Scientists link common weedkiller to convulsions in animals

Scientists have uncovered a link between the world’s most commonly used weedkiller and convulsions in animals — raising questions about the herbicide’s potential impact on the human nervous system as well.  

Exposure to glyphosate, the active ingredient in Roundup, increased seizure-like behavior in soil-dwelling roundworms, according to the researchers, who published their findings in Scientific Reports on Tuesday. 

With glyphosate use expected to rise dramatically over the coming years, understanding its possible effects on human health is critical, according to the study.  

“It is concerning how little we understand the impact of glyphosate on the nervous system,” lead author Akshay Naraine, a Ph.D. candidate at Florida Atlantic University and the International Max Planck Research School for Synapses and Circuits, said in a statement. 

“More evidence is mounting for how prevalent exposure to glyphosate is, so this work hopefully pushes other researchers to expand on these findings and solidify where our concerns should be,” Naraine added.     

Just last month, data from the Centers for Disease Control and Prevention revealed that more than 80 percent of urine sampled by the agency was at or above the detection limit for glyphosate, as The Hill reported. 

Bayer, which manufactures Roundup, has faced thousands of lawsuits alleging that the product causes cancer. While the International Agency for Research on Cancer deemed glyphosate a “probable” carcinogen in 2015, the Environmental Protection Agency said in 2020 that there was insufficient evidence to show that the chemical is a probable or likely carcinogen. 

In Tuesday’s study, Naraine and his colleagues said they used the roundworm C. elegans to test the effects of glyphosate alone and of both the U.S. formulation of Roundup and the British product from two distinct periods. 

The two windows in question were before and after 2016, at which time the United Kingdom banned a surfactant — called polyethoxylated tallowamine — that had been in the earlier formulation. 

These varying conditions, the scientists explained, helped them pinpoint which effects were specific to the active ingredient glyphosate. 

Ultimately, the authors found that glyphosate exacerbated convulsions in C. elegans and concluded that a receptor protein called GABA-A was the neurological target for the observed physiological changes. In humans, these receptors are essential to locomotion and contribute to sleep and mood regulation, according to the authors. 

In response to the findings, a statement from Bayer said that research conducted with worms “does not meet the scientific standards necessary to predict effects on humans or other mammals for the purposes of a pesticide safety assessment.”

Regulatory agencies worldwide have considered whether glyphosate and glyphosate-based herbicides “can harm the nervous system based on data from high dose studies in mammals, not worms,” the statement said. 

“Scientists that have reviewed all of the available data have concluded that neither glyphosate nor the other ingredients in glyphosate-based herbicides will harm the nervous system at doses much greater than what any human would be exposed to,” the company added.

While Tuesday’s study did employ roundworms and not mammals, scientists often study C. elegans to gain an understanding about human diseases and development, as they share a common ancestor. 

The data in the study revealed a significant distinction between exposure to glyphosate alone and Roundup — with exposure to Roundup increasing the percentage of C. elegans that did not recover from seizure activity.

The scientists also used significantly lower levels of glyphosate and Roundup than is suggested on the product — more than 300 times less herbicide than the lowest concentration recommended for consumer use. 

Yet they found that the roundworms convulsed at concentrations that were diluted 1,000 times more than concentrations previously deemed toxic, according to the study. 

“Given how widespread the use of these products is, we must learn as much as we can about the potential negative impacts that may exist,” Ken Dawson-Scully, a professor of neurobiology and Naraine’s faculty mentor, said in a statement. 

“There have been studies done in the past that showed the potential dangers, and our study takes that one step further with some pretty dramatic results,” added Dawson-Scully, who also serves as a senior vice president and associate provost at Nova Southeastern University.

Roundworms already experience convulsions when they encounter thermal stress — and these new findings show that exposure to glyphosate and Roundup can exacerbate these impacts, according to Naraine. 

“This could prove vital as we experience the effects of climate change,” Naraine said.

Dawson-Scully acknowledged that, at this point, there is no insight as to “how exposure to glyphosate and Roundup may affect humans diagnosed with epilepsy or other seizure disorders.” 

“Our study indicates that there is significant disruption in locomotion and should prompt further vertebrate studies,” he said. 

This story was updated at 1:16 p.m.