The idea that pesticides may be to blame has now received a boost from the first large-scale, prospective study to examine this possible link. Alberto Ascherio of the Harvard School of Public Health in Boston, Massachusetts, US, and colleagues looked at data from roughly 143,000 people involved in a cancer and diet study, of whom 413 were diagnosed with Parkinson’s disease in the 1990s.
In 1982 these volunteers each completed an initial questionnaire, providing information about their occupation and levels of exposure to various toxins.
Ascherio and colleagues found that those who reported exposure to pesticides had a 70% greater risk of developing Parkinson’s disease than those who said they had no such exposure. But exposure to other toxic compounds – such as asbestos and formaldehyde – did not increase their chances of acquiring the illness.
Ascherio stresses that the absolute risk of developing Parkinson’s is low. So while about 2% of the population as a whole may be at risk of developing the disease, exposure to pesticides might increase this risk to little more than 3%.
Of the 413 patients with Parkinson’s disease, 43 reported exposure to pesticides. But surprisingly the study found that farmers – many of whom presumably had high levels of exposure – and non-farmers shared a similarly increased risk. This contradicts a previous, smaller study reporting that risk rises with exposure levels (see Exposure to pesticides can cause Parkinson’s).
Ascherio suggests that non-farmers may have encountered pesticides while gardening. “Maybe the pesticides used in agriculture are not the most harmful,” he speculates. He regrets that the initial questionnaire did not include more details about the type, duration and intensity of pesticide exposure.
Experts stress that many people unknowingly consume pesticides on a daily basis. “If you analyse the fruit and vegetables we eat, they’re full of chemicals,” says Serge Przedborski of Columbia University in New York, US. He adds that traces found in such foods can accumulate over a lifetime to potentially harmful levels.
Przedborski describes the new study as “excellent” because researchers collected data about pesticide exposure years before participants developed Parkinson’s disease, ruling out potential bias. But he notes that it does not prove that pesticides are the main cause of Parkinson’s disease.
Moreover, Przedborski explains that because the initial questionnaire did not ask about specific pesticides, we are no closer to knowing which particular chemicals are the culprits. “In reality, we have no idea,” he says.
Journal reference: Annals of Neurology (DOI:10.1002/ana.20904)