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| A pile of problems: The presence of low levels of antibiotics in animal manure can lead to ARG, which contribute to the ineffectiveness of antibiotics in humans |
This could provide a better understanding of ARG, and help in the development of prevention strategies and best management practices in minimizing the effects of animal waste on the spread of ARG.
Antibiotic resistance is a growing human health concern. Several studies have confirmed the presence of antibiotics in animal waste lagoons, surface waters and sediments, municipal wastewater and the groundwater underlying lagoons. The presence of low levels of antibiotics can lead to ARG, which contribute to the ineffectiveness of antibiotics in humans.
It is accepted that the general environment may be a reservoir or transfer pathway for ARG, but little is known about the transport and fate of antibiotics in agricultural pathways and treatment systems, say scientists from Colorado State University (CSU).
As a result, they studied how antibiotics and ARG responded to manure management. They added the antibiotics chlortetracycline, tylosin and monensin to horse manure that contained no antibiotics, as part of a pilot field study to compare it with unspiked controls. The researchers also conducted a large-scale field study to compare manure from a dairy with minimal use of antibiotics with that of a feedlot, where regular subtherapeutic use of the drugs is practiced.
The team found that the manure type and treatment time were the main factors in reducing the levels of antibiotics and ARG in manure before land application.
A treatment time of six months may be needed to reduce initially high levels of the antibiotics and ARG. However, ARG may be present for a longer time in the manure, even after antibiotics had fully dissipated, the researchers said.
High intensity management (HIM) of manure from horses and cattle was also more effective in increasing rates of dissipation of antibiotics in manure compared with low intensity management (LIM). But HIM was not a significant factor in reducing the levels of AGR.
Manure from feedlot cattle needed two additional months of treatment than that of dairy cattle to achieve the same levels of dissipation.
"The feedlot manure had significantly higher levels of ARG than the dairy manure, which was higher than the horse manure," they said. "This is reasonable given that cattle from the feedlot were routinely fed subtherapeutic concentrations of antibiotics, whereas the dairy cattle were only given therapeutic antibiotics during non-lactating periods, and the horses were given no antibiotics."
More research is needed in comparing more contrasting treatments and in monitoring of additional genes such as tetracycline efflux ARG, say the researchers. The results of the study have been published in the Journal of Environmental Quality. The study was funded by the United States Department of Agriculture's Agricultural Experiment Station and the National Science Foundation's Early Career Development program.
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