Researchers in the U.S. say they have found evidence that exposure to pesticides may be linked to the development of Parkinson’s disease (PD).

In a new study a team from Harvard School of Public Health (HSPH) has found that people who stated in 1992 that they had been in contact with pesticides were 70 per cent more likely to develop Parkinson’s within the next 10 years.

Previous studies have suggested a link between PD and low-level exposure to pesticides, but the data is inconclusive.

The study is the first large-scale, prospective study to examine possible links between chronic, low-dose exposure to pesticides and Parkinson’s disease.

The researchers, led by Alberto Ascherio, associate professor of nutrition and epidemiology at HSPH, looked at data from a study begun in 1992 by the American Cancer Society linking diet and cancer.

In a follow-up survey in 2001, some 143,325 participants responded and of these, 413 had developed Parkinson’s since the start of the study.

The team then contacted those who reported a diagnosis of PD and examined their medical records to confirm the diagnosis.

Among the original questions had been one about exposure to pesticides, to which 5,203 men and 2,661 women had said yes.

The researchers found that among those reporting exposure, after adjusting for age, sex, and other risk factors for Parkinson’s disease, there was a 70 percent higher incidence of PD than among people who reported no exposure.

The finding are significant and are validated by the fact that the participants were questioned about their exposure to pesticides long before they developed Parkinson’s disease.

Often when people are questioned after a diagnosis for Parkinson’s has been made, their answers are unreliable and may have been open to suggestion.

Parkinson’s disease is caused when two chemical messengers in the brain, dopamine and acetylcholine, which usually help to transmit messages between nerve cells and muscles, get confused.

As a rule symptoms usually appear after the age of 50, and the risk increases with age; men are more susceptible to the disease than women.

About 10,000 people in Britain are diagnosed with the disease each year and 1 in 20 of these will be aged under 40.

The researchers say that future studies will need to examine which specific pesticides or classes of pesticides are likely to cause Parkinson’s disease.

The study is published in the July issue of Annals of Neurology and also appears online via Wiley Interscience.