After testing the effects of chronic exposure to low doses of glyphosate, either pure or mixed with co-formulants, scientists from the Viral Diseases in Fish Unit at ANSES’s Ploufragan-Plouzané-Niort Laboratory exposed five-month-old trout to a concentration of 500 µg of glyphosate per litre of water over a period of four days. This concentration is 75 times lower than the median lethal dose (causing death in half the population) established by the European Food Safety Authority (EFSA) for rainbow trout (38 mg/l). The experiments, which were carried out using pure glyphosate and two glyphosate-based herbicidal formulations, did not lead to any major physiological changes in the trout.
Exposure to glyphosate and then to a virus: a double challenge
Although the trout did not appear to undergo any significant changes, the effects of exposure to the glyphosate and/or the co-formulants could have a metabolic cost for the fish, making them less able to cope with a second environmental stress factor. Following the chemical exposure test, researchers therefore tested the trout's ability to resist viral infection.
No significant difference in mortality was observed between the group exposed to the chemicals and the non-exposed group, with mortality rates from the virus ranging between 60% and 67%. However, the two stressors combined did affect the activity of a number of enzymes associated with energy metabolism. For example, 96 hours after infection with the virus, the activity of the enzyme G6PDH, involved in repairing oxidative damage, increased by 65% in trout exposed to pure glyphosate and by up to 138% in the group exposed to one of the herbicidal formulations. The activity of another enzyme, acetylcholinesterase, increased by 62% following exposure to pure glyphosate and by 79% following exposure to one of the two herbicidal formulations, while remaining the same as the control group for the group exposed to the other product. These results confirm the importance of taking account of environmental factors, such as infectious agents, in assessing the effects of chemicals on animals.