Traces of antibiotics used to promote the health and increased growth of livestock and poultry are turning up in waterways, new research shows.
While testing at several sites along Colorado's Cache la Poudre River, a research team led by Ken Carlson, a Colorado State University civil engineering professor, discovered the presence of at least three antibiotics used only on food animals.
The compounds were not detected at the river's source near the Continental Divide, or near human waste treatment plants around Fort Colins, Colo. But they were found farther downriver close to large farms and feedlots. The scientists' conclusion: Those operations were almost certainly the source. The livestock industry firmly insists it is able to manage the waste produced at such facilities.
"We're saying that doesn't appear to be completely the case," Carlson said. "I think most people would agree ... these compounds should not be in our rivers."
Study is a first
A 2002 report by the U.S. Geological Survey highlighted the presence of dozens of pharmaceuticals and other chemicals in the nation's streams and rivers, but this study appears to be the first time these chemicals have been pinpointed.
The three substances in question — monensin, salinomycin and narasin — are of interest because they are used exclusively in animal agriculture and are not used on humans. Carlson found these substances not only in river water but in riverbed sediment as well, where concentrations of the antibiotics were up to 1,000 times greater than they were in the water.
The amounts found in the water are nearly infinitesimal, about 50 parts per trillion at most. But their presence raises three concerns: a possible contamination of drinking water; an impact on fish and other aquatic animals susceptible to long-term exposure; and the drugs' potential ability to provide increased resistance to waterborne bacteria.
Monensin is given to cattle as a way to improve the health of their rumens and to help them grow more quickly. Salinomycin and narasin are usually used to help reduce harmful bacteria in chickens' digestive tracts.
The environmental impact for any of the three is not clear, and is certain to prompt additional research and debate.
Industry questions findings
Gary Weber, executive director of regulatory affairs for the National Cattlemen's Beef Association, questioned the study's methodology, particularly the detection of narasin at certain sites despite a lack of nearby poultry farms. He believes the compounds may have been misidentified or may be from other sources.
"We just don't think it's from livestock operations at all," Weber said. "We have a question about what they're finding."
The three substances, part of a group of antibiotics known as ionophores, not only keep food animals healthy but can help them grow faster and with less feed.
Livestock and poultry producers have worked to minimize any remaining traces, or residue, of these drugs from meat intended for human consumption. But most of them strongly support the use of ionophores as a regular part of raising cattle and chickens.
Debate over definition of antibiotics
Many in the industry won't even acknowledge ionophores to be antibiotics, though the Food and Drug Administration classifies them as such. In 2003, members of the cattlemen's group sought to petition the FDA to reclassify ionophores as the bovine equivalent of a food supplement. They eased that stance after an FDA guidance document concluded the human health impacts were likely minimal.
Though agricultural groups maintain these compounds are of no concern to humans, there is little clear research about the long-term health impacts.
"While low concentrations are not necessarily a problem, we just don't know enough to say, 'Oh, these are low concentrations. Let's not worry about that,'" said Margaret Mellon, senior scientist at the Union of Concerned Scientists.
In larger doses, they have proven dangerous to animals for which they are not approved. Monensin has been shown to cause poisoning in horses, and firms that produce it warn against allowing them, or laying hens, any contact with the drug. Pigs and turkeys exposed to narasin have also died. A New Zealand farm worker fell seriously ill after exposure to salinomycin granules.
Worries over greater resistance
Ellen Silbergold, a Johns Hopkins University professor of environmental health sciences who studies the impact of animal antibiotics on humans, said the largest worry is that otherwise harmless bacteria in streams and rivers — which concentrate in sediment, where the largest amounts of antibiotics were detected — might develop better antibiotic resistance when exposed to ambient levels of drugs used by livestock producers.
"The industry is obviously concerned about them because they're trying to keep them out of our consumer products," Silbergold said.
While federal regulations focus on the toxicity of runoff and waste emissions from large animal operations, there is little attention to the residue from medicines, and little monitoring of less apparent sources of possible contamination, including the pools, or "lagoons," used by large livestock operations to process animal waste.
In addition to animal medicines, Carlson found the presence of five tetracycline antibiotics used in both human and veterinary medicine.
Ed Furlong, a USGS research chemist who helped produce the 2002 study, said the latest findings are helpful in narrowing possible sources of contamination and developing better water management policies, but shouldn't be used to start assigning blame.
"Little is known about what effect these concentrations have on humans or ecosystems," Furlong said.
Carlson acknowledged it was not clear whether the antibiotics might be harmful, but he plans additional research to pinpoint exactly how the chemicals made their way into the river.
"There's really only one place they could be coming from," he said. "Now we need to figure out how they're getting there."
Note: An earlier version included a different quote from Ed Furlong, which was revised to more accurately reflect his position.