Just as people take antibiotics when they have an infection, livestock similarly will receive veterinary antibiotics to treat their illnesses. But what happens when manure containing those livestock medicines is applied to cropland? How much runs off fields and enters our waterways?
New Ohio Sea Grant research from Dr. Laura Johnson, director of the National Center for Water Quality Research at Heidelberg University in Tiffin, Ohio, tracked the presence of veterinary antibiotics in Lake Erie tributaries.
“We wanted to better understand the range of different compounds and their average concentrations across a variety of different watersheds,” Johnson said. “Are we still seeing veterinary medicines in streams and rivers? How prevalent are they, and should we be concerned about it?”
About 1.2 million kilograms of antibiotics are produced for U.S. agriculture each year, and up to 90% of those veterinary antibiotics can be excreted by livestock unmetabolized. Once in the environment, antibiotics can contribute to the phenomenon known as antibiotic resistance, in which bacteria adapt to overcome the drugs designed to eliminate them. Food-borne pathogens, such as Salmonella, Campylobacter, and E. coli, already show high resistance to common antibiotics, making it harder to treat diseases.
“If we have antibiotic resistance for bacteria that would also affect humans, that would be a problem,” said Johnson. “And if you think about it from an ecological standpoint, having more antibiotics means that you’re changing the microbial community structure of an aquatic ecosystem. That can just change how it’s functioning in general.”
To find out whether this is a concern in the western basin of Lake Erie, Johnson’s team measured pharmaceuticals at trace levels in the Sandusky and Maumee River watersheds. Researchers used passive samplers — specifically, polar organic chemical integrative samplers — at various points across the watersheds. While typical nutrient sampling involves collecting “grab” samples at a single point in time, passive sampling measures the presence of compounds over time, at very low concentrations.
“It’s basically just a little disc that you put in the water, and it will continually absorb polar compounds for however long you have it there,” Johnson said. “It’s nice because we get a longer viewpoint of the potential amount of antibiotics in the water.”
Over the course of three years during the growing season of April to October, the team continually deployed passive samplers at different stream locations and collected grab samples for additional data. After a slow start in 2020 due to the COVID-19 pandemic, the team expanded to nine sites with varying proximity to animal operations in 2022.
Once collected, the samples were sent to the University of Nebraska-Lincoln’s Water Science Laboratory for extraction and analysis. Researchers used a technique called liquid chromatography tandem mass spectrometry to separate antibiotic compounds and measure their masses. This was largely successful: Almost all samplers worked out, were extracted, and provided good information, Johnson said.
Ultimately, researchers found that some antibiotics, both veterinary and human, were frequently detected in rivers and streams in the Western Basin. For example, in 98% percent of the study’s samples, researchers found lincomycin, an antibiotic mostly used in livestock.
“So everywhere we went to measure it, we found it,” Johnson said.
Interestingly, the samplers also consistently detected sulfamethoxazole, an antibiotic primarily used to treat urinary tract infections in humans. Johnson attributed the findings to upstream septic systems.
“I wasn’t expecting to see human antibiotics in some of our more rural areas,” she said. “I think that kind of helped reflect the mixed nature of sources in some of these watersheds. Even in places where you wouldn’t expect a human source, there can be one.”
The team also confirmed that across watersheds, antibiotic levels were associated with the density of livestock nearby.
“There is definitely a link there that as you have more of a burden of livestock in the watershed, you will see higher antibiotics that are used for those animals,” Johnson said.
Though the study found a prevalence of several antibiotics, the actual concentrations of the compounds were relatively low, with some on the scale of a nanogram — one billionth of a gram — per liter. Johnson found that these levels are likely low enough that they’re not causing antibiotic resistance in the microbial environment of streams and rivers.
“The concern I would have,” Johnson said, “is that if you’re really consistently seeing antibiotics — even at low levels — in our streams and rivers, then that means when they’re applied to the land, they’re at much higher concentrations.”
This could serve as an indicator that antibiotic resistance is still a concern in agricultural fields in the region, she said.
Meanwhile, the use of antibiotics for livestock has seen increasing regulation since 2017, when the U.S. Food and Drug Administration established rules to eliminate the use of such drugs for growth promotion and to require prescriptions from veterinarians for therapeutic uses. Starting this year, any remaining over-the-counter veterinary antibiotics will require a prescription as well. However, Johnson noted that some antibiotics detected in the study — such as monensin, found in 70% of samples — are exempt from this rule.
Johnson added that she hopes the results from the study will inform livestock practices in the state and regulatory bodies like the Ohio Environmental Protection Agency and Ohio Department of Health.
“I think them knowing the potential for exposure to antibiotics in the environment is useful, even if it wasn’t incredibly high,” she said. “Hopefully results like this would encourage farmers and producers to not use antibiotics as often. I think anybody would agree that if there’s a sick animal or other issues, yes, use antibiotics, but use them smartly.”
While antibiotic concentrations weren’t high enough to immediately warrant continued sampling in the Western Basin, Johnson said the subject could be revisited in the future with a follow-up study to see how levels changed.
For more about this Sea Grant-funded research, contact Dr. Johnson at: email@example.com.
Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.