LONDON — Syngenta, the Swiss pesticide giant, claims on its website that data from an influential 2011 study shows that farmers who use the weed killer paraquat are less likely to develop Parkinson’s disease than the general population.
However, Syngenta’s claim is at odds with the actual findings of the study, according to two of its authors.
The 2011 study, carried out by the National Institutes of Health and researchers from other institutions around the world, found that people who used paraquat or another pesticide, called rotenone, were roughly two and a half times more likely to develop Parkinson’s.
The work is known as the Farming and Movement Evaluation, or FAME, study. It drew on a sweeping United States government project called the Agricultural Heath Study, which tracked more than 80,000 farmers and their spouses, as well as other people who applied pesticides, in Iowa and North Carolina.
The FAME researchers identified 115 people from the Agricultural Health Study who developed Parkinson’s, and studied 110 of them who provided information on the pesticides they used.
The study was influential even among some people who had been skeptics of a connection between the chemicals and the disease. Gary W. Miller, a professor of environmental health at Emory University, referred to a link between Parkinson’s and paraquat as a “red herring” in a 2007 publication. But while Dr. Miller said in a recent email exchange that he had concerns about some previous research making the connection, “the FAME data are strong and should be considered.” He said the study “appears to show a connection between paraquat exposure and Parkinson’s disease.”
Because of the prominence of the FAME study, Syngenta addresses it on one of its websites, paraquat.com. Syngenta claims that the study shows that only 115 people had Parkinson’s out of the more than 80,000 people in the broader Agricultural Health Study. Therefore, “the incidence of Parkinson’s disease” in the study “appears to be lower than in the general U.S. population,” Syngenta says.
But the FAME study was not a comprehensive assessment of the incidence of Parkinson’s disease among those in the wider Agricultural Health Study. Rather, researchers picked out a group of people who did have Parkinson’s and specifically studied them against a control group.
“We did not study incidence, so the Syngenta statement is not correct,” said Dr. Caroline M. Tanner, the director of the Parkinson’s Disease Research, Education and Clinical Centers at the San Francisco Veterans Affairs Medical Center, and the lead author of the FAME study.
Syngenta, in response to questions, said the FAME study’s “initial selection process relied on active identification by the researchers, not voluntary enrollment.”
That is not, however, what the study itself says of its methodology, which relied on self-reporting by farmers and state mortality files. Studies of how many people in a population have Parkinson’s typically go beyond reliance on self-reporting or death records because both can fail to capture the full picture.
For example, a 2001 Parkinson’s incidence study conducted by Kaiser Permanente, one of the largest insurers and health providers, with Dr. Tanner, had a far more detailed approach. It monitored outpatient and inpatient visits, as well as out-of-network insurance payments, and it periodically reviewed Kaiser’s pharmacy system for drugs used to treat Parkinson’s. It also alerted neurologists in its system to refer newly diagnosed patients.
Syngenta, the best-known manufacturer of paraquat, is in the process of being acquired by the China National Chemical Corporation. In its statement, Syngenta said it “went to significant lengths to attempt to access the data” underlying the FAME study “in order to gain as complete an understanding as possible of the study in the pursuit of scientific rigor.”
Freya Kamel, a scientist at the National Institute of Environmental Health Sciences, a branch of the National Institutes of Health and a co-author of the FAME study, said, “The analysis they’re suggesting is not appropriate.”
“There were probably quite a few people with Parkinson’s disease who didn’t enroll in our study,” Dr. Kamel said.