Pesticides and dementia

Long-term exposure to pesticides leads to a greater risk of dementia, The Independent has reported.

The news comes from a study of almost 1,000 French vineyard workers. It found that those directly exposed to pesticides over a long period performed less well in tests of mental (cognitive) ability than those who were not exposed at all. When the volunteers were tested again around five years later, the cognitive performance of workers directly exposed to pesticides had declined more in certain tests than that of workers who were not exposed.

This study appears to show an association between long-term pesticide exposure and a slightly greater risk of decline in mental ability, as measured by certain tests. Although the study raises an important issue, it does not show that pesticides cause dementia. Dementia has several risk factors, including family history of the condition and age. It is possible that other factors influenced participants’ mental performance, including age, education and alcohol intake.

The study looked at vineyard workers who were working either directly or indirectly with pesticides in a professional capacity.

Where did the story come from?

The study was carried out by researchers from the Université Victor-Segalen and the Université de Caen in France. It was funded by a number of French organisations, including the Ministry of the Environment, the National Agency for the Improvement of Working Conditions, the Regional Council of Aquitaine and the Association Recherche et Partage.

The study was published in the peer-reviewed medical journal Occupational and Environmental Medicine.

The study was reported accurately by The Independent , which included comments on its limitations by independent experts.

What kind of research was this?

This cohort study of 929 workers investigated the possible effect of long-term exposure to pesticides on mental performance. Cohort studies, which can track people over time, are often used to look at the possible effect of certain events (in this case, exposure to pesticide) and health outcomes.

The researchers say there is growing evidence that pesticides can have harmful effects on health, including contributing to cancer and neurological and reproductive problems. So far, there has been a limited number of studies on the possible effects of long-term pesticide exposure on the working of the brain.

What did the research involve?

In 1997-98, the researchers enrolled 929 farm workers, aged 40-55, from southwest France. The workers had a minimum of 20 years’ experience of working in agriculture. After examining their job calendars, the researchers put them into three groups according to pesticide exposure: not exposed, directly exposed (for example, applying pesticides), and either possibly or certainly indirectly exposed (for example, contact with treated plants).

The participants took nine validated tests of their mental abilities at the start of the study, including the Mini Mental State Examination (MMSE). The tests measured memory, language retrieval and verbal skills, and reaction time speeds. The participants were also interviewed and given questionnaires to complete.

Researchers contacted participants for follow-up tests between 2001 and 2003.

The researchers then used standard statistical methods to assess the possible relationship between pesticide exposure and performance in the tests. They adjusted their findings for other factors, such as age, alcohol intake, education and depression status.

What were the basic results?

Of the original 929 participants, 614 completed the study. Of those who completed, the researchers found that:

One in five (19.4%) had never been exposed to pesticides in their work.
8.5% had been possibly indirectly exposed.
17.4% had been certainly indirectly exposed.
Over half (54%) had been directly exposed to pesticides.

The risk of obtaining a low performance in the tests was higher in participants who had been exposed to pesticides (odds ratios [OR] 1.35-5.60), with those directly exposed at slightly higher risk than those who were indirectly exposed.

At follow-up, across the various tests:

between one-fifth and nearly half of participants improved their performance
between one-sixth and half had worse scores

In seven of the nine tests, participants whose scores deteriorated the most at follow-up were more often those who had been exposed to pesticides. In particular, those exposed to pesticides were over twice as likely to score two points lower on the MMSE (OR 2.15, 95% confidence interval 1.18 to 3.94).

How did the researchers interpret the results?

The researchers say their results suggest that chronic exposure to pesticides has long-term cognitive effects, and that this could be linked to the development of dementia.

Conclusion

This study seems to have found an association between long-term exposure to pesticides and a mild risk of worsening cognitive ability, compared to non-exposure. It’s important to note that the study’s limitations could have affected the results:

The study had a high drop-out rate, with about one-third of participants not available at follow-up. This increases the risk that the results are biased.
Although researchers tried to take account of other risk factors (called confounders), such as age and education, it is possible that these could still have influenced the results.
Although all the participants were agricultural workers, their exposure may have been linked to the relative status of their job. In other words, workers who handled pesticides may have had a lower rank than those who did not. Job rank may in turn have been linked to wealth and lifestyle, which might influence cognitive abilities.
As the authors note, they cannot rule out the possible effects of unidentified risk factors, such as exposure to solvents and metals.
The authors also point out that there may have been inaccuracies in categorising participants in the direct and indirect exposure groups.
Some participants had better results at follow-up than at the start of the study, which could indicate that they benefitted from “practice” when the test was repeated.
The researchers measured cognitive impairment. While this may be linked to dementia, the exact relationship between the two is unclear. It is probably incorrect to claim that this study was about dementia itself.

The study was unable to identify the specific pesticides to which workers had been exposed. It is possible that farmers now use different pesticide to those used at the time the study was carried out, so its findings may not be relevant to current pesticide use.

In conclusion, this study raises an important issue. However, further research is required to establish any association between pesticide use and mental ability.