The notorious weed-killer, Roundup, originally made by Monsanto and acquired by Bayer, has been found to cause seizure-like convulsions in roundworms.
When a soil-dwelling species, Caenorhabditis elegans, was exposed to highly diluted samples of Roundup – 300 times lower than the lowest concentration recommended for consumer use – researchers found the herbicide elicited prolonged paroxysms.
In a third of the worm models, the toxic effects of Roundup and its main ingredient, glyphosate, could only be halted with drug intervention.
Researchers say their findings are "pretty dramatic" – and they come at a crucial moment for Bayer and the future of Roundup.
"It is concerning how little we understand the impact of glyphosate on the nervous system," says neuroscientist Akshay Naraine from Florida Atlantic University.
"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."
Glyphosate is the world's most commonly used herbicide. A recent federal report from the United States found traces of this weed-killer in more than 80 percent of urine samples given by children and adults.
Yet the health effects of such widespread exposure are extremely contentious.
Leaders at Monsanto and now Bayer have persistently argued that Roundup is safe and non-carcinogenic for humans. But in recent years, scientists, policymakers and the public have started to question those assertions, which tend to be based on industry-led research and not on independent scientific investigation.
Bayer is currently embroiled in several legal battles with people who say they have developed cancer from using Roundup. Recently, the United Kingdom banned a specific ingredient, POE-tallowamine, from being used in Roundup's trademarked formula after learning of its potentially toxic effects.
The United States, however, has taken no action against Roundup or its ingredients. In 2020, the Environmental Protection Agency (EPA) once again determined that it could find no risks to human health from the appropriate use of Roundup.
Two years later, some critics are unconvinced. Recently, a federal court ruled that this Trump-era decision had overlooked important, emerging evidence on the toxic effects of Roundup.
The EPA now has until October to reconsider its decision. At the same time, the European Union is also mulling over whether to reauthorize the use of glyphosate past 2022.
The current study among worms is worth considering, even if it is based on animal models. While much research has focused on the possible carcinogenic effects of Roundup, there may be neurotoxic effects, too.
One study previously found that the level of toxicity measured in nematode brains is highly correlated with neurotoxicity in mammals.
That's part of what makes the current findings so concerning.
When researchers in Florida tested the US version of Roundup on worms, they found it caused convulsions that went for just over a minute, nearly twice as long as the effect of glyphosate on its own.
The UK formula also caused more seizures than glyphosate on its own, suggesting there were other dangerous ingredients in Roundup interacting with other chemicals to overstimulate the animal brain.
When researchers bred worms without GABA-A receptors, they did not have seizures when exposed to glyphosate or Roundup.
This suggests that the chemicals are acting on receptors in the brain associated with movement to exacerbate seizures. Interestingly, drugs that target these receptors are common anti-epileptic treatments in humans.
"As of now, there is no information for how exposure to glyphosate and Roundup may affect humans diagnosed with epilepsy or other seizure disorders," clarifies neuroscientist Ken Dawson-Scully from Nova Southeastern University in Florida.
"Our study indicates that there is significant disruption in locomotion and should prompt further vertebrate studies."
With glyphosate use in the US projected to increase 200-fold in the future, some scientists are understandably worried by how little independent research has been done.
Very few studies have investigated the neurotoxic effects of glyphosate and Roundup exposure.
A study on buff-tailed bumblebees (Bombus terrestris) published earlier this year found that those who consumed glyphosate-laden sugar water found it harder to keep their hives warm enough to incubate larvae.
Another study published in 2018 suggested glyphosate impacted the honeybee's microbiome, which the researchers said may be contributing to colony collapse disorder.
It may not just be bees and worms that are falling prey to herbicides meant to kill weeds. We need to know more.
The study was published in Scientific Reports.