The common horse drench ivermectin spells death to most horse parasites, but what effect does it have on dung-loving organisms once the drug has passed through the animal?
Researchers from the University of Zurich devised a manure-based experiment conducted in four countries to provide insights into the issue.
They devised what they said was a new approach for environmental compatibility testing of livestock drugs.
Drugs for livestock are known to carry the potential to harm beneficial organisms that break down dung. Therefore new medical substances need to be tested on single species in laboratory settings.
An international research group including evolutionary biologists from the University of Zurich have been scrutinizing the reliability of such laboratory tests. In doing so, they evaluated a new field test which they used to assess ivermectin at four locations with different climates.
In the case of ivermectin, too high a dosage is known to harm coprophilous organisms (those growing or living on dung).
The toxicity of new livestock medications therefore needed to be verified in ecotoxicological tests with individual animal species such as the common yellow dung fly, the barn fly or a dung beetle.
This involves determining the lethal dose leading to the death of half the maggots (the LD50 test). However, sensitivity to toxic substances is known to vary significantly, even among closely related dung-dwelling organisms, which begs the question as to how representative the reaction of any individual animal species actually is in such laboratory tests.
After all, the researchers noted, there is a high risk that more sensitive species will continue to be harmed by the substance, jeopardizing key ecosystem functions in the long run.
An international research group, including University of Zurich evolutionary biologist Wolf Blanckenhorn, recently proposed extending the testing scheme to a representative selection of all organisms that break down dung, ideally in their natural environment.
The scientists have now disclosed details of what they describe as a successful and more comprehensive higher-tier ecotoxicological field test. Their study provided important insights into minimizing the risks of drug residues in nature.
For their feasibility study, the scientists worked on cattle pastures in the Canadian Prairie and the agricultural landscapes of southern France, the Netherlands and Switzerland – four locations with very different climatic conditions.
On these pastures, they distributed cow-dung pats with different concentrations of ivermectin.
“As expected, the overall number and diversity of dung beetles, dung flies and parasitoid wasps decreased as the ivermectin concentration increased,” Blanckenhorn said.
However, several species also proved to be resistant: earthworms and springtails living in the ground underneath the cowpats were not notably affected, and a parallel test ultimately revealed that dung degradation was not significantly impaired.
“Evidently, beneficial organisms not affected as much by the drug, such as earthworms, were apparently able to compensate for the loss of other organisms,” Blanckenhorn said.
Despite diverse environmental conditions and methodological details, the results were very similar and reproducible in all four habitats.
“Our field approach was therefore a success and in principle can be recommended. The regulation authorities responsible, such as the European Medicines Agency EMA, now have to decide whether this more conclusive yet more complex test should be required in the future.”
The amount of effort involved in determining the many dung organisms is substantial and impossible without expert biological knowledge.
“Classifying species via so-called DNA barcoding, based on each organism’s unique genetic fingerprint, is possible in principle and will probably be more cost-effective in the future,” he said.
“However, this approach requires the establishment of a complete database for coprophilous organisms, which does not yet exist.”