Pesticides linked to increased risk of developing MND

The results from a study looking at the possible links between exposure to environmental toxins (found in pesticides) and motor neurone disease (MND) was published yesterday (9 May) in the journal JAMA Neurology.

A group of researchers from the University of Michigan, led by Dr Feng-Chiao Su and Dr Eva Feldman, have found that exposure to pesticides is associated with an increased risk of developing MND.

What did the study involve?

156 people with amyotrophic lateral sclerosis (ALS, a type of MND) and 129 healthy ‘control’ participants from Michigan, USA completed questionnaires on their occupation history, gave blood samples, or did both.

The questionnaire asked about their occupations over four windows of time; at any point during their life, in the last 10 years, in the last 10-30 years, and over 30 years ago. From their answers, the researchers worked out the likelihood of each participant’s exposure to pesticides.

The levels of 122 persistent environmental pollutants (including organochlorine pesticides or OCPs) were tested for in blood samples taken from participants.

Persistent environmental pollutants are those with a long half-life, meaning that they break down slowly. This meant that they could be tested for in the blood, even if exposure happened several years ago. However, the blood sample cannot tell us the source of the pollutants, such as if it was through work, at home or even from eating fruit and vegetables that had been sprayed with pesticides.

What do the study results say?

The results from the questionnaire indicated that exposure to pesticides at work during any point of a person’s life is linked to an increased risk of MND.

Blood samples showed that concentrations of five environmental pollutants were also associated with increased risk of developing MND.

In studies such as this, the measure of this link between the two factors is given as an odds ratio. The odds ratio represents the chances that an outcome (in this case risk of developing MND) will occur after being exposed to something (in this case pesticides). A ratio of greater than one indicates that there are higher odds of the two being associated.

The odds ratio for reported exposure at work was 5.09, whereas the odds ratio for the five chemicals in blood samples ranged from 1.81 to 5.74 (for cis-Chlordane, a type of OCP). This equates to up to a five times increase in risk of developing MND (though the relative risk is still extremely low).

What does this mean for me?

There are many different types of pesticide. The group of pesticide that the researchers found increased the risk of developing MND are an old type of pesticide that is known to stay in the body many years after they enter it. The effects of these are well known, and have led to a ban on the use of several pesticides of this type such as DDT.

Modern pesticide use is strictly regulated and the chemicals used do not last as long in the body as the old type. The new types of pesticide could not be picked up in participants’ blood samples, and so we do not know if they present a risk factor for MND.

Commenting on the results from this study, Dr Belinda Cupid, Head of Research at the MND Association said:

“We know that people get MND due to a combination of many environmental, lifestyle and genetic factors.

“It’s unlikely that any single factor on its own will cause MND and the results in this research paper aren’t a ‘eureka’ moment. But the evidence that exposure to pesticides is a contributory risk factor towards getting MND is stacking up and I’m sure will be the focus of future research.”

What do we still need to know?

Even though this study does contribute to our knowledge on risk factors that may contribute to MND, there are still questions that need answering, such as:

• When does exposure to pesticides need to occur to increase the risk of developing MND (such as exposure within the last X years)?
• What level of exposure equates to an increase in risk?
• Are all types of pesticide deemed a risk factor or is it specific ones (older versus modern pesticides)?
• How do pesticides contribute to motor neurone damage?

More information:

Journal article: Feng-Chiao Su et al. (2016) Association of Environmental Toxins With Amyotrophic Lateral Sclerosis. JAMA Neurology. Published online May 9, 2016 doi: 10.1001/jamaneurol.2016.0594 (published open access)

Editorial in JAMA Neurology: J Cragg, M Cudkowicz, M Weisskopf. The Role of Environmental Toxins in Amyotrophic Lateral Sclerosis Risk. doi: 10.1001/jamaneurol.2016.1038