seriously.
In the early 2000s, upon learning that there were some new studies reporting losses in dopaminergic neurons, the degeneration of which are thought to play a role in Parkinson’s disease, Syngenta asked one of its scientists to conduct several studies to determine if these findings could be replicated as part of Syngenta’s paraquat safety evaluation. Specifically, the researcher conducted four studies in which a certain breed of mouse was injected with near-fatal doses of paraquat. Of those four studies, one was unable to replicate the loss in dopaminergic neurons, two were identical to what was already in the public literature, and the fourth was materially identical to what was in the public literature but had a slight difference in dosing regimen. Only the fourth study presented new information for purposes of Syngenta’s reporting obligations – which Syngenta timely reported to the EPA in 2006.
The other studies did not add any new information to the publicly available body of science that had already existed. And when Syngenta later did submit the studies, the EPA confirmed they were not relevant given the route of administration, as well as the information then already existing on paraquat. In the EPA’s words: “Additional animal and in vitro publications were identified in the public comments that were not included in the literature database compiled for the PD systematic review. . . The agency reviewed all newly identified and submitted studies and concluded that they would not impact the PD systematic review conclusions.” (EPA 2020).
To be clear, Syngenta believed that epidemiology (evaluating humans actually using paraquat occupationally) was likely to be far more instructive than high-dose primate or mouse injection studies. Nevertheless, because Syngenta’s work in animal studies yielded contradictory results, it continued the research to even more thoroughly assess the mouse model. Ultimately, the results of these efforts were published in 2013. (Breckenridge 2013). Dr. Breckenridge et al. found that the injection of “near-lethal doses of PQ” failed to show the “constellation of effects” that would suggest neuron death in the relevant part of the brain.
Regarding the Beck 2012 study referenced in the program, APVMA had concluded that “the reproducibility and/or reliability of findings in other studies showing effects in the brain following injection of paraquat are questionable for regulatory purposes.” 2
Regarding the Anderson 2020 paper coauthored by Dr. Deborah Cory-Slechta and others, the authors provided only speculative arguments and no experimental evidence, that the loss of olfaction (smell) in the mice was indicative of the development of Parkinson’s in the brain.
This study exposed mice to paraquat that had been aerosolized/vaporized (a form in which paraquat is not applied occupationally) in order to increase exposure far beyond what happens in occupational settings. Specifically, Anderson 2020 put mice in a whole-body inhalation chamber where they were exposed to aerosolized paraquat for four hours a day, five days a week, for four straight weeks. As the authors note, exposure studies showing airborne paraquat in occupational exposure settings is “ten fold lower than the concentration used in this study.”3
Please find here links to scientific studies conducted both independently and by Syngenta:
Does Paraquat cause Parkinson’s Disease? A Review of Reviews (Weed, 2021)
Mortality from Parkinson’s Disease and other causes among a workforce manufacturing paraquat: An updated retrospective cohort study (Tomenson, 2021)
Pesticide use and incident Parkinson’s disease in a cohort of farmers and their spouses (Shrestha, 2020)
1 Shrestha S, Parks CG, Umbach DM, Richards-Barber M, Hofmann JN, Chen H, Blair A, Beane Freeman LE, Sandler DP. Pesticide use and incident Parkinson's disease in a cohort of farmers and their spouses. Environ Res. 2020 Dec;191:110186. doi: 10.1016/j.envres.2020.110186
2 APVMA Review of the Mammalian Toxicology and Metabolism/Toxokinetics of Paraquat, Supplement II: NEUROTOXICOLOGY, October 2016, available at: https://www.apvma.gov.au/sites/default/files/publication/20771-paraquat-toxicology-report-supplement-ii-neurotoxicology-1-26-10-2016.doc
3 Paraquat Inhalation, a Translationally Relevant Route of Exposure: Disposition to the Brain and Male-Specific Olfactory Impairment in Mice, available at: 10.1093/toxsci/kfaa183