Being diagnosed with amyotrophic lateral sclerosis ( ALS ), also known as Lou Gehrig’s disease, can be devastating.
Author
- Rachel Tan
Associate Professor in Neuroscience, The Brain and Mind Centre, University of Sydney
ALS is the most common form of motor neurone disease, a neurodegenerative condition that affects people’s ability to speak, eat, breathe and swallow. It is debilitating and progresses rapidly: on average, people survive just 2-3 years after a diagnosis. Most people are diagnosed between the ages of 40 and 70.
A new study, published today , has found that being exposed to pesticide at work can increase the risk of developing ALS by 61-71%. The study brings together the findings of eight other studies – but not all of them assess risk in the same way. Here’s why we need to interpret the findings with caution.
What we know about ALS causes
Men are around 20% more likely to develop ALS than women.
The disease occurs sporadically in over 90% of patients, meaning the large majority have no known family history or identified genetic cause. The remaining 10% of cases are familial, meaning the person has a known genetic variant or family history of ALS.
However, it’s not well understood exactly what triggers both the sporadic and familial forms of the disease, although an interaction between genes and environment is likely.
Over the last few decades, researchers have been investigating exposure to pesticides as a possible trigger.
Several studies have found associations between being exposed to pesticides and a higher risk of developing ALS. However, a causal relationship has not been established.
What the new study did and what it found
This new study pooled data from eight separate studies, five from the United States, one spanning Canada and France, one from Italy, and one from Australia.
It asked a simple but important question: does having ever being exposed to pesticides at work raise someone’s risk of developing ALS?
The answer, based on this combined analysis, was yes.
Overall, people who had ever had any exposure to pesticides (including fungicides, herbicides and insecticides) at work were 61% more likely to develop ALS, compared to those who hadn’t.
Pesticide exposure appears more strongly linked to ALS risk in men than in women in this analysis. But the evidence is too limited to know whether this reflects a true biological difference, or gaps in how exposure was measured for women.
The risk was slightly higher with exposure to herbicides (a type of pesticide), which was associated with a 71% heightened risk of developing ALS.
ALS affects an estimated 8 in every 100,000 Australians. So a 61-71% increase would mean a small number of new cases.
But these numbers don’t reflect how devastating and debilitating this disease is for families and communities affected by ALS. There is currently no cure.
But here’s why we should be cautious
This review was able to assess the link between pesticide and ALS in a large sample, by combining 1,734 ALS cases across the eight studies. This is more than any single study could provide alone.
But combining studies in this way inevitably introduces some inconsistencies, as each one has been conducted independently, using different methods. So it’s hard to control for the differences.
Several of the studies in the review found a link between developing ALS and other separate risk factors, such as head trauma or being exposed to other kinds of toxins. But the analysis couldn’t fully disentangle these risk factors from pesticide exposure. This means we can’t tell how much someone’s exposure to pesticide contributed to them later developing ALS, or whether it was due to the other contributing factors too.
One study also assessed military veterans exposed to Agent Orange , a herbicide the US used as a weapon during the Vietnam war. This kind of exposure is likely quite different to typical occupational pesticide exposure today. But the new review didn’t assess this separately.
Most of the included studies also relied on self-reported exposure, meaning participants had to recall and estimate how much pesticide they’d likely been exposed to. These studies tended to show a stronger link between pesticides and ALS than studies that independently measured exposure.
As the authors of the new study noted, this means recall bias may have played a role. People who already have ALS, or who already suspect a link between pesticides and the disease, may be more likely to remember and report being exposed to pesticides.
So, what does this all mean?
Overall, the study supports the case that being exposed to pesticide at work increases your risk of developing ALS.
These findings highlight the need for more rigorous research into the specific details. We need to understand which chemicals or pesticide ingredients may be responsible, and whether the risk is related to dose. This means understanding whether being exposed to pesticides over a longer time, or at a higher dose, might further increase your risk.
Someone’s risk of developing ALS is likely influenced by a combination of factors, including genetics and environment.
The way multiple exposures to risk factors accumulate and interact over a person’s lifetime may also play a role; for example, if you work with pesticides every day and have also been exposed to other toxins, had a severe head injury and/or a have genetic predisposition.
This is known as a “multistep” model of disease development and it isn’t currently possible to determine someone’s personal risk profile with precision.
The picture is still evolving. More evidence will help regulators and employers make better decisions about safe exposure levels and potential risk mitigation strategies.
