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NEW YORK (Reuters Health) - Farmers who used weedkillers were more than twice as likely to be treated for depression than farmers who didn't use the chemicals in a new study from France.
Whether the weedkillers are causing depression "is not clear," said Marc Weisskopf, the study's lead author and an associate professor at the Harvard School of Public Health. "But (the result) suggests we should not be ignoring herbicides just because they're targeting plants."
Earlier research on depression and pesticides has focused on insecticides, particularly organophosphates, which are known to be toxic to nerve cells, said Weisskopf.
Monocrotophos, the insecticide that killed 23 school children in India this month, is an organophosphate, for example.
The use of pesticides has also been linked to Parkinson's disease among farmers (see Reuters Health story of May 28, 2013 here:).
As part of a study on Parkinson's disease, Weisskopf and his colleagues assessed the risks for depression with exposure to any kind of pesticide by surveying 567 French farmers about their use of fungicides, insecticides and herbicides.
The team conducted home visits to get a detailed assessment of chemical exposures, including going over bills for pesticide purchases, looking through farming calendars and inspecting old pesticide containers.
They also asked the farmers whether they had ever been treated for depression.
Weisskopf's group reports in the American Journal of Epidemiology that 83 farmers, about 15 percent, said they had been treated for depression. Forty-seven of them had never used pesticides, while 36 had.
Among the farmers without Parkinson's disease, 37 who had never used herbicides and 20 who had used the weedkillers reported being treated for depression.
There was no difference in the risk of having depression among the farmers who had used fungicides or insecticides, compared to those who hadn't used any pesticide.
But when the researchers took into account factors linked with depression, such as age and cigarette smoking, they determined that those farmers exposed to weedkillers were nearly two and a half times as likely to have had depression.
Furthermore, farmers who had greater exposure - either more hours or longer years using herbicides - also had a greater chance of having depression than farmers who had used weedkillers less.
That kind of dose-response relationship is usually thought to support a connection - in this case, between the chemicals and depression. But this type of study cannot prove cause and effect.
One possibility that wasn't ruled out is that the exposed farmers might have had other health conditions that affected their ability to work, which in turn made them vulnerable to depression.
"The health of the farmer is critical. If they can't work, they get depressed," said Cheryl Beseler, a researcher at Colorado State University, who was not involved in this study.
She said the study was otherwise very well done in terms of collecting information about the farmers' past pesticide use.
The results do not apply to the average gardener, although Weisskopf said it will be valuable to better understand herbicides' safety in farming and non-farming settings.
"It's very important given their widespread use around the home," he said.
"I think people tend to not take (the risks of pesticides) seriously when they're gardeners," she told Reuters Health.
Weisskopf said one explanation for his finding that insecticides and other pesticides were not tied to depression is that farmers might be aware of their potential hazards and they take greater precautions to avoid being exposed.
Another possibility is that the chemicals simply don't cause depression.
He said that more research is needed to determine the safety of herbicides, and meanwhile it would be wise for farmers and gardeners to be just as diligent about protecting themselves as they are with other chemicals.
"If (herbicides) are considered in general safer and people take less precautions because people think they're not as bad, then that poses a problem," he told Reuters Health.
"This still has to be considered a relatively first, small study. There's more work to do, but it raises concerns that need to be looked into more fully," Weisskopf said.
SOURCE: bit.ly/1dXuiro American Journal of Epidemiology, online July 12, 2013.