Modelling of agricultural wastewater shows that footbaths are a source of antimicrobial resistance

New research has mapped farm wastewater flows and revealed where spikes in antibiotic-resistant bacteria occur in manure, and shows that water from copper and zinc footbaths used by dairy cows can cause fluctuations.

Researchers at the University of Nottingham developed mathematical models and conducted on-farm surveys to investigate the impact of wastewater flows and management practices on antimicrobial resistance (AMR) in manure. The research is the first to examine the impact of farm division, farming practices in different areas of the farm and their potential impact on the emergence and spread of AMR across the farm.

Temporal variations in cephalosporin-resistant Escherichia coli were observed and attributed to agricultural activities, particularly the disposal of spent copper and zinc footbath into the manure system. The results were published in NPJ antibiotics and resistance.

The results highlight agriculture-specific opportunities to reduce AMR burden beyond reducing antibiotic use, including careful disposal or recycling of antimicrobial metal waste.

If improperly stored, milk manure can be a source of environmental contamination with antimicrobial resistance genes and bacteria that can eventually reach humans via water or plants.

Previous modelling studies from the University of Nottingham showed that slurry tanks left alone for at least 60 days without further waste input reduced the spread of antibiotic-resistant bacteria (ARB) as the bacteria died in the hostile environment. The researchers also found that there were times when there were peaks in cephalosporin-resistant Escherichia coli.

“We first found that the slurry tank is not as dangerous a place for the spread of antimicrobial resistance genes as we thought. In fact, the bacteria would die in such a hostile environment if left unattended for a while. But what was also interesting was that we noticed fluctuations in a particularly problematic drug-resistant bacteria called Escherichia coli.

“When we investigated this further in this study using computer modelling and on-farm surveys, we found that there was a direct link between emptying the water from the zinc and copper footbaths in the slurry tanks and an increase in Escherichia coli populations,” said Dov Stekel, professor of computational biology at the University of Nottingham’s School of Biosciences.

In addition to antibiotics, other antimicrobials such as metals (copper and zinc) and other chemicals (e.g. formalin, disinfectants) are used on farms around the world, particularly in foot baths, to prevent lameness in farm animals.

“Metals and other antimicrobials (such as formalin and glutaraldehyde) are known to have a coselective effect on antibiotic resistance, meaning that ARBs can remain in the slurry even after the antibiotics have already been degraded,” said Dr. Jon Hobman, Associate Professor of Microbiology in the School of Biosciences.

Professor Stekel added: “Mapping the antibiotic-resistant bacteria in this way allows us to understand their exact source and, more importantly, their path through the farm. We hope this information will lead to wastewater management practices that can be developed to curb this.”

Engineers at the University of Nottingham have begun investigating how to remove copper and zinc from cow footbath wastewater. They have found that layered double hydroxides successfully remove copper and zinc from cow footbaths. This was the first successful investigation into the removal of copper and zinc from a commercial cow footbath powder mix.