A common organic pesticide has been linked to Parkinson's disease in new research which backs up earlier findings that people with long-term exposure to weed-killers, such as farmers, are more likely to develop the disorder.

A common organic pesticide has been linked to Parkinson's disease in new research which backs up earlier findings that people with long-term exposure to weed-killers, such as farmers, are more likely to develop the disorder.

Scientists at Emory University in Georgia discovered that rats which were exposed to doses of the pesticide over a period of weeks gradually lost function of the brain cells which produce dopamine, used to transmit signals inside the brain. The rats then developed the symptoms of Parkinson's, including limb tremors and occasional rigidity.

Dr Timothy Greenamyre, who led the research, said the findings "support the idea" that chronic exposure to pesticides may raise the chances of getting the degenerative brain disease. About 1 per cent of people over 65 develop Parkinson's.

The possibility of the link was raised earlier this year by a team at Stanford University in California, which studied more than 1,000 people, half of whom had Parkinson's, and found that those who had been frequently exposed to pesticides were twice as likely to develop the disease.

Rotenone is a naturally occurring pesticide used by organic farmers both to kill insects and as a means of killing fish in water management programmes. The scientists warned that their findings, published today in the journal Nature Neuroscience, do not prove that rotenone causes Parkinson's, but do "raise new questions about its safety".

"A likely explanation is that rotenone acts by causing the mitochondria, which power the cell, to produce free radicals - chemicals which react with anything they can," Dr Greenamyre said. "Those free radicals produce damage in all sorts of contexts and have been implicated in many human degenerative diseases.

"Our findings are consistent with the idea that chronic exposure to low levels of toxin may cause cumulative damage to the brain's dopamine system, eventually leading to clinical symptoms."

The Soil Association, which oversees and certifies organic farming and products in the UK, said: "Rotenone is a natural extract from plants but it is mainly used as a last resort, when everything else has failed to work." European regulations do allow its use, the spokesman said, but applications were "very rare". He added: "If it was found to have harmful effects then it could be withdrawn."

The findings add another piece to the jigsaw about the effects of pesticides. The Government advises people to minimise their exposure to pesticides and insecticides and to wash produce before eating.

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